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

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Message-ID: <20240502135941.136ad639@collabora.com>
Date: Thu, 2 May 2024 13:59:41 +0200
From: Boris Brezillon <boris.brezillon@...labora.com>
To: Thomas Zimmermann <tzimmermann@...e.de>
Cc: Adrián Larumbe <adrian.larumbe@...labora.com>, 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

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).

Regards,

Boris