On Tue, Apr 23, 2019 at 06:36:32AM +0000, Wojciech Macek wrote: > Author: wma > Date: Tue Apr 23 06:36:32 2019 > New Revision: 346593 > URL: https://svnweb.freebsd.org/changeset/base/346593 > > Log: > This patch offers a workaround to buf_ring reordering > visible on armv7 and armv8. Similar issue to rS302292. > > Obtained from: Semihalf > Authored by: Michal Krawczyk <m...@semihalf.com> > Approved by: wma > Differential Revision: https://reviews.freebsd.org/D19932 > > Modified: > head/sys/sys/buf_ring.h > > Modified: head/sys/sys/buf_ring.h > ============================================================================== > --- head/sys/sys/buf_ring.h Tue Apr 23 04:06:26 2019 (r346592) > +++ head/sys/sys/buf_ring.h Tue Apr 23 06:36:32 2019 (r346593) > @@ -310,14 +310,23 @@ buf_ring_peek_clear_sc(struct buf_ring *br) > if (!mtx_owned(br->br_lock)) > panic("lock not held on single consumer dequeue"); > #endif > - /* > - * I believe it is safe to not have a memory barrier > - * here because we control cons and tail is worst case > - * a lagging indicator so we worst case we might > - * return NULL immediately after a buffer has been enqueued > - */ > + > if (br->br_cons_head == br->br_prod_tail) > return (NULL); > + > +#if defined(__arm__) || defined(__aarch64__) > + /* > + * The barrier is required there on ARM and ARM64 to ensure, that > + * br->br_ring[br->br_cons_head] will not be fetched before the above > + * condition is checked. > + * Without the barrier, it is possible, that buffer will be fetched > + * before the enqueue will put mbuf into br, then, in the meantime, the > + * enqueue will update the array and the br_prod_tail, and the > + * conditional check will be true, so we will return previously fetched > + * (and invalid) buffer. > + */ > + atomic_thread_fence_acq(); > +#endif
Why is it specific to ARM? _______________________________________________ svn-src-all@freebsd.org mailing list https://lists.freebsd.org/mailman/listinfo/svn-src-all To unsubscribe, send any mail to "svn-src-all-unsubscr...@freebsd.org"
