[PATCH] drm/amdgpu: Harden against potential deadlock

[Philipp Stanner]

amdgpu's KFD eviction fence's dma_fence ops->enable_signaling() contains
a dma_fence_is_signaled() check.

ops->enable_signaling() is currently consistently being called with the
dma_fence lock held. dma_fence_is_signaled() might try to take this lock
in its dma_fence_signal() call.

This deadlock can currently not occur because the amdkfd_fence_ops do
not implement ops->signaled. Should that ever be added, however,
deadlocks might occur occasionally once the fastpath check in
dma_fence_is_signaled() is skipped.

Replace dma_fence_is_signaled() with dma_fence_is_signaled_locked().

Signed-off-by: Philipp Stanner <phasta@xxxxxxxxxx>
---
As detailed in my recent discussions with Christian [1], I also think
that drivers should be very tightly constrained as to what they do in
the fence_ops. I think at the very least they shouldn't use any
dma_fence functions there.

So we could also remove this check here completely;
__dma_fence_enable_signaling() already checks the signaled-flag, after
all, but I'm not sure whether that might break amdgpu.


P.

[1] https://lore.kernel.org/dri-devel/320b6e7c75da6f05c015e89df0e2c7d11f20d70a.camel@xxxxxxxxxxx/
---
 drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_fence.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_fence.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_fence.c
index 9b10d015671c..61621eebb2b0 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_fence.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_fence.c
@@ -123,7 +123,8 @@ static bool amdkfd_fence_enable_signaling(struct dma_fence *f)
 	if (!fence)
 		return false;
 
-	if (dma_fence_is_signaled(f))
+	/* The enable_signaling() ops is called under lock protection. */
+	if (dma_fence_is_signaled_locked(f))
 		return true;
 
 	if (!kgd2kfd_schedule_evict_and_restore_process(fence->mm, fence->context_id, f))
-- 
2.56.0