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

[Paul E. McKenney]

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);
> >
> > .
>