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Tue, 8 Sep 2026 13:56:05 +0000 Message-ID: <1081fa83-d2f7-40be-816d-29f28c6e7576@amd.com> Date: Tue, 8 Sep 2026 15:56:01 +0200 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] drm/drm_exec: fix contention when num_objects is 0 To: Sunil Khatri , Alex Deucher Cc: dri-devel@lists.freedesktop.org, stable@vger.kernel.org References: <20260908091729.2749399-1-sunil.khatri@amd.com> Content-Language: en-US From: =?UTF-8?Q?Christian_K=C3=B6nig?= In-Reply-To: <20260908091729.2749399-1-sunil.khatri@amd.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-ClientProxiedBy: FR2P281CA0135.DEUP281.PROD.OUTLOOK.COM (2603:10a6:d10:9e::19) To PH7PR12MB5685.namprd12.prod.outlook.com (2603:10b6:510:13c::22) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: PH7PR12MB5685:EE_|BL1PR12MB5924:EE_ X-MS-Office365-Filtering-Correlation-Id: 4b444c36-6852-4cfa-bae6-08df0db0eccc X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; 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This breaks the > invariant upheld by drm_exec_lock_obj(), where every entry point into > the locking sequence must first attempt to lock any previously > contended object before proceeding. > > Drivers that chain multiple drm_exec_prepare_array() calls per > drm_exec_until_all_locked() iteration (e.g. amdgpu's userq signal/wait > ioctls, which prepare separate read and write BO arrays) can pass an > empty array for one of the two calls. If contention is hit while > preparing the non-empty array, exec->contended is set and the loop > retries; on retry, the empty-array call preceding it is a no-op that > never clears exec->contended, so drm_exec_retry_on_contention() > immediately jumps back to the top of the loop without ever reaching > the call that would resolve the contention. This spins forever. > > Fix it by having drm_exec_prepare_array() call drm_exec_lock_contended() > directly when num_objects is zero, so a pending contended object dont > loop infinitely. > > Fixes: 09593216bff1 ("drm: execution context for GEM buffers v7") > CC: stable@vger.kernel.org I've added a # v6.6+ here. BTW: If you identified the commit you can use "git tag --contains=09593216bff1" to figure that out. > Signed-off-by: Sunil Khatri Reviewed-by: Christian König I'm going to push this to drm-misc-fixes, please sync with Alex to get that cherry picked into amd-staging-drm-next as well. Thanks, Christian. > --- > drivers/gpu/drm/drm_exec.c | 13 +++++++++++++ > 1 file changed, 13 insertions(+) > > diff --git a/drivers/gpu/drm/drm_exec.c b/drivers/gpu/drm/drm_exec.c > index fa923852fae4..f4503ab82c66 100644 > --- a/drivers/gpu/drm/drm_exec.c > +++ b/drivers/gpu/drm/drm_exec.c > @@ -322,6 +322,19 @@ int drm_exec_prepare_array(struct drm_exec *exec, > { > int ret; > > + /* > + * Make sure to lock a contended object even when no objects are > + * given, otherwise drm_exec_retry_on_contention() would loop > + * forever on patterns like: > + * > + * ret = drm_exec_prepare_array(exec, objs, num_objects, ...); > + * drm_exec_retry_on_contention(exec); > + * > + * with num_objects == 0. > + */ > + if (!num_objects) > + return drm_exec_lock_contended(exec); > + > for (unsigned int i = 0; i < num_objects; ++i) { > ret = drm_exec_prepare_obj(exec, objects[i], num_fences); > if (unlikely(ret))