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[ Upstream commit4cb5ac2626] Shrikanth noted that the per-cpu reference counter was still some 10% slower than the old immutable option (which removes the reference counting entirely). Further optimize the per-cpu reference counter by: - switching from RCU to preempt; - using __this_cpu_*() since we now have preempt disabled; - switching from smp_load_acquire() to READ_ONCE(). This is all safe because disabling preemption inhibits the RCU grace period exactly like rcu_read_lock(). Having preemption disabled allows using __this_cpu_*() provided the only access to the variable is in task context -- which is the case here. Furthermore, since we know changing fph->state to FR_ATOMIC demands a full RCU grace period we can rely on the implied smp_mb() from that to replace the acquire barrier(). This is very similar to the percpu_down_read_internal() fast-path. The reason this is significant for PowerPC is that it uses the generic this_cpu_*() implementation which relies on local_irq_disable() (the x86 implementation relies on it being a single memop instruction to be IRQ-safe). Switching to preempt_disable() and __this_cpu*() avoids this IRQ state swizzling. Also, PowerPC needs LWSYNC for the ACQUIRE barrier, not having to use explicit barriers safes a bunch. Combined this reduces the performance gap by half, down to some 5%. Fixes:760e6f7bef("futex: Remove support for IMMUTABLE") Reported-by: Shrikanth Hegde <sshegde@linux.ibm.com> Tested-by: Shrikanth Hegde <sshegde@linux.ibm.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> Link: https://patch.msgid.link/20251106092929.GR4067720@noisy.programming.kicks-ass.net Signed-off-by: Sasha Levin <sashal@kernel.org>