From 06646da01458682023321bdc7553b8140e95d077 Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Wed, 18 Oct 2023 15:28:32 -0700 Subject: [PATCH] bpf: Fold smp_mb__before_atomic() into atomic_set_release() The bpf_user_ringbuf_drain() BPF_CALL function uses an atomic_set() immediately preceded by smp_mb__before_atomic() so as to order storing of ring-buffer consumer and producer positions prior to the atomic_set() call's clearing of the ->busy flag, as follows: smp_mb__before_atomic(); atomic_set(&rb->busy, 0); Although this works given current architectures and implementations, and given that this only needs to order prior writes against a later write. However, it does so by accident because the smp_mb__before_atomic() is only guaranteed to work with read-modify-write atomic operations, and not at all with things like atomic_set() and atomic_read(). Note especially that smp_mb__before_atomic() will not, repeat *not*, order the prior write to "a" before the subsequent non-read-modify-write atomic read from "b", even on strongly ordered systems such as x86: WRITE_ONCE(a, 1); smp_mb__before_atomic(); r1 = atomic_read(&b); Therefore, replace the smp_mb__before_atomic() and atomic_set() with atomic_set_release() as follows: atomic_set_release(&rb->busy, 0); This is no slower (and sometimes is faster) than the original, and also provides a formal guarantee of ordering that the original lacks. Signed-off-by: Paul E. McKenney Signed-off-by: Daniel Borkmann Acked-by: David Vernet Link: https://lore.kernel.org/bpf/ec86d38e-cfb4-44aa-8fdb-6c925922d93c@paulmck-laptop --- kernel/bpf/ringbuf.c | 3 +-- 1 file changed, 1 insertion(+), 2 deletions(-) diff --git a/kernel/bpf/ringbuf.c b/kernel/bpf/ringbuf.c index f045fde632e5..0ee653a936ea 100644 --- a/kernel/bpf/ringbuf.c +++ b/kernel/bpf/ringbuf.c @@ -770,8 +770,7 @@ schedule_work_return: /* Prevent the clearing of the busy-bit from being reordered before the * storing of any rb consumer or producer positions. */ - smp_mb__before_atomic(); - atomic_set(&rb->busy, 0); + atomic_set_release(&rb->busy, 0); if (flags & BPF_RB_FORCE_WAKEUP) irq_work_queue(&rb->work);