On Mon, Aug 29, 2016 at 08:08:32AM +0100, Chris Wilson wrote:

In order to be completely generic, we have to double check the read seqlock after acquiring a reference to the fence. If the driver is allocating fences from a SLAB_DESTROY_BY_RCU, or similar freelist, then within an RCU grace period a fence may be freed and reallocated. The RCU read side critical section does not prevent this reallocation, instead we have to inspect the reservation's seqlock to double check if the fences have been reassigned as we were acquiring our reference.

Signed-off-by: Chris Wilson chris@chris-wilson.co.uk Cc: Daniel Vetter daniel.vetter@ffwll.ch Cc: Maarten Lankhorst maarten.lankhorst@linux.intel.com Cc: Christian König christian.koenig@amd.com Cc: Alex Deucher alexander.deucher@amd.com Cc: Sumit Semwal sumit.semwal@linaro.org Cc: linux-media@vger.kernel.org Cc: dri-devel@lists.freedesktop.org Cc: linaro-mm-sig@lists.linaro.org

Reviewed-by: Daniel Vetter daniel.vetter@ffwll.ch

---

drivers/dma-buf/reservation.c | 30 ++++++++++-------------------- 1 file changed, 10 insertions(+), 20 deletions(-)

diff --git a/drivers/dma-buf/reservation.c b/drivers/dma-buf/reservation.c index 3369e4668e96..e74493e7332b 100644 --- a/drivers/dma-buf/reservation.c +++ b/drivers/dma-buf/reservation.c @@ -474,12 +474,13 @@ bool reservation_object_test_signaled_rcu(struct reservation_object *obj, bool test_all) { unsigned seq, shared_count;

• int ret = true;

• int ret;

• rcu_read_lock();

retry:

• ret = true; shared_count = 0; seq = read_seqcount_begin(&obj->seq);

• rcu_read_lock();

if (test_all) { unsigned i; @@ -490,46 +491,35 @@ retry: if (fobj) shared_count = fobj->shared_count;

• if (read_seqcount_retry(&obj->seq, seq))
  

• 	goto unlock_retry;
  

• for (i = 0; i < shared_count; ++i) { struct fence *fence = rcu_dereference(fobj->shared[i]);

ret = reservation_object_test_signaled_single(fence); if (ret < 0)

• 		goto unlock_retry;
  

• 		goto retry;
  else if (!ret)
  	break;
  

  }

• /*
  

•  * There could be a read_seqcount_retry here, but nothing cares
  

•  * about whether it's the old or newer fence pointers that are
  

•  * signaled. That race could still have happened after checking
  

•  * read_seqcount_retry. If you care, use ww_mutex_lock.
  

•  */
  

• if (read_seqcount_retry(&obj->seq, seq))
  

• 	goto retry;
  

  }

if (!shared_count) { struct fence *fence_excl = rcu_dereference(obj->fence_excl);

• if (read_seqcount_retry(&obj->seq, seq))
  

• 	goto unlock_retry;
  

• if (fence_excl) { ret = reservation_object_test_signaled_single( fence_excl); if (ret < 0)

• 		goto unlock_retry;
  

• 		goto retry;
  

• 	if (read_seqcount_retry(&obj->seq, seq))
  

• 		goto retry;
  

  } }

rcu_read_unlock(); return ret;

-unlock_retry:

• rcu_read_unlock();
• goto retry;

} EXPORT_SYMBOL_GPL(reservation_object_test_signaled_rcu); -- 2.9.3