Date: Fri, 4 Sep 2020 11:22:18 +0000 (UTC) From: Marcin Wojtas <mw@FreeBSD.org> To: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-stable@freebsd.org, svn-src-stable-12@freebsd.org Subject: svn commit: r365326 - stable/12/sys/sys Message-ID: <202009041122.084BMIYn040628@repo.freebsd.org>
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Author: mw Date: Fri Sep 4 11:22:18 2020 New Revision: 365326 URL: https://svnweb.freebsd.org/changeset/base/365326 Log: MFC: r346593 Add barrier in buf ring peek function to prevent race in ARM and ARM64. Obtained from: Semihalf Sponsored by: Amazon, Inc. Modified: stable/12/sys/sys/buf_ring.h Directory Properties: stable/12/ (props changed) Modified: stable/12/sys/sys/buf_ring.h ============================================================================== --- stable/12/sys/sys/buf_ring.h Fri Sep 4 04:31:56 2020 (r365325) +++ stable/12/sys/sys/buf_ring.h Fri Sep 4 11:22:18 2020 (r365326) @@ -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 #ifdef DEBUG_BUFRING /*
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