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From: Daniel Vetter <daniel@ffwll.ch>
To: Viresh Kumar <viresh.kumar@linaro.org>
Cc: Rob Clark <robdclark@gmail.com>,
	dri-devel <dri-devel@lists.freedesktop.org>,
	Rob Clark <robdclark@chromium.org>, Sean Paul <sean@poorly.run>,
	David Airlie <airlied@linux.ie>,
	"open list:DRM DRIVER FOR MSM ADRENO GPU" 
	<linux-arm-msm@vger.kernel.org>,
	"open list:DRM DRIVER FOR MSM ADRENO GPU" 
	<freedreno@lists.freedesktop.org>,
	open list <linux-kernel@vger.kernel.org>,
	"Menon, Nishanth" <nm@ti.com>
Subject: Re: [PATCH v2 07/22] drm/msm: Do rpm get sooner in the submit path
Date: Tue, 20 Oct 2020 13:42:33 +0200	[thread overview]
Message-ID: <CAKMK7uHYBCoBvTGyMHGMrt2YRrB7RFt+maWNRjApgz621hu8JA@mail.gmail.com> (raw)
In-Reply-To: <20201020112413.xbk2vow2kgjky3pb@vireshk-i7>

On Tue, Oct 20, 2020 at 1:24 PM Viresh Kumar <viresh.kumar@linaro.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

  reply	other threads:[~2020-10-20 11:42 UTC|newest]

Thread overview: 50+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-10-12  2:09 [PATCH 00/14] drm/msm: de-struct_mutex-ification Rob Clark
2020-10-12  2:09 ` [PATCH v2 01/22] drm/msm/gem: Add obj->lock wrappers Rob Clark
2020-10-12  2:09 ` [PATCH v2 02/22] drm/msm/gem: Rename internal get_iova_locked helper Rob Clark
2020-10-12  2:09 ` [PATCH v2 03/22] drm/msm/gem: Move prototypes to msm_gem.h Rob Clark
2020-10-12  2:09 ` [PATCH v2 04/22] drm/msm/gem: Add some _locked() helpers Rob Clark
2020-10-12  2:09 ` [PATCH v2 05/22] drm/msm/gem: Move locking in shrinker path Rob Clark
2020-10-12  2:09 ` [PATCH v2 06/22] drm/msm/submit: Move copy_from_user ahead of locking bos Rob Clark
2020-10-12  2:09 ` [PATCH v2 07/22] drm/msm: Do rpm get sooner in the submit path Rob Clark
2020-10-12 14:35   ` Daniel Vetter
2020-10-12 15:43     ` Rob Clark
2020-10-20  9:07       ` Viresh Kumar
2020-10-20 10:56         ` Daniel Vetter
2020-10-20 11:24           ` Viresh Kumar
2020-10-20 11:42             ` Daniel Vetter [this message]
2020-10-20 14:13             ` Rob Clark
2020-10-22  8:06               ` Viresh Kumar
2020-10-25 17:39                 ` Rob Clark
2020-10-27 11:35                   ` Viresh Kumar
2020-11-03  5:47                     ` Viresh Kumar
2020-11-03 16:50                       ` Rob Clark
2020-11-04  3:03                         ` Viresh Kumar
2020-11-05 19:24                           ` Rob Clark
2020-11-06  7:16                             ` Viresh Kumar
2020-11-17 10:03                               ` Viresh Kumar
2020-11-17 17:02                               ` Rob Clark
2020-11-18  5:28                                 ` Viresh Kumar
2020-11-18 16:53                                   ` Rob Clark
2020-11-19  6:05                                     ` Viresh Kumar
2020-12-07  6:16                                       ` Viresh Kumar
2020-12-16  5:22                                         ` Viresh Kumar
2020-10-12  2:09 ` [PATCH v2 08/22] drm/msm/gem: Switch over to obj->resv for locking Rob Clark
2020-10-12  2:09 ` [PATCH v2 09/22] drm/msm: Use correct drm_gem_object_put() in fail case Rob Clark
2020-10-12  2:09 ` [PATCH v2 10/22] drm/msm: Drop chatty trace Rob Clark
2020-10-12  2:09 ` [PATCH v2 11/22] drm/msm: Move update_fences() Rob Clark
2020-10-12  2:09 ` [PATCH v2 12/22] drm/msm: Add priv->mm_lock to protect active/inactive lists Rob Clark
2020-10-12  2:09 ` [PATCH v2 13/22] drm/msm: Document and rename preempt_lock Rob Clark
2020-10-12  2:09 ` [PATCH v2 14/22] drm/msm: Protect ring->submits with it's own lock Rob Clark
2020-10-12  2:09 ` [PATCH v2 15/22] drm/msm: Refcount submits Rob Clark
2020-10-12  2:09 ` [PATCH v2 16/22] drm/msm: Remove obj->gpu Rob Clark
2020-10-12  2:09 ` [PATCH v2 17/22] drm/msm: Drop struct_mutex from the retire path Rob Clark
2020-10-12  2:09 ` [PATCH v2 18/22] drm/msm: Drop struct_mutex in free_object() path Rob Clark
2020-10-12  2:09 ` [PATCH v2 19/22] drm/msm: remove msm_gem_free_work Rob Clark
2020-10-12  2:09 ` [PATCH v2 20/22] drm/msm: drop struct_mutex in madvise path Rob Clark
2020-10-12  2:09 ` [PATCH v2 21/22] drm/msm: Drop struct_mutex in shrinker path Rob Clark
2020-10-12  2:09 ` [PATCH v2 22/22] drm/msm: Don't implicit-sync if only a single ring Rob Clark
2020-10-12 14:40   ` Daniel Vetter
2020-10-12 15:07     ` Rob Clark
2020-10-13 11:08       ` Daniel Vetter
2020-10-13 16:15         ` [Freedreno] " Rob Clark
2020-10-15  8:22           ` Daniel Vetter

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