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Date: Mon, 22 Apr 2024 08:41:03 +0200
From: Boris Brezillon <boris.brezillon@collabora.com>
To: =?UTF-8?B?QWRyacOhbg==?= Larumbe <adrian.larumbe@collabora.com>
Cc: Rob Herring <robh@kernel.org>, Steven Price <steven.price@arm.com>,
 Maarten Lankhorst <maarten.lankhorst@linux.intel.com>, Maxime Ripard
 <mripard@kernel.org>, Thomas Zimmermann <tzimmermann@suse.de>, David Airlie
 <airlied@gmail.com>, Daniel Vetter <daniel@ffwll.ch>, Sumit Semwal
 <sumit.semwal@linaro.org>, Christian =?UTF-8?B?S8O2bmln?=
 <christian.koenig@amd.com>, Zack Rusin <zack.rusin@broadcom.com>, Dmitry
 Osipenko <dmitry.osipenko@collabora.com>, kernel@collabora.com,
 dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org,
 linux-media@vger.kernel.org, linaro-mm-sig@lists.linaro.org
Subject: Re: [PATCH] drm/panfrost: Fix dma_resv deadlock at drm object pin
 time
Message-ID: <20240422084103.1a9fbef6@collabora.com>
In-Reply-To: <20240421163951.3398622-1-adrian.larumbe@collabora.com>
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On Sun, 21 Apr 2024 17:39:47 +0100
Adri=C3=A1n Larumbe <adrian.larumbe@collabora.com> wrote:

> When Panfrost must pin an object that is being prepared a dma-buf
> attachment for on behalf of another driver, the core drm gem object pinni=
ng
> code already takes a lock on the object's dma reservation.
>=20
> However, Panfrost GEM object's pinning callback would eventually try taki=
ng
> the lock on the same dma reservation when delegating pinning of the object
> onto the shmem subsystem, which led to a deadlock.
>=20
> This can be shown by enabling CONFIG_DEBUG_WW_MUTEX_SLOWPATH, which throws
> the following recursive locking situation:
>=20
> weston/3440 is trying to acquire lock:
> ffff000000e235a0 (reservation_ww_class_mutex){+.+.}-{3:3}, at: drm_gem_sh=
mem_pin+0x34/0xb8 [drm_shmem_helper]
> but task is already holding lock:
> ffff000000e235a0 (reservation_ww_class_mutex){+.+.}-{3:3}, at: drm_gem_pi=
n+0x2c/0x80 [drm]
>=20
> Fix it by assuming the object's reservation had already been locked by the
> time we reach panfrost_gem_pin.
>=20
> Cc: Thomas Zimmermann <tzimmermann@suse.de>
> Cc: Dmitry Osipenko <dmitry.osipenko@collabora.com>
> Cc: Boris Brezillon <boris.brezillon@collabora.com>
> Cc: Steven Price <steven.price@arm.com>
> Fixes: a78027847226 ("drm/gem: Acquire reservation lock in drm_gem_{pin/u=
npin}()")
> Signed-off-by: Adri=C3=A1n Larumbe <adrian.larumbe@collabora.com>

Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>

> ---
>  drivers/gpu/drm/panfrost/panfrost_gem.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
>=20
> diff --git a/drivers/gpu/drm/panfrost/panfrost_gem.c b/drivers/gpu/drm/pa=
nfrost/panfrost_gem.c
> index d47b40b82b0b..6c26652d425d 100644
> --- a/drivers/gpu/drm/panfrost/panfrost_gem.c
> +++ b/drivers/gpu/drm/panfrost/panfrost_gem.c
> @@ -192,7 +192,12 @@ static int panfrost_gem_pin(struct drm_gem_object *o=
bj)
>  	if (bo->is_heap)
>  		return -EINVAL;
> =20
> -	return drm_gem_shmem_pin(&bo->base);
> +	/*
> +	 * Pinning can only happen in response to a prime attachment request fr=
om
> +	 * another driver, but that's already being handled by drm_gem_pin
> +	 */
> +	drm_WARN_ON(obj->dev, obj->import_attach);
> +	return drm_gem_shmem_pin_locked(&bo->base);
>  }
> =20
>  static enum drm_gem_object_status panfrost_gem_status(struct drm_gem_obj=
ect *obj)
>=20
> base-commit: 04687bff66b8a4b22748aa7215d3baef0b318e5b