Messages in this thread Patch in this message | | | Date | Wed, 18 Oct 2023 15:28:32 -0700 | From | "Paul E. McKenney" <> | Subject | [PATCH bpf] Fold smp_mb__before_atomic() into atomic_set_release() |
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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 <paulmck@kernel.org> Acked-by: David Vernet <void@manifault.com> Cc: Andrii Nakryiko <andrii@kernel.org> Cc: Alexei Starovoitov <ast@kernel.org> Cc: Daniel Borkmann <daniel@iogearbox.net> Cc: Martin KaFai Lau <martin.lau@linux.dev> Cc: Song Liu <song@kernel.org> Cc: Yonghong Song <yonghong.song@linux.dev> Cc: John Fastabend <john.fastabend@gmail.com> Cc: KP Singh <kpsingh@kernel.org> Cc: Stanislav Fomichev <sdf@google.com> Cc: Hao Luo <haoluo@google.com> Cc: Jiri Olsa <jolsa@kernel.org> Cc: <bpf@vger.kernel.org>
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 @@ BPF_CALL_4(bpf_user_ringbuf_drain, struct bpf_map *, map, /* 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);
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