Hi Paul, On 10/19/2023 6:28 AM, Paul E. McKenney wrote: > 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); The reason is smp_mb__before_atomic() is defined as noop and atomic_read() in x86-64 is just READ_ONCE(), right ? And it seems that I also used smp_mb__before_atomic() in a wrong way for patch [1]. The memory order in the posted patch is process X process Y atomic64_dec_and_test(&map->usercnt) READ_ONCE(timer->timer) timer->time = t // it won't work smp_mb__before_atomic() atomic64_read(&map->usercnt) For the problem, it seems I need to replace smp_mb__before_atomic() by smp_mb() to fix the memory order, right ? Regards, Hou [1]: https://lore.kernel.org/bpf/20231017125717.241101-2-houtao@xxxxxxxxxxxxxxx/ > > 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); > > .
