commit 61399e0c54 upstream.
RCU re-initializes the deferred QS irq work everytime before attempting
to queue it. However there are situations where the irq work is
attempted to be queued even though it is already queued. In that case
re-initializing messes-up with the irq work queue that is about to be
handled.
The chances for that to happen are higher when the architecture doesn't
support self-IPIs and irq work are then all lazy, such as with the
following sequence:
1) rcu_read_unlock() is called when IRQs are disabled and there is a
grace period involving blocked tasks on the node. The irq work
is then initialized and queued.
2) The related tasks are unblocked and the CPU quiescent state
is reported. rdp->defer_qs_iw_pending is reset to DEFER_QS_IDLE,
allowing the irq work to be requeued in the future (note the previous
one hasn't fired yet).
3) A new grace period starts and the node has blocked tasks.
4) rcu_read_unlock() is called when IRQs are disabled again. The irq work
is re-initialized (but it's queued! and its node is cleared) and
requeued. Which means it's requeued to itself.
5) The irq work finally fires with the tick. But since it was requeued
to itself, it loops and hangs.
Fix this with initializing the irq work only once before the CPU boots.
Fixes: b41642c877 ("rcu: Fix rcu_read_unlock() deadloop due to IRQ work")
Reported-by: kernel test robot <oliver.sang@intel.com>
Closes: https://lore.kernel.org/oe-lkp/202508071303.c1134cce-lkp@intel.com
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Reviewed-by: Joel Fernandes <joelagnelf@nvidia.com>
Signed-off-by: Neeraj Upadhyay (AMD) <neeraj.upadhyay@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
[ Upstream commit 33b6a1f155 ]
Currently the call_rcu() API does not check whether a callback
pointer is NULL. If NULL is passed, rcu_core() will try to invoke
it, resulting in NULL pointer dereference and a kernel crash.
To prevent this and improve debuggability, this patch adds a check
for NULL and emits a kernel stack trace to help identify a faulty
caller.
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Reviewed-by: Joel Fernandes <joelagnelf@nvidia.com>
Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
[ Upstream commit 85aad7cc41 ]
The get_state_synchronize_rcu_full() and poll_state_synchronize_rcu_full()
functions use the root rcu_node structure's ->gp_seq field to detect
the beginnings and ends of grace periods, respectively. This choice is
necessary for the poll_state_synchronize_rcu_full() function because
(give or take counter wrap), the following sequence is guaranteed not
to trigger:
get_state_synchronize_rcu_full(&rgos);
synchronize_rcu();
WARN_ON_ONCE(!poll_state_synchronize_rcu_full(&rgos));
The RCU callbacks that awaken synchronize_rcu() instances are
guaranteed not to be invoked before the root rcu_node structure's
->gp_seq field is updated to indicate the end of the grace period.
However, these callbacks might start being invoked immediately
thereafter, in particular, before rcu_state.gp_seq has been updated.
Therefore, poll_state_synchronize_rcu_full() must refer to the
root rcu_node structure's ->gp_seq field. Because this field is
updated under this structure's ->lock, any code following a call to
poll_state_synchronize_rcu_full() will be fully ordered after the
full grace-period computation, as is required by RCU's memory-ordering
semantics.
By symmetry, the get_state_synchronize_rcu_full() function should also
use this same root rcu_node structure's ->gp_seq field. But it turns out
that symmetry is profoundly (though extremely infrequently) destructive
in this case. To see this, consider the following sequence of events:
1. CPU 0 starts a new grace period, and updates rcu_state.gp_seq
accordingly.
2. As its first step of grace-period initialization, CPU 0 examines
the current CPU hotplug state and decides that it need not wait
for CPU 1, which is currently offline.
3. CPU 1 comes online, and updates its state. But this does not
affect the current grace period, but rather the one after that.
After all, CPU 1 was offline when the current grace period
started, so all pre-existing RCU readers on CPU 1 must have
completed or been preempted before it last went offline.
The current grace period therefore has nothing it needs to wait
for on CPU 1.
4. CPU 1 switches to an rcutorture kthread which is running
rcutorture's rcu_torture_reader() function, which starts a new
RCU reader.
5. CPU 2 is running rcutorture's rcu_torture_writer() function
and collects a new polled grace-period "cookie" using
get_state_synchronize_rcu_full(). Because the newly started
grace period has not completed initialization, the root rcu_node
structure's ->gp_seq field has not yet been updated to indicate
that this new grace period has already started.
This cookie is therefore set up for the end of the current grace
period (rather than the end of the following grace period).
6. CPU 0 finishes grace-period initialization.
7. If CPU 1’s rcutorture reader is preempted, it will be added to
the ->blkd_tasks list, but because CPU 1’s ->qsmask bit is not
set in CPU 1's leaf rcu_node structure, the ->gp_tasks pointer
will not be updated. Thus, this grace period will not wait on
it. Which is only fair, given that the CPU did not come online
until after the grace period officially started.
8. CPUs 0 and 2 then detect the new grace period and then report
a quiescent state to the RCU core.
9. Because CPU 1 was offline at the start of the current grace
period, CPUs 0 and 2 are the only CPUs that this grace period
needs to wait on. So the grace period ends and post-grace-period
cleanup starts. In particular, the root rcu_node structure's
->gp_seq field is updated to indicate that this grace period
has now ended.
10. CPU 2 continues running rcu_torture_writer() and sees that,
from the viewpoint of the root rcu_node structure consulted by
the poll_state_synchronize_rcu_full() function, the grace period
has ended. It therefore updates state accordingly.
11. CPU 1 is still running the same RCU reader, which notices this
update and thus complains about the too-short grace period.
The fix is for the get_state_synchronize_rcu_full() function to use
rcu_state.gp_seq instead of the root rcu_node structure's ->gp_seq field.
With this change in place, if step 5's cookie indicates that the grace
period has not yet started, then any prior code executed by CPU 2 must
have happened before CPU 1 came online. This will in turn prevent CPU
1's code in steps 3 and 11 from spanning CPU 2's grace-period wait,
thus preventing CPU 1 from being subjected to a too-short grace period.
This commit therefore makes this change. Note that there is no change to
the poll_state_synchronize_rcu_full() function, which as noted above,
must continue to use the root rcu_node structure's ->gp_seq field.
This is of course an asymmetry between these two functions, but is an
asymmetry that is absolutely required for correct operation. It is a
common human tendency to greatly value symmetry, and sometimes symmetry
is a wonderful thing. Other times, symmetry results in poor performance.
But in this case, symmetry is just plain wrong.
Nevertheless, the asymmetry does require an additional adjustment.
It is possible for get_state_synchronize_rcu_full() to see a given
grace period as having started, but for an immediately following
poll_state_synchronize_rcu_full() to see it as having not yet started.
Given the current rcu_seq_done_exact() implementation, this will
result in a false-positive indication that the grace period is done
from poll_state_synchronize_rcu_full(). This is dealt with by making
rcu_seq_done_exact() reach back three grace periods rather than just
two of them.
However, simply changing get_state_synchronize_rcu_full() function to
use rcu_state.gp_seq instead of the root rcu_node structure's ->gp_seq
field results in a theoretical bug in kernels booted with
rcutree.rcu_normal_wake_from_gp=1 due to the following sequence of
events:
o The rcu_gp_init() function invokes rcu_seq_start() to officially
start a new grace period.
o A new RCU reader begins, referencing X from some RCU-protected
list. The new grace period is not obligated to wait for this
reader.
o An updater removes X, then calls synchronize_rcu(), which queues
a wait element.
o The grace period ends, awakening the updater, which frees X
while the reader is still referencing it.
The reason that this is theoretical is that although the grace period
has officially started, none of the CPUs are officially aware of this,
and thus will have to assume that the RCU reader pre-dated the start of
the grace period. Detailed explanation can be found at [2] and [3].
Except for kernels built with CONFIG_PROVE_RCU=y, which use the polled
grace-period APIs, which can and do complain bitterly when this sequence
of events occurs. Not only that, there might be some future RCU
grace-period mechanism that pulls this sequence of events from theory
into practice. This commit therefore also pulls the call to
rcu_sr_normal_gp_init() to precede that to rcu_seq_start().
Although this fixes commit 91a967fd69 ("rcu: Add full-sized polling
for get_completed*() and poll_state*()"), it is not clear that it is
worth backporting this commit. First, it took me many weeks to convince
rcutorture to reproduce this more frequently than once per year.
Second, this cannot be reproduced at all without frequent CPU-hotplug
operations, as in waiting all of 50 milliseconds from the end of the
previous operation until starting the next one. Third, the TREE03.boot
settings cause multi-millisecond delays during RCU grace-period
initialization, which greatly increase the probability of the above
sequence of events. (Don't do this in production workloads!) Fourth,
the TREE03 rcutorture scenario was modified to use four-CPU guest OSes,
to have a single-rcu_node combining tree, no testing of RCU priority
boosting, and no random preemption, and these modifications were
necessary to reproduce this issue in a reasonable timeframe. Fifth,
extremely heavy use of get_state_synchronize_rcu_full() and/or
poll_state_synchronize_rcu_full() is required to reproduce this, and as
of v6.12, only kfree_rcu() uses it, and even then not particularly
heavily.
[boqun: Apply the fix [1], and add the comment before the moved
rcu_sr_normal_gp_init(). Additional links are added for explanation.]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Tested-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Link: https://lore.kernel.org/rcu/d90bd6d9-d15c-4b9b-8a69-95336e74e8f4@paulmck-laptop/ [1]
Link: https://lore.kernel.org/rcu/20250303001507.GA3994772@joelnvbox/ [2]
Link: https://lore.kernel.org/rcu/Z8bcUsZ9IpRi1QoP@pc636/ [3]
Reviewed-by: Joel Fernandes <joelagnelf@nvidia.com>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
[ Upstream commit a23da88c6c ]
KCSAN reports a data race when access the krcp->monitor_work.timer.expires
variable in the schedule_delayed_monitor_work() function:
<snip>
BUG: KCSAN: data-race in __mod_timer / kvfree_call_rcu
read to 0xffff888237d1cce8 of 8 bytes by task 10149 on cpu 1:
schedule_delayed_monitor_work kernel/rcu/tree.c:3520 [inline]
kvfree_call_rcu+0x3b8/0x510 kernel/rcu/tree.c:3839
trie_update_elem+0x47c/0x620 kernel/bpf/lpm_trie.c:441
bpf_map_update_value+0x324/0x350 kernel/bpf/syscall.c:203
generic_map_update_batch+0x401/0x520 kernel/bpf/syscall.c:1849
bpf_map_do_batch+0x28c/0x3f0 kernel/bpf/syscall.c:5143
__sys_bpf+0x2e5/0x7a0
__do_sys_bpf kernel/bpf/syscall.c:5741 [inline]
__se_sys_bpf kernel/bpf/syscall.c:5739 [inline]
__x64_sys_bpf+0x43/0x50 kernel/bpf/syscall.c:5739
x64_sys_call+0x2625/0x2d60 arch/x86/include/generated/asm/syscalls_64.h:322
do_syscall_x64 arch/x86/entry/common.c:52 [inline]
do_syscall_64+0xc9/0x1c0 arch/x86/entry/common.c:83
entry_SYSCALL_64_after_hwframe+0x77/0x7f
write to 0xffff888237d1cce8 of 8 bytes by task 56 on cpu 0:
__mod_timer+0x578/0x7f0 kernel/time/timer.c:1173
add_timer_global+0x51/0x70 kernel/time/timer.c:1330
__queue_delayed_work+0x127/0x1a0 kernel/workqueue.c:2523
queue_delayed_work_on+0xdf/0x190 kernel/workqueue.c:2552
queue_delayed_work include/linux/workqueue.h:677 [inline]
schedule_delayed_monitor_work kernel/rcu/tree.c:3525 [inline]
kfree_rcu_monitor+0x5e8/0x660 kernel/rcu/tree.c:3643
process_one_work kernel/workqueue.c:3229 [inline]
process_scheduled_works+0x483/0x9a0 kernel/workqueue.c:3310
worker_thread+0x51d/0x6f0 kernel/workqueue.c:3391
kthread+0x1d1/0x210 kernel/kthread.c:389
ret_from_fork+0x4b/0x60 arch/x86/kernel/process.c:147
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:244
Reported by Kernel Concurrency Sanitizer on:
CPU: 0 UID: 0 PID: 56 Comm: kworker/u8:4 Not tainted 6.12.0-rc2-syzkaller-00050-g5b7c893ed5ed #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 09/13/2024
Workqueue: events_unbound kfree_rcu_monitor
<snip>
kfree_rcu_monitor() rearms the work if a "krcp" has to be still
offloaded and this is done without holding krcp->lock, whereas
the kvfree_call_rcu() holds it.
Fix it by acquiring the "krcp->lock" for kfree_rcu_monitor() so
both functions do not race anymore.
Reported-by: syzbot+061d370693bdd99f9d34@syzkaller.appspotmail.com
Link: https://lore.kernel.org/lkml/ZxZ68KmHDQYU0yfD@pc636/T/
Fixes: 8fc5494ad5 ("rcu/kvfree: Move need_offload_krc() out of krcp->lock")
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Reviewed-by: Neeraj Upadhyay <Neeraj.Upadhyay@amd.com>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Improve readability of kvfree_rcu_queue_batch() function
in away that, after a first batch queuing, the loop is break
and success value is returned to a caller.
There is no reason to loop and check batches further as all
outstanding objects have already been picked and attached to
a certain batch to complete an offloading.
Fixes: 2b55d6a42d ("rcu/kvfree: Add kvfree_rcu_barrier() API")
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Closes: https://lore.kernel.org/lkml/ZvWUt2oyXRsvJRNc@pc636/T/
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Pull slab updates from Vlastimil Babka:
"This time it's mostly refactoring and improving APIs for slab users in
the kernel, along with some debugging improvements.
- kmem_cache_create() refactoring (Christian Brauner)
Over the years have been growing new parameters to
kmem_cache_create() where most of them are needed only for a small
number of caches - most recently the rcu_freeptr_offset parameter.
To avoid adding new parameters to kmem_cache_create() and adjusting
all its callers, or creating new wrappers such as
kmem_cache_create_rcu(), we can now pass extra parameters using the
new struct kmem_cache_args. Not explicitly initialized fields
default to values interpreted as unused.
kmem_cache_create() is for now a wrapper that works both with the
new form: kmem_cache_create(name, object_size, args, flags) and the
legacy form: kmem_cache_create(name, object_size, align, flags,
ctor)
- kmem_cache_destroy() waits for kfree_rcu()'s in flight (Vlastimil
Babka, Uladislau Rezki)
Since SLOB removal, kfree() is allowed for freeing objects
allocated by kmem_cache_create(). By extension kfree_rcu() as
allowed as well, which can allow converting simple call_rcu()
callbacks that only do kmem_cache_free(), as there was never a
kmem_cache_free_rcu() variant. However, for caches that can be
destroyed e.g. on module removal, the cache owners knew to issue
rcu_barrier() first to wait for the pending call_rcu()'s, and this
is not sufficient for pending kfree_rcu()'s due to its internal
batching optimizations. Ulad has provided a new
kvfree_rcu_barrier() and to make the usage less error-prone,
kmem_cache_destroy() calls it. Additionally, destroying
SLAB_TYPESAFE_BY_RCU caches now again issues rcu_barrier()
synchronously instead of using an async work, because the past
motivation for async work no longer applies. Users of custom
call_rcu() callbacks should however keep calling rcu_barrier()
before cache destruction.
- Debugging use-after-free in SLAB_TYPESAFE_BY_RCU caches (Jann Horn)
Currently, KASAN cannot catch UAFs in such caches as it is legal to
access them within a grace period, and we only track the grace
period when trying to free the underlying slab page. The new
CONFIG_SLUB_RCU_DEBUG option changes the freeing of individual
object to be RCU-delayed, after which KASAN can poison them.
- Delayed memcg charging (Shakeel Butt)
In some cases, the memcg is uknown at allocation time, such as
receiving network packets in softirq context. With
kmem_cache_charge() these may be now charged later when the user
and its memcg is known.
- Misc fixes and improvements (Pedro Falcato, Axel Rasmussen,
Christoph Lameter, Yan Zhen, Peng Fan, Xavier)"
* tag 'slab-for-6.12' of git://git.kernel.org/pub/scm/linux/kernel/git/vbabka/slab: (34 commits)
mm, slab: restore kerneldoc for kmem_cache_create()
io_uring: port to struct kmem_cache_args
slab: make __kmem_cache_create() static inline
slab: make kmem_cache_create_usercopy() static inline
slab: remove kmem_cache_create_rcu()
file: port to struct kmem_cache_args
slab: create kmem_cache_create() compatibility layer
slab: port KMEM_CACHE_USERCOPY() to struct kmem_cache_args
slab: port KMEM_CACHE() to struct kmem_cache_args
slab: remove rcu_freeptr_offset from struct kmem_cache
slab: pass struct kmem_cache_args to do_kmem_cache_create()
slab: pull kmem_cache_open() into do_kmem_cache_create()
slab: pass struct kmem_cache_args to create_cache()
slab: port kmem_cache_create_usercopy() to struct kmem_cache_args
slab: port kmem_cache_create_rcu() to struct kmem_cache_args
slab: port kmem_cache_create() to struct kmem_cache_args
slab: add struct kmem_cache_args
slab: s/__kmem_cache_create/do_kmem_cache_create/g
memcg: add charging of already allocated slab objects
mm/slab: Optimize the code logic in find_mergeable()
...
Pull RCU updates from Neeraj Upadhyay:
"Context tracking:
- rename context tracking state related symbols and remove references
to "dynticks" in various context tracking state variables and
related helpers
- force context_tracking_enabled_this_cpu() to be inlined to avoid
leaving a noinstr section
CSD lock:
- enhance CSD-lock diagnostic reports
- add an API to provide an indication of ongoing CSD-lock stall
nocb:
- update and simplify RCU nocb code to handle (de-)offloading of
callbacks only for offline CPUs
- fix RT throttling hrtimer being armed from offline CPU
rcutorture:
- remove redundant rcu_torture_ops get_gp_completed fields
- add SRCU ->same_gp_state and ->get_comp_state functions
- add generic test for NUM_ACTIVE_*RCU_POLL* for testing RCU and SRCU
polled grace periods
- add CFcommon.arch for arch-specific Kconfig options
- print number of update types in rcu_torture_write_types()
- add rcutree.nohz_full_patience_delay testing to the TREE07 scenario
- add a stall_cpu_repeat module parameter to test repeated CPU stalls
- add argument to limit number of CPUs a guest OS can use in
torture.sh
rcustall:
- abbreviate RCU CPU stall warnings during CSD-lock stalls
- Allow dump_cpu_task() to be called without disabling preemption
- defer printing stall-warning backtrace when holding rcu_node lock
srcu:
- make SRCU gp seq wrap-around faster
- add KCSAN checks for concurrent updates to ->srcu_n_exp_nodelay and
->reschedule_count which are used in heuristics governing
auto-expediting of normal SRCU grace periods and
grace-period-state-machine delays
- mark idle SRCU-barrier callbacks to help identify stuck
SRCU-barrier callback
rcu tasks:
- remove RCU Tasks Rude asynchronous APIs as they are no longer used
- stop testing RCU Tasks Rude asynchronous APIs
- fix access to non-existent percpu regions
- check processor-ID assumptions during chosen CPU calculation for
callback enqueuing
- update description of rtp->tasks_gp_seq grace-period sequence
number
- add rcu_barrier_cb_is_done() to identify whether a given
rcu_barrier callback is stuck
- mark idle Tasks-RCU-barrier callbacks
- add *torture_stats_print() functions to print detailed diagnostics
for Tasks-RCU variants
- capture start time of rcu_barrier_tasks*() operation to help
distinguish a hung barrier operation from a long series of barrier
operations
refscale:
- add a TINY scenario to support tests of Tiny RCU and Tiny
SRCU
- optimize process_durations() operation
rcuscale:
- dump stacks of stalled rcu_scale_writer() instances and
grace-period statistics when rcu_scale_writer() stalls
- mark idle RCU-barrier callbacks to identify stuck RCU-barrier
callbacks
- print detailed grace-period and barrier diagnostics on
rcu_scale_writer() hangs for Tasks-RCU variants
- warn if async module parameter is specified for RCU implementations
that do not have async primitives such as RCU Tasks Rude
- make all writer tasks report upon hang
- tolerate repeated GFP_KERNEL failure in rcu_scale_writer()
- use special allocator for rcu_scale_writer()
- NULL out top-level pointers to heap memory to avoid double-free
bugs on modprobe failures
- maintain per-task instead of per-CPU callbacks count to avoid any
issues with migration of either tasks or callbacks
- constify struct ref_scale_ops
Fixes:
- use system_unbound_wq for kfree_rcu work to avoid disturbing
isolated CPUs
Misc:
- warn on unexpected rcu_state.srs_done_tail state
- better define "atomic" for list_replace_rcu() and
hlist_replace_rcu() routines
- annotate struct kvfree_rcu_bulk_data with __counted_by()"
* tag 'rcu.release.v6.12' of git://git.kernel.org/pub/scm/linux/kernel/git/rcu/linux: (90 commits)
rcu: Defer printing stall-warning backtrace when holding rcu_node lock
rcu/nocb: Remove superfluous memory barrier after bypass enqueue
rcu/nocb: Conditionally wake up rcuo if not already waiting on GP
rcu/nocb: Fix RT throttling hrtimer armed from offline CPU
rcu/nocb: Simplify (de-)offloading state machine
context_tracking: Tag context_tracking_enabled_this_cpu() __always_inline
context_tracking, rcu: Rename rcu_dyntick trace event into rcu_watching
rcu: Update stray documentation references to rcu_dynticks_eqs_{enter, exit}()
rcu: Rename rcu_momentary_dyntick_idle() into rcu_momentary_eqs()
rcu: Rename rcu_implicit_dynticks_qs() into rcu_watching_snap_recheck()
rcu: Rename dyntick_save_progress_counter() into rcu_watching_snap_save()
rcu: Rename struct rcu_data .exp_dynticks_snap into .exp_watching_snap
rcu: Rename struct rcu_data .dynticks_snap into .watching_snap
rcu: Rename rcu_dynticks_zero_in_eqs() into rcu_watching_zero_in_eqs()
rcu: Rename rcu_dynticks_in_eqs_since() into rcu_watching_snap_stopped_since()
rcu: Rename rcu_dynticks_in_eqs() into rcu_watching_snap_in_eqs()
rcu: Rename rcu_dynticks_eqs_online() into rcu_watching_online()
context_tracking, rcu: Rename rcu_dynticks_curr_cpu_in_eqs() into rcu_is_watching_curr_cpu()
context_tracking, rcu: Rename rcu_dynticks_task*() into rcu_task*()
refscale: Constify struct ref_scale_ops
...
Add a kvfree_rcu_barrier() function. It waits until all
in-flight pointers are freed over RCU machinery. It does
not wait any GP completion and it is within its right to
return immediately if there are no outstanding pointers.
This function is useful when there is a need to guarantee
that a memory is fully freed before destroying memory caches.
For example, during unloading a kernel module.
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
When soft interrupt actions are called, they are passed a pointer to the
struct softirq action which contains the action's function pointer.
This pointer isn't useful, as the action callback already knows what
function it is. And since each callback handles a specific soft interrupt,
the callback also knows which soft interrupt number is running.
No soft interrupt action callback actually uses this parameter, so remove
it from the function pointer signature. This clarifies that soft interrupt
actions are global routines and makes it slightly cheaper to call them.
Signed-off-by: Caleb Sander Mateos <csander@purestorage.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Link: https://lore.kernel.org/all/20240815171549.3260003-1-csander@purestorage.com
The context_tracking.state RCU_DYNTICKS subvariable has been renamed to
RCU_WATCHING, replace "dyntick_idle" into "eqs" to drop the dyntick
reference.
Signed-off-by: Valentin Schneider <vschneid@redhat.com>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Neeraj Upadhyay <neeraj.upadhyay@kernel.org>
The context_tracking.state RCU_DYNTICKS subvariable has been renamed to
RCU_WATCHING, drop the dyntick reference and update the name of this helper
to express that it rechecks rdp->watching_snap after an earlier
rcu_watching_snap_save().
Signed-off-by: Valentin Schneider <vschneid@redhat.com>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Neeraj Upadhyay <neeraj.upadhyay@kernel.org>
The context_tracking.state RCU_DYNTICKS subvariable has been renamed to
RCU_WATCHING, and the 'dynticks' prefix can be dropped without losing any
meaning.
Suggested-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Valentin Schneider <vschneid@redhat.com>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Neeraj Upadhyay <neeraj.upadhyay@kernel.org>
The context_tracking.state RCU_DYNTICKS subvariable has been renamed to
RCU_WATCHING, and the snapshot helpers are now prefix by
"rcu_watching". Reflect that change into the storage variables for these
snapshots.
Signed-off-by: Valentin Schneider <vschneid@redhat.com>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Neeraj Upadhyay <neeraj.upadhyay@kernel.org>
The context_tracking.state RCU_DYNTICKS subvariable has been renamed to
RCU_WATCHING, reflect that change in the related helpers.
Signed-off-by: Valentin Schneider <vschneid@redhat.com>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Neeraj Upadhyay <neeraj.upadhyay@kernel.org>
The context_tracking.state RCU_DYNTICKS subvariable has been renamed to
RCU_WATCHING, the dynticks prefix can go.
While at it, this helper is only meant to be called after failing an
earlier call to rcu_watching_snap_in_eqs(), document this in the comments
and add a WARN_ON_ONCE() for good measure.
Signed-off-by: Valentin Schneider <vschneid@redhat.com>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Neeraj Upadhyay <neeraj.upadhyay@kernel.org>
The context_tracking.state RCU_DYNTICKS subvariable has been renamed to
RCU_WATCHING, reflect that change in the related helpers.
While at it, update a comment that still refers to rcu_dynticks_snap(),
which was removed by commit:
7be2e6323b9b ("rcu: Remove full memory barrier on RCU stall printout")
Signed-off-by: Valentin Schneider <vschneid@redhat.com>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Neeraj Upadhyay <neeraj.upadhyay@kernel.org>
The context_tracking.state RCU_DYNTICKS subvariable has been renamed to
RCU_WATCHING, reflect that change in the related helpers.
Signed-off-by: Valentin Schneider <vschneid@redhat.com>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Neeraj Upadhyay <neeraj.upadhyay@kernel.org>
The context_tracking.state RCU_DYNTICKS subvariable has been renamed to
RCU_WATCHING, reflect that change in the related helpers.
Note that "watching" is the opposite of "in EQS", so the negation is lifted
out of the helper and into the callsites.
Signed-off-by: Valentin Schneider <vschneid@redhat.com>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Neeraj Upadhyay <neeraj.upadhyay@kernel.org>
RCU keeps a count of the number of callbacks that the current
rcu_barrier() is waiting on, but there is currently no easy way to
work out which callback is stuck. One way to do this is to mark idle
RCU-barrier callbacks by making the ->next pointer point to the callback
itself, and this commit does just that.
Later commits will use this for debug output.
Signed-off-by: "Paul E. McKenney" <paulmck@kernel.org>
Signed-off-by: Neeraj Upadhyay <neeraj.upadhyay@kernel.org>
It was discovered that isolated CPUs could sometimes be disturbed by
kworkers processing kfree_rcu() works causing higher than expected
latency. It is because the RCU core uses "system_wq" which doesn't have
the WQ_UNBOUND flag to handle all its work items. Fix this violation of
latency limits by using "system_unbound_wq" in the RCU core instead.
This will ensure that those work items will not be run on CPUs marked
as isolated.
Beside the WQ_UNBOUND flag, the other major difference between system_wq
and system_unbound_wq is their max_active count. The system_unbound_wq
has a max_active of WQ_MAX_ACTIVE (512) while system_wq's max_active
is WQ_DFL_ACTIVE (256) which is half of WQ_MAX_ACTIVE.
Reported-by: Vratislav Bendel <vbendel@redhat.com>
Closes: https://issues.redhat.com/browse/RHEL-50220
Signed-off-by: Waiman Long <longman@redhat.com>
Reviewed-by: "Uladzislau Rezki (Sony)" <urezki@gmail.com>
Tested-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Neeraj Upadhyay <neeraj.upadhyay@kernel.org>
Add the __counted_by compiler attribute to the flexible array member
records to improve access bounds-checking via CONFIG_UBSAN_BOUNDS and
CONFIG_FORTIFY_SOURCE.
Increment nr_records before adding a new pointer to the records array.
Signed-off-by: Thorsten Blum <thorsten.blum@toblux.com>
Reviewed-by: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Reviewed-by: "Uladzislau Rezki (Sony)" <urezki@gmail.com>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Neeraj Upadhyay <neeraj.upadhyay@kernel.org>
The context_tracking.state RCU_DYNTICKS subvariable has been renamed to
RCU_WATCHING, and the 'dynticks' prefix can be dropped without losing any
meaning.
Signed-off-by: Valentin Schneider <vschneid@redhat.com>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Neeraj Upadhyay <neeraj.upadhyay@kernel.org>
The context_tracking.state RCU_DYNTICKS subvariable has been renamed to
RCU_WATCHING, and the 'dynticks' prefix can be dropped without losing any
meaning.
Suggested-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Valentin Schneider <vschneid@redhat.com>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Neeraj Upadhyay <neeraj.upadhyay@kernel.org>
The context_tracking.state RCU_DYNTICKS subvariable has been renamed to
RCU_WATCHING, and the 'dynticks' prefix can be dropped without losing any
meaning.
[ neeraj.upadhyay: Fix htmldocs build error reported by Stephen Rothwell ]
Suggested-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Valentin Schneider <vschneid@redhat.com>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Neeraj Upadhyay <neeraj.upadhyay@kernel.org>
The context_tracking.state RCU_DYNTICKS subvariable has been renamed to
RCU_WATCHING, reflect that change in the related helpers.
Suggested-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Valentin Schneider <vschneid@redhat.com>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Neeraj Upadhyay <neeraj.upadhyay@kernel.org>
The context_tracking.state RCU_DYNTICKS subvariable has been renamed to
RCU_WATCHING, reflect that change in the related helpers.
[ neeraj.upadhyay: Fix htmldocs build error reported by Stephen Rothwell ]
Suggested-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Valentin Schneider <vschneid@redhat.com>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Neeraj Upadhyay <neeraj.upadhyay@kernel.org>
Within the rcu_sr_normal_gp_cleanup_work() function, there is an acquire
load from rcu_state.srs_done_tail, which is expected to be non-NULL.
This commit adds a WARN_ON_ONCE() to check this expectation.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Neeraj Upadhyay <neeraj.upadhyay@kernel.org>
RCU core can't be running anymore while in the middle of (de-)offloading
since this sort of transition now only applies to offline CPUs.
The SEGCBLIST_RCU_CORE state can therefore be removed.
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Neeraj Upadhyay <neeraj.upadhyay@kernel.org>
RCU core can't be running anymore while in the middle of (de-)offloading
since this sort of transition now only applies to offline CPUs.
The locked callback acceleration handling during the transition can
therefore be removed, along with concurrent batch execution.
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Neeraj Upadhyay <neeraj.upadhyay@kernel.org>
RCU core can't be running anymore while in the middle of (de-)offloading
since this sort of transition now only applies to offline CPUs.
The locked callback acceleration handling during the transition can
therefore be removed.
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Neeraj Upadhyay <neeraj.upadhyay@kernel.org>
The barrier_mutex is used currently to protect (de-)offloading
operations and prevent from nocb_lock locking imbalance in rcu_barrier()
and shrinker, and also from misordered RCU barrier invocation.
Now since RCU (de-)offloading is going to happen on offline CPUs, an RCU
barrier will have to be executed while transitionning from offloaded to
de-offloaded state. And this can't happen while holding the
barrier_mutex.
Introduce a NOCB mutex to protect (de-)offloading transitions. The
barrier_mutex is still held for now when necessary to avoid barrier
callbacks reordering and nocb_lock imbalance.
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Neeraj Upadhyay <neeraj.upadhyay@kernel.org>
The context_tracking.state RCU_DYNTICKS subvariable has been renamed to
RCU_WATCHING, reflect that change in the related helpers.
Signed-off-by: Valentin Schneider <vschneid@redhat.com>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Neeraj Upadhyay <neeraj.upadhyay@kernel.org>
The context_tracking.state RCU_DYNTICKS subvariable has been renamed to
RCU_WATCHING, reflect that change in the related helpers.
Signed-off-by: Valentin Schneider <vschneid@redhat.com>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Neeraj Upadhyay <neeraj.upadhyay@kernel.org>
The context_tracking.state RCU_DYNTICKS subvariable has been renamed to
RCU_WATCHING, reflect that change in the related helpers.
Signed-off-by: Valentin Schneider <vschneid@redhat.com>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Neeraj Upadhyay <neeraj.upadhyay@kernel.org>
The symbols relating to the CT_STATE part of context_tracking.state are now
all prefixed with CT_STATE.
The RCU dynticks counter part of that atomic variable still involves
symbols with different prefixes, align them all to be prefixed with
CT_RCU_WATCHING.
Suggested-by: "Paul E. McKenney" <paulmck@kernel.org>
Signed-off-by: Valentin Schneider <vschneid@redhat.com>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Neeraj Upadhyay <neeraj.upadhyay@kernel.org>
When rcu_barrier() calls rcu_rdp_cpu_online() and observes a CPU off
rnp->qsmaskinitnext, it means that all accesses from the offline CPU
preceding the CPUHP_TEARDOWN_CPU are visible to RCU barrier, including
callbacks expiration and counter updates.
However interrupts can still fire after stop_machine() re-enables
interrupts and before rcutree_report_cpu_dead(). The related accesses
happening between CPUHP_TEARDOWN_CPU and rnp->qsmaskinitnext clearing
are _NOT_ guaranteed to be seen by rcu_barrier() without proper
ordering, especially when callbacks are invoked there to the end, making
rcutree_migrate_callback() bypass barrier_lock.
The following theoretical race example can make rcu_barrier() hang:
CPU 0 CPU 1
----- -----
//cpu_down()
smpboot_park_threads()
//ksoftirqd is parked now
<IRQ>
rcu_sched_clock_irq()
invoke_rcu_core()
do_softirq()
rcu_core()
rcu_do_batch()
// callback storm
// rcu_do_batch() returns
// before completing all
// of them
// do_softirq also returns early because of
// timeout. It defers to ksoftirqd but
// it's parked
</IRQ>
stop_machine()
take_cpu_down()
rcu_barrier()
spin_lock(barrier_lock)
// observes rcu_segcblist_n_cbs(&rdp->cblist) != 0
<IRQ>
do_softirq()
rcu_core()
rcu_do_batch()
//completes all pending callbacks
//smp_mb() implied _after_ callback number dec
</IRQ>
rcutree_report_cpu_dead()
rnp->qsmaskinitnext &= ~rdp->grpmask;
rcutree_migrate_callback()
// no callback, early return without locking
// barrier_lock
//observes !rcu_rdp_cpu_online(rdp)
rcu_barrier_entrain()
rcu_segcblist_entrain()
// Observe rcu_segcblist_n_cbs(rsclp) == 0
// because no barrier between reading
// rnp->qsmaskinitnext and rsclp->len
rcu_segcblist_add_len()
smp_mb__before_atomic()
// will now observe the 0 count and empty
// list, but too late, we enqueue regardless
WRITE_ONCE(rsclp->len, rsclp->len + v);
// ignored barrier callback
// rcu barrier stall...
This could be solved with a read memory barrier, enforcing the message
passing between rnp->qsmaskinitnext and rsclp->len, matching the full
memory barrier after rsclp->len addition in rcu_segcblist_add_len()
performed at the end of rcu_do_batch().
However the rcu_barrier() is complicated enough and probably doesn't
need too many more subtleties. CPU down is a slowpath and the
barrier_lock seldom contended. Solve the issue with unconditionally
locking the barrier_lock on rcutree_migrate_callbacks(). This makes sure
that either rcu_barrier() sees the empty queue or its entrained
callback will be migrated.
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
If a CPU is running either a userspace application or a guest OS in
nohz_full mode, it is possible for a system call to occur just as an
RCU grace period is starting. If that CPU also has the scheduling-clock
tick enabled for any reason (such as a second runnable task), and if the
system was booted with rcutree.use_softirq=0, then RCU can add insult to
injury by awakening that CPU's rcuc kthread, resulting in yet another
task and yet more OS jitter due to switching to that task, running it,
and switching back.
In addition, in the common case where that system call is not of
excessively long duration, awakening the rcuc task is pointless.
This pointlessness is due to the fact that the CPU will enter an extended
quiescent state upon returning to the userspace application or guest OS.
In this case, the rcuc kthread cannot do anything that the main RCU
grace-period kthread cannot do on its behalf, at least if it is given
a few additional milliseconds (for example, given the time duration
specified by rcutree.jiffies_till_first_fqs, give or take scheduling
delays).
This commit therefore adds a rcutree.nohz_full_patience_delay kernel
boot parameter that specifies the grace period age (in milliseconds,
rounded to jiffies) before which RCU will refrain from awakening the
rcuc kthread. Preliminary experimentation suggests a value of 1000,
that is, one second. Increasing rcutree.nohz_full_patience_delay will
increase grace-period latency and in turn increase memory footprint,
so systems with constrained memory might choose a smaller value.
Systems with less-aggressive OS-jitter requirements might choose the
default value of zero, which keeps the traditional immediate-wakeup
behavior, thus avoiding increases in grace-period latency.
[ paulmck: Apply Leonardo Bras feedback. ]
Link: https://lore.kernel.org/all/20240328171949.743211-1-leobras@redhat.com/
Reported-by: Leonardo Bras <leobras@redhat.com>
Suggested-by: Leonardo Bras <leobras@redhat.com>
Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Leonardo Bras <leobras@redhat.com>
RCU stall printout fetches the EQS state of a CPU with a preceding full
memory barrier. However there is nothing to order this read against at
this debugging stage. It is inherently racy when performed remotely.
Do a plain read instead.
This was the last user of rcu_dynticks_snap().
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Reviewed-by: Neeraj Upadhyay <neeraj.upadhyay@kernel.org>
When the boot CPU initializes the per-CPU data on behalf of all possible
CPUs, a sanity check is performed on each of them to make sure none is
initialized in an extended quiescent state.
This check involves a full memory barrier which is useless at this early
boot stage.
Do a plain access instead.
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Reviewed-by: Neeraj Upadhyay <neeraj.upadhyay@kernel.org>
When the grace period kthread checks the extended quiescent state
counter of a CPU, full ordering is necessary to ensure that either:
* If the GP kthread observes the remote target in an extended quiescent
state, then that target must observe all accesses prior to the current
grace period, including the current grace period sequence number, once
it exits that extended quiescent state.
or:
* If the GP kthread observes the remote target NOT in an extended
quiescent state, then the target further entering in an extended
quiescent state must observe all accesses prior to the current
grace period, including the current grace period sequence number, once
it enters that extended quiescent state.
This ordering is enforced through a full memory barrier placed right
before taking the first EQS snapshot. However this is superfluous
because the snapshot is taken while holding the target's rnp lock which
provides the necessary ordering through its chain of
smp_mb__after_unlock_lock().
Remove the needless explicit barrier before the snapshot and put a
comment about the implicit barrier newly relied upon here.
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Reviewed-by: Neeraj Upadhyay <neeraj.upadhyay@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
When the grace period kthread checks the extended quiescent state
counter of a CPU, full ordering is necessary to ensure that either:
* If the GP kthread observes the remote target in an extended quiescent
state, then that target must observe all accesses prior to the current
grace period, including the current grace period sequence number, once
it exits that extended quiescent state. Also the GP kthread must
observe all accesses performed by the target prior it entering in
EQS.
or:
* If the GP kthread observes the remote target NOT in an extended
quiescent state, then the target further entering in an extended
quiescent state must observe all accesses prior to the current
grace period, including the current grace period sequence number, once
it enters that extended quiescent state. Also the GP kthread later
observing that EQS must also observe all accesses performed by the
target prior it entering in EQS.
This ordering is explicitly performed both on the first EQS snapshot
and on the second one as well through the combination of a preceding
full barrier followed by an acquire read. However the second snapshot's
full memory barrier is redundant and not needed to enforce the above
guarantees:
GP kthread Remote target
---- -----
// Access prior GP
WRITE_ONCE(A, 1)
// first snapshot
smp_mb()
x = smp_load_acquire(EQS)
// Access prior GP
WRITE_ONCE(B, 1)
// EQS enter
// implied full barrier by atomic_add_return()
atomic_add_return(RCU_DYNTICKS_IDX, EQS)
// implied full barrier by atomic_add_return()
READ_ONCE(A)
// second snapshot
y = smp_load_acquire(EQS)
z = READ_ONCE(B)
If the GP kthread above fails to observe the remote target in EQS
(x not in EQS), the remote target will observe A == 1 after further
entering in EQS. Then the second snapshot taken by the GP kthread only
need to be an acquire read in order to observe z == 1.
Therefore remove the needless full memory barrier on second snapshot.
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Reviewed-by: Neeraj Upadhyay <neeraj.upadhyay@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
In the synchronize_rcu() common case, we will have less than
SR_MAX_USERS_WAKE_FROM_GP number of users per GP. Waking up the kworker
is pointless just to free the last injected wait head since at that point,
all the users have already been awakened.
Introduce a new counter to track this and prevent the wakeup in the
common case.
[ paulmck: Remove atomic_dec_return_release in cannot-happen state. ]
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Reviewed-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
This commit make rcu-tasks related rcutorture test support rcu-tasks
gp state printing when the writer stall occurs or the at the end of
rcutorture test, and generate rcu_ops->get_gp_data() operation to
simplify the acquisition of gp state for different types of rcutorture
tests.
Signed-off-by: Zqiang <qiang.zhang1211@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
synchronize_rcu() users have to be processed regardless
of memory pressure so our private WQ needs to have at least
one execution context what WQ_MEM_RECLAIM flag guarantees.
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
