linux-kernel - Re: [PATCH v2 07/22] drm/msm: Do rpm get sooner in the submit path

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Date:   Tue, 20 Oct 2020 13:42:33 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     Rob Clark <robdclark@...il.com>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Rob Clark <robdclark@...omium.org>,
        Sean Paul <sean@...rly.run>, David Airlie <airlied@...ux.ie>,
        "open list:DRM DRIVER FOR MSM ADRENO GPU" 
        <linux-arm-msm@...r.kernel.org>,
        "open list:DRM DRIVER FOR MSM ADRENO GPU" 
        <freedreno@...ts.freedesktop.org>,
        open list <linux-kernel@...r.kernel.org>,
        "Menon, Nishanth" <nm@...com>
Subject: Re: [PATCH v2 07/22] drm/msm: Do rpm get sooner in the submit path

On Tue, Oct 20, 2020 at 1:24 PM Viresh Kumar <viresh.kumar@...aro.org> wrote:
>
> On 20-10-20, 12:56, Daniel Vetter wrote:
> > Yeah that's bad practice. Generally you shouldn't need to hold locks
> > in setup/teardown code, since there's no other thread which can
> > possible hold a reference to anything your touching anymore. Ofc
> > excluding quickly grabbing/dropping a lock to insert/remove objects
> > into lists and stuff.
> >
> > The other reason is that especially with anything related to sysfs or
> > debugfs, the locking dependencies you're pulling in are enormous: vfs
> > locks pull in mm locks (due to mmap) and at that point there's pretty
> > much nothing left you're allowed to hold while acquiring such a lock.
> > For simple drivers this is no issue, but for fancy drivers (like gpu
> > drivers) which need to interact with core mm) this means your
> > subsystem is a major pain to use.
> >
> > Usually the correct fix is to only hold your subsystem locks in
> > setup/teardown when absolutely required, and fix any data
> > inconsistency issues by reordering your setup/teardown code: When you
> > register as the last step and unregister as the first step, there's no
> > need for any additional locking. And hence no need to call debugfs
> > functions while holding your subsystem locks.
> >
> > The catch phrase I use for this is "don't solve object lifetime issues
> > with locking". Instead use refcounting and careful ordering in
> > setup/teardown code.
>
> This is exactly what I have done in the OPP core, the locks were taken
> only when really necessary, though as we have seen now I have missed
> that at a single place and that should be fixed as well. Will do that,
> thanks.

Excellent. If the fix is small enough can you push it into 5.10? That
way drm/msm doesn't have to carry the temporary solution for 5.11 (the
issue only pops up with the locking rework, which teaches lockdep a
few more things about what's going on as a side effect).
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch