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drm/xe/xe_sync: avoid race during ufence signaling

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Marking ufence as signalled after copy_to_user() is too late.
Worker thread which signals ufence by memory write might be raced
with another userspace vm-bind call. In map/unmap scenario unmap
may still see ufence is not signalled causing -EBUSY. Change the
order of marking / write to user-fence fixes this issue.

Fixes: 977e5b82e0 ("drm/xe: Expose user fence from xe_sync_entry")
Link: https://gitlab.freedesktop.org/drm/xe/kernel/-/issues/5536
Signed-off-by: Zbigniew Kempczyński <zbigniew.kempczynski@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Link: https://lore.kernel.org/r/20250820083903.2109891-2-zbigniew.kempczynski@intel.com
(cherry picked from commit 8ae04fe9ff)
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
This commit is contained in:
Zbigniew Kempczyński 2025-08-20 10:39:04 +02:00 committed by Rodrigo Vivi
parent 1b237f190e
commit 04e1f683cd
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GPG Key ID: FA625F640EEB13CA
1 changed files with 1 additions and 1 deletions
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2
drivers/gpu/drm/xe/xe_sync.c
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@ -77,6 +77,7 @@ static void user_fence_worker(struct work_struct *w)
{
struct xe_user_fence *ufence = container_of(w, struct xe_user_fence, worker);
WRITE_ONCE(ufence->signalled, 1);
if (mmget_not_zero(ufence->mm)) {
kthread_use_mm(ufence->mm);
if (copy_to_user(ufence->addr, &ufence->value, sizeof(ufence->value)))
@ -91,7 +92,6 @@ static void user_fence_worker(struct work_struct *w)
* Wake up waiters only after updating the ufence state, allowing the UMD
* to safely reuse the same ufence without encountering -EBUSY errors.
*/
WRITE_ONCE(ufence->signalled, 1);
wake_up_all(&ufence->xe->ufence_wq);
user_fence_put(ufence);
}