drm/v3d: Fix race-condition between sysfs/fdinfo and interrupt handler

In V3D, the conclusion of a job is indicated by a IRQ. When a job
finishes, then we update the local and the global GPU stats of that
queue. But, while the GPU stats are being updated, a user might be
reading the stats from sysfs or fdinfo.

For example, on `gpu_stats_show()`, we could think about a scenario where
`v3d->queue[queue].start_ns != 0`, then an interrupt happens, we update
the value of `v3d->queue[queue].start_ns` to 0, we come back to
`gpu_stats_show()` to calculate `active_runtime` and now,
`active_runtime = timestamp`.

In this simple example, the user would see a spike in the queue usage,
that didn't match reality.

In order to address this issue properly, use a seqcount to protect read
and write sections of the code.

Fixes: 09a93cc4f7d1 ("drm/v3d: Implement show_fdinfo() callback for GPU usage stats")
Reported-by: Tvrtko Ursulin <[email protected]>
Signed-off-by: Maíra Canal <[email protected]>
Reviewed-by: Tvrtko Ursulin <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

1 files changed, 7 insertions, 0 deletions

diff --git a/drivers/gpu/drm/v3d/v3d_sched.c b/drivers/gpu/drm/v3d/v3d_sched.c
index b9614944931c..7cd8c335cd9b 100644
--- a/drivers/gpu/drm/v3d/v3d_sched.c
+++ b/drivers/gpu/drm/v3d/v3d_sched.c
@@ -114,16 +114,23 @@ v3d_job_start_stats(struct v3d_job *job, enum v3d_queue queue)
 	struct v3d_stats *local_stats = &file->stats[queue];
 	u64 now = local_clock();
 
+	write_seqcount_begin(&local_stats->lock);
 	local_stats->start_ns = now;
+	write_seqcount_end(&local_stats->lock);
+
+	write_seqcount_begin(&global_stats->lock);
 	global_stats->start_ns = now;
+	write_seqcount_end(&global_stats->lock);
 }
 
 static void
 v3d_stats_update(struct v3d_stats *stats, u64 now)
 {
+	write_seqcount_begin(&stats->lock);
 	stats->enabled_ns += now - stats->start_ns;
 	stats->jobs_completed++;
 	stats->start_ns = 0;
+	write_seqcount_end(&stats->lock);
 }
 
 void