Re: [PATCH 1/2] drm/v3d: Disable preemption while updating GPU stats

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Hi Tvrtko,

On 8/12/24 06:12, Tvrtko Ursulin wrote:
From: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxx>

We forgot to disable preemption around the write_seqcount_begin/end() pair
while updating GPU stats:

   [ ] WARNING: CPU: 2 PID: 12 at include/linux/seqlock.h:221 __seqprop_assert.isra.0+0x128/0x150 [v3d]
   [ ] Workqueue: v3d_bin drm_sched_run_job_work [gpu_sched]
  <...snip...>
   [ ] Call trace:
   [ ]  __seqprop_assert.isra.0+0x128/0x150 [v3d]
   [ ]  v3d_job_start_stats.isra.0+0x90/0x218 [v3d]
   [ ]  v3d_bin_job_run+0x23c/0x388 [v3d]
   [ ]  drm_sched_run_job_work+0x520/0x6d0 [gpu_sched]
   [ ]  process_one_work+0x62c/0xb48
   [ ]  worker_thread+0x468/0x5b0
   [ ]  kthread+0x1c4/0x1e0
   [ ]  ret_from_fork+0x10/0x20

Fix it.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxx>
Fixes: 6abe93b621ab ("drm/v3d: Fix race-condition between sysfs/fdinfo and interrupt handler")
Cc: Maíra Canal <mcanal@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> # v6.10+
---
  drivers/gpu/drm/v3d/v3d_sched.c | 6 ++++++
  1 file changed, 6 insertions(+)

diff --git a/drivers/gpu/drm/v3d/v3d_sched.c b/drivers/gpu/drm/v3d/v3d_sched.c
index 42d4f4a2dba2..cc2e5a89467b 100644
--- a/drivers/gpu/drm/v3d/v3d_sched.c
+++ b/drivers/gpu/drm/v3d/v3d_sched.c
@@ -136,6 +136,8 @@ v3d_job_start_stats(struct v3d_job *job, enum v3d_queue queue)
  	struct v3d_stats *local_stats = &file->stats[queue];
  	u64 now = local_clock();
+ preempt_disable();
+
  	write_seqcount_begin(&local_stats->lock);
  	local_stats->start_ns = now;
  	write_seqcount_end(&local_stats->lock);
@@ -143,6 +145,8 @@ v3d_job_start_stats(struct v3d_job *job, enum v3d_queue queue)
  	write_seqcount_begin(&global_stats->lock);
  	global_stats->start_ns = now;
  	write_seqcount_end(&global_stats->lock);
+
+	preempt_enable();
  }
static void
@@ -164,8 +168,10 @@ v3d_job_update_stats(struct v3d_job *job, enum v3d_queue queue)
  	struct v3d_stats *local_stats = &file->stats[queue];
  	u64 now = local_clock();
+ preempt_disable();
  	v3d_stats_update(local_stats, now);
  	v3d_stats_update(global_stats, now);
+	preempt_enable();

Although `v3d_stats_update()` is only used here, shouldn't we move `preempt_disable()` and `preempt_enable()` inside of the
`v3d_stats_update()` function? This way, we can guarantee that any
future uses will disable preemption.

With that, you can add my:

Acked-by: Maíra Canal <mcanal@xxxxxxxxxx>

Best Regards,
- Maíra

  }
static struct dma_fence *v3d_bin_job_run(struct drm_sched_job *sched_job)



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