@@ -963,9 +963,16 @@ i915_request_await_start(struct i915_request *rq, struct i915_request *signal)
if (i915_request_started(signal))
return 0;
+ /*
+ * The caller holds a reference on @signal, but we do not serialise
+ * against it being retired and removed from the lists.
+ *
+ * We do not hold a reference to the request before @signal, and
+ * so must be very careful to ensure that it is not _recycled_ as
+ * we follow the link backwards.
+ */
fence = NULL;
rcu_read_lock();
- spin_lock_irq(&signal->lock);
do {
struct list_head *pos = READ_ONCE(signal->link.prev);
struct i915_request *prev;
@@ -996,7 +1003,6 @@ i915_request_await_start(struct i915_request *rq, struct i915_request *signal)
fence = &prev->fence;
} while (0);
- spin_unlock_irq(&signal->lock);
rcu_read_unlock();
if (!fence)
return 0;
Originally, we used the signal->lock as a means of following the previous link in its timeline and peeking at the previous fence. However, we have replaced the explicit serialisation with a series of very careful probes that anticipate the links being deleted and the fences recycled before we are able to acquire a strong reference to it. We do not need the signal->lock crutch anymore, nor want the contention. Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com> --- drivers/gpu/drm/i915/i915_request.c | 10 ++++++++-- 1 file changed, 8 insertions(+), 2 deletions(-)