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 <tursulin@igalia.com>
Signed-off-by: Maíra Canal <mcanal@igalia.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240420213632.339941-7-mcanal@igalia.com

1 files changed, 10 insertions, 4 deletions

diff --git a/drivers/gpu/drm/v3d/v3d_drv.c b/drivers/gpu/drm/v3d/v3d_drv.c
index 2ec359ed2def..28b7ddce7747 100644
--- a/drivers/gpu/drm/v3d/v3d_drv.c
+++ b/drivers/gpu/drm/v3d/v3d_drv.c
@@ -121,6 +121,7 @@ v3d_open(struct drm_device *dev, struct drm_file *file)
 				      1, NULL);
 
 		memset(&v3d_priv->stats[i], 0, sizeof(v3d_priv->stats[i]));
+		seqcount_init(&v3d_priv->stats[i].lock);
 	}
 
 	v3d_perfmon_open_file(v3d_priv);
@@ -145,10 +146,15 @@ v3d_postclose(struct drm_device *dev, struct drm_file *file)
 void v3d_get_stats(const struct v3d_stats *stats, u64 timestamp,
 		   u64 *active_runtime, u64 *jobs_completed)
 {
-	*active_runtime = stats->enabled_ns;
-	if (stats->start_ns)
-		*active_runtime += timestamp - stats->start_ns;
-	*jobs_completed = stats->jobs_completed;
+	unsigned int seq;
+
+	do {
+		seq = read_seqcount_begin(&stats->lock);
+		*active_runtime = stats->enabled_ns;
+		if (stats->start_ns)
+			*active_runtime += timestamp - stats->start_ns;
+		*jobs_completed = stats->jobs_completed;
+	} while (read_seqcount_retry(&stats->lock, seq));
 }
 
 static void v3d_show_fdinfo(struct drm_printer *p, struct drm_file *file)