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

[Dhinakaran Pandiyan]

On Sun, 2018-06-24 at 09:54 +0100, Chris Wilson wrote:
> Quoting Dhinakaran Pandiyan (2018-06-23 05:45:06)
> > 
> > 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.
> Ok, that would explain the second work.
> 
> > 
> > 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?
> WARN_ON(active), yup. Seems reasonable to keep for the moment.
> WARN_ON(hw_state) would be worth keeping for some time. Hmm,
> shouldn't
> it be checking both PSR_CTL irrespective of intended mode. After
> switching psr1 to psr2 we should check that psr1 is disabled as well,
> and vice versa.
> 
Yeah, I've sent a patch to check both PSR_CTL and PSR2_CTL. Thanks for
the review.

-DK

> > 
> > 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>
> Reviewed-by: Chris Wilson <chris at chris-wilson.co.uk>
> -Chris
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