Quoting Chris Wilson (2020-07-15 13:21:43) > Quoting Daniel Vetter (2020-07-15 13:10:22) > > On Wed, Jul 15, 2020 at 11:49:05AM +0100, Chris Wilson wrote: > > > When waiting with a callback on the stack, we must remove the callback > > > upon wait completion. Since this will be notified by the fence signal > > > callback, the removal often contends with the fence->lock being held by > > > the signaler. We can look at the list entry to see if the callback was > > > already signaled before we take the contended lock. > > > > > > Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> > > > --- > > > drivers/dma-buf/dma-fence.c | 3 +++ > > > 1 file changed, 3 insertions(+) > > > > > > diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c > > > index 8d5bdfce638e..b910d7bc0854 100644 > > > --- a/drivers/dma-buf/dma-fence.c > > > +++ b/drivers/dma-buf/dma-fence.c > > > @@ -420,6 +420,9 @@ dma_fence_remove_callback(struct dma_fence *fence, struct dma_fence_cb *cb) > > > unsigned long flags; > > > bool ret; > > > > > > + if (list_empty(&cb->node)) > > > > I was about to say "but the races" but then noticed that Paul fixed > > list_empty to use READ_ONCE like 5 years ago :-) > > I'm always going "when exactly do we need list_empty_careful()"? > > We can rule out a concurrent dma_fence_add_callback() for the same > dma_fence_cb, as that is a lost cause. So we only have to worry about > the concurrent list_del_init() from dma_fence_signal_locked(). So it's > the timing of > list_del_init(): WRITE_ONCE(list->next, list) > vs > READ_ONCE(list->next) == list > and we don't need to care about the trailing instructions in > list_del_init()... > > Wait that trailing instruction is actually important here if the > dma_fence_cb is on the stack, or other imminent free. > > Ok, this does need to be list_empty_careful! There's a further problem in that we call INIT_LIST_HEAD on the dma_fence_cb before the signal callback. So even if list_empty_careful() confirms the dma_fence_cb to be completely decoupled, the containing struct may still be inuse. -Chris _______________________________________________ Intel-gfx mailing list Intel-gfx@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/intel-gfx
