From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from gabe.freedesktop.org (gabe.freedesktop.org [131.252.210.177]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id C49E8CD3442 for ; Thu, 7 May 2026 10:01:58 +0000 (UTC) Received: from gabe.freedesktop.org (localhost [127.0.0.1]) by gabe.freedesktop.org (Postfix) with ESMTP id 33BE610EFB1; Thu, 7 May 2026 10:01:58 +0000 (UTC) Authentication-Results: gabe.freedesktop.org; dkim=fail reason="signature verification failed" (1024-bit key; unprotected) header.d=arm.com header.i=@arm.com header.b="fhalT9yd"; dkim-atps=neutral Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by gabe.freedesktop.org (Postfix) with ESMTP id B02D110EFB1 for ; Thu, 7 May 2026 10:01:56 +0000 (UTC) Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id B3E283532 for ; Thu, 7 May 2026 03:01:50 -0700 (PDT) Received: from [192.168.0.1] (usa-sjc-imap-foss1.foss.arm.com [10.121.207.14]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPA id 944873F763 for ; Thu, 7 May 2026 03:01:55 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=arm.com; s=foss; t=1778148115; bh=V6SSn9JDAt3Sb2yowSSRXctJvfC9kF3/lLVSaXdPYEQ=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=fhalT9ydBzVprPEoO+3PuS3GrYMixs3EpBZcsY4Yy93kFXSlmfX9Ybq4MVz0YsPzK a3ljzoZJ7I0UZtfyYjXk9sEVrKqBOrdx9aZPnkOo3DD5Y+LVZ6eVWH20DPFgZRArEV HLVDzSxWbFvQHoF0eXZy6XyzDOV9ovQA7BpDD/6Y= Date: Thu, 7 May 2026 11:01:25 +0100 From: Liviu Dudau To: Boris Brezillon Cc: Steven Price , Dmitry Osipenko , Maarten Lankhorst , Maxime Ripard , Thomas Zimmermann , David Airlie , Simona Vetter , Akash Goel , Chia-I Wu , Rob Clark , Dmitry Baryshkov , Abhinav Kumar , Jessica Zhang , Sean Paul , Marijn Suijten , linux-arm-msm@vger.kernel.org, freedreno@lists.freedesktop.org, dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/3] drm/panthor: Don't use the racy drm_gem_lru_remove() helper Message-ID: References: <20260506-panthor-shrinker-fixes-v1-0-e7721526de96@collabora.com> <20260506-panthor-shrinker-fixes-v1-1-e7721526de96@collabora.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20260506-panthor-shrinker-fixes-v1-1-e7721526de96@collabora.com> X-BeenThere: dri-devel@lists.freedesktop.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Direct Rendering Infrastructure - Development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dri-devel-bounces@lists.freedesktop.org Sender: "dri-devel" On Wed, May 06, 2026 at 02:16:26PM +0200, Boris Brezillon wrote: > drm_gem_lru_remove() dereference stores drm_gem_object::lru in a local > variable that's then dereferenced to acquire the LRU lock. Because this > assignment in done without the LRU lock held, it can race with > drm_gem_lru_scan() where drm_gem_object::lru is temporarily assigned > a stack-allcated LRU that goes away when leaving the function. By > the time we dereference this local lru variable, the object might already > be gone. > > It feels like drm_gem_lru_move_tail() was never meant to be used this > way, because there's no easy way we can avoid this race unless we defer > the locking to the caller. Let's add an explicit LRU for unreclaimable > BOs instead, and have all BOs added to this LRU at creation time. I would argue that drm_gem_lru_scan() is broken by design. If you're going to release the LRU lock in the middle of a loop you can expect that someone will get hold of your stack-allocated LRU and end up picking the pieces. This patch is fine in itself by trying to avoid stepping into the fight, but I think we should also add a warning in drm_gem_lru_scan() for future users to be aware of the dangers. Reviewed-by: Liviu Dudau Best regards, Liviu > > Fixes: fb42964e2a76 ("drm/panthor: Add a GEM shrinker") > Reported-by: Chia-I Wu > Closes: https://gitlab.freedesktop.org/panfrost/linux/-/work_items/86 > Signed-off-by: Boris Brezillon > Reviewed-by: Chia-I Wu > --- > drivers/gpu/drm/panthor/panthor_device.h | 10 ++++++++++ > drivers/gpu/drm/panthor/panthor_gem.c | 5 ++++- > 2 files changed, 14 insertions(+), 1 deletion(-) > > diff --git a/drivers/gpu/drm/panthor/panthor_device.h b/drivers/gpu/drm/panthor/panthor_device.h > index 4e4607bca7cc..45b71546f83c 100644 > --- a/drivers/gpu/drm/panthor/panthor_device.h > +++ b/drivers/gpu/drm/panthor/panthor_device.h > @@ -190,6 +190,16 @@ struct panthor_device { > /** @reclaim.lock: Lock protecting all LRUs */ > struct mutex lock; > > + /** > + * @reclaim.unreclaimable: unreclaimable BOs > + * > + * Either the BO is unreclaimable because it has no pages allocated, > + * or it's unreclaimable because pages are pinned. > + * > + * All BOs start in that list at creation time. > + */ > + struct drm_gem_lru unreclaimable; > + > /** > * @reclaim.unused: BOs with unused pages > * > diff --git a/drivers/gpu/drm/panthor/panthor_gem.c b/drivers/gpu/drm/panthor/panthor_gem.c > index 13295d7a593d..8e31740126e7 100644 > --- a/drivers/gpu/drm/panthor/panthor_gem.c > +++ b/drivers/gpu/drm/panthor/panthor_gem.c > @@ -204,7 +204,7 @@ void panthor_gem_update_reclaim_state_locked(struct panthor_gem_object *bo, > drm_gem_lru_move_tail(&ptdev->reclaim.gpu_mapped_shared, &bo->base); > break; > case PANTHOR_GEM_UNRECLAIMABLE: > - drm_gem_lru_remove(&bo->base); > + drm_gem_lru_move_tail(&ptdev->reclaim.unreclaimable, &bo->base); > break; > default: > drm_WARN(&ptdev->base, true, "invalid GEM reclaim state (%d)\n", new_state); > @@ -994,6 +994,7 @@ static struct panthor_gem_object * > panthor_gem_create(struct drm_device *dev, size_t size, uint32_t flags, > struct panthor_vm *exclusive_vm, u32 usage_flags) > { > + struct panthor_device *ptdev = container_of(dev, struct panthor_device, base); > struct panthor_gem_object *bo; > int ret; > > @@ -1026,6 +1027,7 @@ panthor_gem_create(struct drm_device *dev, size_t size, uint32_t flags, > } > > panthor_gem_debugfs_set_usage_flags(bo, usage_flags); > + drm_gem_lru_move_tail(&ptdev->reclaim.unreclaimable, &bo->base); > return bo; > > err_put: > @@ -1551,6 +1553,7 @@ int panthor_gem_shrinker_init(struct panthor_device *ptdev) > return ret; > > INIT_LIST_HEAD(&ptdev->reclaim.vms); > + drm_gem_lru_init(&ptdev->reclaim.unreclaimable, &ptdev->reclaim.lock); > drm_gem_lru_init(&ptdev->reclaim.unused, &ptdev->reclaim.lock); > drm_gem_lru_init(&ptdev->reclaim.mmapped, &ptdev->reclaim.lock); > drm_gem_lru_init(&ptdev->reclaim.gpu_mapped_shared, &ptdev->reclaim.lock); > > -- > 2.54.0 > -- ==================== | I would like to | | fix the world, | | but they're not | | giving me the | \ source code! / --------------- ¯\_(ツ)_/¯