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: 09a93cc4f7 ("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

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)
 {
+	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)

drivers/gpu/drm/v3d/v3d_drv.h

@@ -40,6 +40,13 @@ struct v3d_stats {
 	u64 start_ns;
 	u64 enabled_ns;
 	u64 jobs_completed;
+
+	/*
+	 * This seqcount is used to protect the access to the GPU stats
+	 * variables. It must be used as, while we are reading the stats,
+	 * IRQs can happen and the stats can be updated.
+	 */
+	seqcount_t lock;
 };
 
 struct v3d_queue_state {

drivers/gpu/drm/v3d/v3d_gem.c

@@ -251,6 +251,7 @@ v3d_gem_init(struct drm_device *dev)
 
 		queue->fence_context = dma_fence_context_alloc(1);
 		memset(&queue->stats, 0, sizeof(queue->stats));
+		seqcount_init(&queue->stats.lock);
 	}
 
 	spin_lock_init(&v3d->mm_lock);

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