[Intel-gfx] [PATCH] drm/i915/psr: Fix race in intel_psr_work()

[Dhinakaran Pandiyan]

commit 5422b37c907e ("drm/i915/psr: Kill delays when activating psr
back.") switched from delayed work to the plain variant and while doing so
remove the check for work_busy() before scheduling a PSR activation.
This appears to cause consecutive executions of psr_activate() in this
scenario - after a worker picks up the PSR work item for execution and
before the work function can acquire the PSR mutex, a psr_flush() can
get hold of the mutex and schedule another PSR work. Without a psr_exit()
between two psr_activate() calls, the warning messages get printed.
Further, since we drop the mutex in the midst of psr_work() to wait for
PSR to idle, another work item can also get scheduled. Fix this by
returning if PSR was already active.

Note:
I am not 100% sure this is what is going on as I could not reproduce
the bug (https://bugs.freedesktop.org/show_bug.cgi?id=106948)

This patch sort of defeats the point of the WARN_ON()s in psr_activate()
now, do we still need them?

Cc: Rodrigo Vivi <rodrigo.vivi at intel.com>
Cc: Chris Wilson <chris at chris-wilson.co.uk>
Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan at intel.com>
---
 drivers/gpu/drm/i915/intel_psr.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/intel_psr.c b/drivers/gpu/drm/i915/intel_psr.c
index aea81ace854b..7aa324f0d1f7 100644
--- a/drivers/gpu/drm/i915/intel_psr.c
+++ b/drivers/gpu/drm/i915/intel_psr.c
@@ -811,7 +811,7 @@ static void intel_psr_work(struct work_struct *work)
 	 * recheck. Since psr_flush first clears this and then reschedules we
 	 * won't ever miss a flush when bailing out here.
 	 */
-	if (dev_priv->psr.busy_frontbuffer_bits)
+	if (dev_priv->psr.busy_frontbuffer_bits || dev_priv->psr.active)
 		goto unlock;
 
 	intel_psr_activate(dev_priv->psr.enabled);
-- 
2.14.1