Re: [PATCH] drm/msm/kms: Make a lock_class_key for each crtc mutex

[Daniel Vetter]

On Wed, Feb 03, 2021 at 02:11:09PM -0800, Rob Clark wrote:
> On Wed, Feb 3, 2021 at 1:58 PM Stephen Boyd <swboyd@xxxxxxxxxxxx> wrote:
> >
> > Quoting Rob Clark (2021-02-03 09:29:09)
> > > On Wed, Feb 3, 2021 at 2:10 AM Daniel Vetter <daniel@xxxxxxxx> wrote:
> > > >
> > > > On Tue, Feb 02, 2021 at 08:51:25AM -0800, Rob Clark wrote:
> > > > > On Tue, Feb 2, 2021 at 7:46 AM Daniel Vetter <daniel@xxxxxxxx> wrote:
> > > > > >
> > > > > > On Mon, Jan 25, 2021 at 03:49:01PM -0800, Stephen Boyd wrote:
> > > > > > > This is because lockdep thinks all the locks taken in lock_crtcs() are
> > > > > > > the same lock, when they actually aren't. That's because we call
> > > > > > > mutex_init() in msm_kms_init() and that assigns on static key for every
> > > > > > > lock initialized in this loop. Let's allocate a dynamic number of
> > > > > > > lock_class_keys and assign them to each lock so that lockdep can figure
> > > > > > > out an AA deadlock isn't possible here.
> > > > > > >
> > > > > > > Fixes: b3d91800d9ac ("drm/msm: Fix race condition in msm driver with async layer updates")
> > > > > > > Cc: Krishna Manikandan <mkrishn@xxxxxxxxxxxxxx>
> > > > > > > Signed-off-by: Stephen Boyd <swboyd@xxxxxxxxxxxx>
> > > > > >
> > > > > > This smells like throwing more bad after initial bad code ...
> > > > > >
> > > > > > First a rant: https://blog.ffwll.ch/2020/08/lockdep-false-positives.html
> > > >
> > > > Some technical on the patch itself: I think you want
> > > > mutex_lock_nested(crtc->lock, drm_crtc_index(crtc)), not your own locking
> > > > classes hand-rolled. It's defacto the same, but much more obviously
> > > > correct since self-documenting.
> > >
> > > hmm, yeah, that is a bit cleaner.. but this patch is already on
> > > msm-next, maybe I'll add a patch on top to change it
> >
> > How many CRTCs are there? The subclass number tops out at 8, per
> > MAX_LOCKDEP_SUBCLASSES so if we have more than that many bits possible
> > then it will fail.

Hm good point, tbh the mutex_lock_nested annotations isn't super awesome
either, it would be kinda neat if we could put that annotation into
mutex_lock_init fairly statically (and at that point we could allos resize
the array fairly easily I think at runtime).

The nice thing with the nesting index is just that it makes it a bit more
obvious that there's a static nesting going on and why it's ok.
-Daniel

> conveniently MAX_CRTCS is 8.. realistically I don't *think* you'd ever
> see more than 2 or 3
> 
> BR,
> -R
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch