On Thu, Oct 19, 2023 at 09:07:07AM +0800, Hou Tao wrote: > 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 ? The real reason is that smp_mb__before_atomic() is not defined to do anything unless followed by an atomic read-modify-write operation, and atomic_read(), atomic_64read(), atomic_set(), and so on are not read-modify-write operations. As you point out, one implementation consequence of this is that smp_mb__before_atomic() is nothingness on x86. > 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 The above two lines are supposed to be accessing the same field, correct? If so, process Y's store really should be WRITE_ONCE(). > // 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 ? Yes, because smp_mb() will order the prior store against that later load. Thanx, Paul > 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); > > > > . >
