[PATCH bpf] Fold smp_mb__before_atomic() into atomic_set_release()

[Paul E. McKenney]

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@xxxxxxxxxx>
Acked-by: David Vernet <void@xxxxxxxxxxxxx>
Cc: Andrii Nakryiko <andrii@xxxxxxxxxx>
Cc: Alexei Starovoitov <ast@xxxxxxxxxx>
Cc: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
Cc: Martin KaFai Lau <martin.lau@xxxxxxxxx>
Cc: Song Liu <song@xxxxxxxxxx>
Cc: Yonghong Song <yonghong.song@xxxxxxxxx>
Cc: John Fastabend <john.fastabend@xxxxxxxxx>
Cc: KP Singh <kpsingh@xxxxxxxxxx>
Cc: Stanislav Fomichev <sdf@xxxxxxxxxx>
Cc: Hao Luo <haoluo@xxxxxxxxxx>
Cc: Jiri Olsa <jolsa@xxxxxxxxxx>
Cc: <bpf@xxxxxxxxxxxxxxx>

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);