Re: [PATCH] snd/hda: Only get/put display_power once

[Takashi Iwai <tiwai@suse.de>]

On Wed, 10 Apr 2019 12:24:24 +0200,
Chris Wilson wrote:
> 
> Quoting Takashi Iwai (2019-04-10 11:09:47)
> > On Wed, 10 Apr 2019 10:17:33 +0200,
> > Chris Wilson wrote:
> > > 
> > > While we only allow a single display power reference, the current
> > > acquisition/release is racy and a direct call may run concurrently with
> > > a runtime-pm worker. Prevent the double unreference by atomically
> > > tracking the display_power_active cookie.
> > > 
> > > Testcase: igt/i915_pm_rpm/module-reload #glk-dsi
> > > Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> > > Cc: Takashi Iwai <tiwai@suse.de>
> > > Cc: Imre Deak <imre.deak@intel.com>
> > 
> > I rather prefer a more straightforward conversion, e.g. something like
> > below.  Checking the returned cookie as the state flag is not quite
> > intuitive, so revive the boolean state flag, and handle it
> > atomically.
> 
> Access to the cookie itself is not atomic there, and theoretically
> there could be a get/put/get running concurrently. Are you sure don't
> want a refcount and lock here? :)

The refcount is what we want to reduce, so the suitable option would
be a (yet another) mutex although the cmpxchg() should be enough for
normal cases.

> Your call. For the case CI is hitting, it should do the trick (as we are
> only seeing the race on put/put I think). CI will answer in a hour or
> two.

OK, once when it seems working, I'll respin a patch with a mutex
instead.  We have already a one that is used for the link state
change, and this can be reused.


thanks,

Takashi
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