drm/v3d: Address race-condition between per-fd GPU stats and fd release

When the file descriptor is closed while a job is still running,
there's a race condition between the job completion callback and the
file descriptor cleanup. This can lead to accessing freed memory when
updating per-fd GPU stats, such as the following example:

[56120.512903] Unable to handle kernel paging request at virtual address 0000330a92b9688a
[56120.520881] Mem abort info:
[56120.523687] ESR = 0x0000000096000005
[56120.527454] EC = 0x25: DABT (current EL), IL = 32 bits
[56120.532785] SET = 0, FnV = 0
[56120.535847] EA = 0, S1PTW = 0
[56120.538995] FSC = 0x05: level 1 translation fault
[56120.543891] Data abort info:
[56120.546778] ISV = 0, ISS = 0x00000005, ISS2 = 0x00000000
[56120.552289] CM = 0, WnR = 0, TnD = 0, TagAccess = 0
[56120.557362] GCS = 0, Overlay = 0, DirtyBit = 0, Xs = 0
[56120.562690] user pgtable: 16k pages, 47-bit VAs, pgdp=0000000023f54000
[56120.569239] [0000330a92b9688a] pgd=0000000000000000, p4d=0000000000000000, pud=0000000000000000
[56120.577975] Internal error: Oops: 0000000096000005 [#1] PREEMPT SMP
 	       CPU: 0 UID: 1000 PID: 1497409 Comm: mpv Not tainted 6.12.37-ncvm5+ #1
 	       Hardware name: Raspberry Pi 5 Model B Rev 1.0 (DT)
 	       pstate: 604000c9 (nZCv daIF +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
 	       pc : v3d_job_update_stats+0x64/0x168 [v3d]
 	       lr : v3d_job_update_stats+0x40/0x168 [v3d]
 	       sp : ffffc00080003e60
 	       x29: ffffc00080003e60 x28: ffff800002860000 x27: 0000000000000000
 	       x26: 0000000000000000 x25: ffff800002860000 x24: ffff800002630800
 	       x23: ffff800060786000 x22: 0000330a933c31fb x21: 0000000000000001
 	       x20: 0000330a92b96302 x19: ffff800060786b10 x18: 0000000000000000
 	       x17: ffffaf90506a0000 x16: ffffd06fce57c360 x15: 0000000000000000
 	       x14: 0000000000000000 x13: 0000000000000000 x12: 0000000000000000
 	       x11: 0000000000000000 x10: 0000000000000000 x9 : ffffd06f5d0fec40
 	       x8 : 0000000000000000 x7 : 0000000000000000 x6 : 000002978dbd535a
 	       x5 : 00ffffffffffffff x4 : 0000000000000015 x3 : 0000300001fddf88
 	       x2 : 0000000000000020 x1 : 0000000000010001 x0 : 0000330a92b96872
 	       Call trace:
		 v3d_job_update_stats+0x64/0x168 [v3d]
		 v3d_irq+0x118/0x2e0 [v3d]
		 __handle_irq_event_percpu+0x60/0x220

Fix such an issue by protecting all accesses to `job->file_priv` with
the queue's lock. With that, we can clear `job->file_priv` before the
V3D per-fd structure is freed and assure that `job->file_priv` exists
during the per-fd GPU stats updates.

Fixes: e1bc3a13bd ("drm/v3d: Avoid NULL pointer dereference in `v3d_job_update_stats()`")
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Link: https://lore.kernel.org/r/20250826-v3d-queue-lock-v3-4-979efc43e490@igalia.com
Signed-off-by: Maíra Canal <mcanal@igalia.com>

drivers/gpu/drm/v3d/v3d_drv.c

@@ -157,12 +157,24 @@ v3d_open(struct drm_device *dev, struct drm_file *file)
 static void
 v3d_postclose(struct drm_device *dev, struct drm_file *file)
 {
+	struct v3d_dev *v3d = to_v3d_dev(dev);
 	struct v3d_file_priv *v3d_priv = file->driver_priv;
+	unsigned long irqflags;
 	enum v3d_queue q;
 
-	for (q = 0; q < V3D_MAX_QUEUES; q++)
+	for (q = 0; q < V3D_MAX_QUEUES; q++) {
+		struct v3d_queue_state *queue = &v3d->queue[q];
+		struct v3d_job *job = queue->active_job;
+
 		drm_sched_entity_destroy(&v3d_priv->sched_entity[q]);
 
+		if (job && job->base.entity == &v3d_priv->sched_entity[q]) {
+			spin_lock_irqsave(&queue->queue_lock, irqflags);
+			job->file_priv = NULL;
+			spin_unlock_irqrestore(&queue->queue_lock, irqflags);
+		}
+	}
+
 	v3d_perfmon_close_file(v3d_priv);
 	kfree(v3d_priv);
 }

drivers/gpu/drm/v3d/v3d_drv.h

@@ -608,7 +608,7 @@ void v3d_timestamp_query_info_free(struct v3d_timestamp_query_info *query_info,
 				   unsigned int count);
 void v3d_performance_query_info_free(struct v3d_performance_query_info *query_info,
 				     unsigned int count);
-void v3d_job_update_stats(struct v3d_job *job, enum v3d_queue queue);
+void v3d_job_update_stats(struct v3d_job *job, enum v3d_queue q);
 int v3d_sched_init(struct v3d_dev *v3d);
 void v3d_sched_fini(struct v3d_dev *v3d);
 

drivers/gpu/drm/v3d/v3d_sched.c

@@ -194,11 +194,11 @@ v3d_stats_update(struct v3d_stats *stats, u64 now)
 }
 
 void
-v3d_job_update_stats(struct v3d_job *job, enum v3d_queue queue)
+v3d_job_update_stats(struct v3d_job *job, enum v3d_queue q)
 {
 	struct v3d_dev *v3d = job->v3d;
-	struct v3d_file_priv *file = job->file_priv;
+	struct v3d_queue_state *queue = &v3d->queue[q];
-	struct v3d_stats *global_stats = &v3d->queue[queue].stats;
+	struct v3d_stats *global_stats = &queue->stats;
 	u64 now = local_clock();
 	unsigned long flags;
 
@@ -209,10 +209,10 @@ v3d_job_update_stats(struct v3d_job *job, enum v3d_queue queue)
 		preempt_disable();
 
 	/* Don't update the local stats if the file context has already closed */
-	if (file)
+	spin_lock(&queue->queue_lock);
-		v3d_stats_update(&file->stats[queue], now);
+	if (job->file_priv)
-	else
+		v3d_stats_update(&job->file_priv->stats[q], now);
-		drm_dbg(&v3d->drm, "The file descriptor was closed before job completion\n");
+	spin_unlock(&queue->queue_lock);
 
 	v3d_stats_update(global_stats, now);