On 10/23/26 12:00, Philipp Stanner wrote: > There have been repeated discussions about why the signaled-bit is set > before a fence is actually done signaling. Repeatedly, people try to > change that. > > As long as the bit can be read locklessly, for example by > dma_fence_is_signaled(), the bit cannot be set at any other place. > > Document that to reduce further noise about the topic. > > Signed-off-by: Philipp Stanner <[email protected]>
Reviewed-by: Christian König <[email protected]> > --- > I thought that if people repeatedly try to make a change, and it > repeatedly gets rejected, then we can all save ourselves time and work > by quickly documenting it :-) > > This way, we also remember in 5 years *why* something was or wasn't > done. > > Philipp > --- > drivers/dma-buf/dma-fence.c | 10 ++++++++++ > 1 file changed, 10 insertions(+) > > diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c > index bd58688b81a7..a71e18a9e346 100644 > --- a/drivers/dma-buf/dma-fence.c > +++ b/drivers/dma-buf/dma-fence.c > @@ -359,6 +359,16 @@ void dma_fence_signal_timestamp_locked(struct dma_fence > *fence, > > dma_fence_assert_held(fence); > > + /* > + * It would be better to set the bit after the callbacks ran so that the > + * fast-path-check in dma_fence_is_signaled() cannot return true while > + * they are running. > + * > + * That is, however, impossible because many parties now check for the > + * signaled-state of a fence in their callbacks (or in the work_items > + * kicked by them). So we have to keep setting it here as long as the > + * signaled-bit is read locklessly by anyone. > + */ > if (unlikely(test_and_set_bit(DMA_FENCE_FLAG_SIGNALED_BIT, > &fence->flags))) > return; > > base-commit: cf5d8e390e2ac2cfa24a50645efdbb9f0eebe65f
