linux-kernel - Re: [PATCH v3 0/2] drm: Fix dma_resv deadlock at drm object pin time

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Date: Tue, 21 May 2024 18:18:17 +0200
From: Boris Brezillon <boris.brezillon@...labora.com>
To: Adrián Larumbe <adrian.larumbe@...labora.com>
Cc: Thomas Zimmermann <tzimmermann@...e.de>, Qiang Yu <yuq825@...il.com>,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, Maxime Ripard
 <mripard@...nel.org>, David Airlie <airlied@...il.com>, Daniel Vetter
 <daniel@...ll.ch>, Rob Herring <robh@...nel.org>, Steven Price
 <steven.price@....com>, Sumit Semwal <sumit.semwal@...aro.org>, Christian
 Koenig= <christian.koenig@....com>, Dmitry Osipenko
 <dmitry.osipenko@...labora.com>, Zack Rusin <zack.rusin@...adcom.com>,
 kernel@...labora.com, dri-devel@...ts.freedesktop.org,
 lima@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
 linux-media@...r.kernel.org, linaro-mm-sig@...ts.linaro.org
Subject: Re: [PATCH v3 0/2] drm: Fix dma_resv deadlock at drm object pin
 time

On Fri, 17 May 2024 19:16:21 +0100
Adrián Larumbe <adrian.larumbe@...labora.com> wrote:

> Hi Boris and Thomas,
> 
> On 02.05.2024 14:18, Thomas Zimmermann wrote:
> > Hi
> > 
> > Am 02.05.24 um 14:00 schrieb Boris Brezillon:  
> > > On Thu, 2 May 2024 13:59:41 +0200
> > > Boris Brezillon <boris.brezillon@...labora.com> wrote:
> > >   
> > > > Hi Thomas,
> > > > 
> > > > On Thu, 2 May 2024 13:51:16 +0200
> > > > Thomas Zimmermann <tzimmermann@...e.de> wrote:
> > > >   
> > > > > Hi,
> > > > > 
> > > > > ignoring my r-b on patch 1, I'd like to rethink the current patches in
> > > > > general.
> > > > > 
> > > > > I think drm_gem_shmem_pin() should become the locked version of _pin(),
> > > > > so that drm_gem_shmem_object_pin() can call it directly. The existing
> > > > > _pin_unlocked() would not be needed any longer. Same for the _unpin()
> > > > > functions. This change would also fix the consistency with the semantics
> > > > > of the shmem _vmap() functions, which never take reservation locks.
> > > > > 
> > > > > There are only two external callers of drm_gem_shmem_pin(): the test
> > > > > case and panthor. These assume that drm_gem_shmem_pin() acquires the
> > > > > reservation lock. The test case should likely call drm_gem_pin()
> > > > > instead. That would acquire the reservation lock and the test would
> > > > > validate that shmem's pin helper integrates well into the overall GEM
> > > > > framework. The way panthor uses drm_gem_shmem_pin() looks wrong to me.
> > > > > For now, it could receive a wrapper that takes the lock and that's it.  
> > > > I do agree that the current inconsistencies in the naming is
> > > > troublesome (sometimes _unlocked, sometimes _locked, with the version
> > > > without any suffix meaning either _locked or _unlocked depending on
> > > > what the suffixed version does), and that's the very reason I asked
> > > > Dmitry to address that in his shrinker series [1]. So, ideally I'd
> > > > prefer if patches from Dmitry's series were applied instead of
> > > > trying to fix that here (IIRC, we had an ack from Maxime).  
> > > With the link this time :-).
> > > 
> > > [1]https://lore.kernel.org/lkml/20240105184624.508603-1-dmitry.osipenko@collabora.com/T/  
> > 
> > Thanks. I remember these patches. Somehow I thought they would have been
> > merged already. I wasn't super happy about the naming changes in patch 5,
> > because the names of the GEM object callbacks do no longer correspond with
> > their implementations. But anyway.
> > 
> > If we go that direction, we should here simply push drm_gem_shmem_pin() and
> > drm_gem_shmem_unpin() into panthor and update the shmem tests with
> > drm_gem_pin(). Panfrost and lima would call drm_gem_shmem_pin_locked(). IMHO
> > we should not promote the use of drm_gem_shmem_object_*() functions, as they
> > are meant to be callbacks for struct drm_gem_object_funcs. (Auto-generating
> > them would be nice.)  
> 
> I'll be doing this in the next patch series iteration, casting the pin function's
> drm object parameter to an shmem object.
> 
> Also for the sake of leaving things in a consistent state, and against Boris' advice,
> I think I'll leave the drm WARN statement inside drm_gem_shmem_pin_locked.

Sure, that's fine.