dma-buf: Relax the write-seqlock for reallocating the shared fence list

As the set of shared fences is not being changed during reallocation of
the reservation list, we can skip updating the write_seqlock.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190712080314.21018-2-chris@chris-wilson.co.uk

1 files changed, 7 insertions, 7 deletions

diff --git a/drivers/dma-buf/reservation.c b/drivers/dma-buf/reservation.c
index 80ecc1283d15..c71b85c8c159 100644
--- a/drivers/dma-buf/reservation.c
+++ b/drivers/dma-buf/reservation.c
@@ -157,15 +157,15 @@ int reservation_object_reserve_shared(struct reservation_object *obj,
                 (ksize(new) - offsetof(typeof(*new), shared)) /
                 sizeof(*new->shared);
 
-        preempt_disable();
-        write_seqcount_begin(&obj->seq);
         /*
-         * RCU_INIT_POINTER can be used here,
-         * seqcount provides the necessary barriers
+         * We are not changing the effective set of fences here so can
+         * merely update the pointer to the new array; both existing
+         * readers and new readers will see exactly the same set of
+         * active (unsignaled) shared fences. Individual fences and the
+         * old array are protected by RCU and so will not vanish under
+         * the gaze of the rcu_read_lock() readers.
          */
-        RCU_INIT_POINTER(obj->fence, new);
-        write_seqcount_end(&obj->seq);
-        preempt_enable();
+        rcu_assign_pointer(obj->fence, new);
 
         if (!old)
                 return 0;