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);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@huaweicloud.com/ > > 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); > > .
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@huaweicloud.com/ > > > > > > 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); > > > > . >
Hi Paul,
On 10/19/2023 12:54 PM, Paul E. McKenney wrote:
> 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.
I see. Thanks for explanation. It seems I did not read
Documentation/atomic_t.txt carefully, it said:
The barriers:
smp_mb__{before,after}_atomic()
only apply to the RMW atomic ops and can be used to augment/upgrade the
ordering inherent to the op.
>
> 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().
Yes. These two processes are accessing the same field (namely
timer->timer). Is WRITE_ONCE(xx) still necessary when the write of
timer->time in process Y is protected by a spin-lock ?
>
>> // 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.
Thanks. Will fix the patch.
Regards,
Hou
>
> Thanx, Paul
>
>> Regards,
>> Hou
>>
>> [1]:
>> https://lore.kernel.org/bpf/20231017125717.241101-2-houtao@huaweicloud.com/
>>
>>
>>> 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);
>>>
>>> .
On Thu, Oct 19, 2023 at 02:20:35PM +0800, Hou Tao wrote:
> Hi Paul,
>
> On 10/19/2023 12:54 PM, Paul E. McKenney wrote:
> > 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.
>
> I see. Thanks for explanation. It seems I did not read
> Documentation/atomic_t.txt carefully, it said:
>
> The barriers:
>
> smp_mb__{before,after}_atomic()
>
> only apply to the RMW atomic ops and can be used to augment/upgrade the
> ordering inherent to the op.
That is the place!
> > 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().
>
> Yes. These two processes are accessing the same field (namely
> timer->timer). Is WRITE_ONCE(xx) still necessary when the write of
> timer->time in process Y is protected by a spin-lock ?
If there is any possibility of a concurrent reader, that is, a reader
not holding that same lock, then yes, you should use WRITE_ONCE().
Compilers can do pretty vicious things to unmarked reads and writes.
But don't take my word for it, here are a few writeups:
o "Who's afraid of a big bad optimizing compiler?" (series)
https://lwn.net/Articles/793253, https://lwn.net/Articles/799218
o "An introduction to lockless algorithms" (Paolo Bonzini series)
https://lwn.net/Articles/844224, https://lwn.net/Articles/846700,
https://lwn.net/Articles/847481, https://lwn.net/Articles/847973,
https://lwn.net/Articles/849237, https://lwn.net/Articles/850202
o "Is Parallel Programming Hard, And, If So, What Can You Do About It?"
Section 4.3.4 ("Accessing Shared Variables")
https://mirrors.edge.kernel.org/pub/linux/kernel/people/paulmck/perfbook/
perfbook.html
> >> // 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.
>
> Thanks. Will fix the patch.
Very good!
Thanx, Paul
> Regards,
> Hou
> >
> > Thanx, Paul
> >
> >> Regards,
> >> Hou
> >>
> >> [1]:
> >> https://lore.kernel.org/bpf/20231017125717.241101-2-houtao@huaweicloud.com/
> >>
> >>
> >>> 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);
> >>>
> >>> .
>
Hi Paul,
On 10/19/2023 10:25 PM, Paul E. McKenney wrote:
> On Thu, Oct 19, 2023 at 02:20:35PM +0800, Hou Tao wrote:
>> Hi Paul,
>>
>> On 10/19/2023 12:54 PM, Paul E. McKenney wrote:
>>> 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.
>> I see. Thanks for explanation. It seems I did not read
>> Documentation/atomic_t.txt carefully, it said:
>>
>> The barriers:
>>
>> smp_mb__{before,after}_atomic()
>>
>> only apply to the RMW atomic ops and can be used to augment/upgrade the
>> ordering inherent to the op.
> That is the place!
>
>>> 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().
>> Yes. These two processes are accessing the same field (namely
>> timer->timer). Is WRITE_ONCE(xx) still necessary when the write of
>> timer->time in process Y is protected by a spin-lock ?
> If there is any possibility of a concurrent reader, that is, a reader
> not holding that same lock, then yes, you should use WRITE_ONCE().
Got it. Will do.
>
> Compilers can do pretty vicious things to unmarked reads and writes.
> But don't take my word for it, here are a few writeups:
>
> o "Who's afraid of a big bad optimizing compiler?" (series)
> https://lwn.net/Articles/793253, https://lwn.net/Articles/799218
>
> o "An introduction to lockless algorithms" (Paolo Bonzini series)
> https://lwn.net/Articles/844224, https://lwn.net/Articles/846700,
> https://lwn.net/Articles/847481, https://lwn.net/Articles/847973,
> https://lwn.net/Articles/849237, https://lwn.net/Articles/850202
>
> o "Is Parallel Programming Hard, And, If So, What Can You Do About It?"
> Section 4.3.4 ("Accessing Shared Variables")
> https://mirrors.edge.kernel.org/pub/linux/kernel/people/paulmck/perfbook/
> perfbook.html
Thanks for these excellent articles. Will read these articles carefully
this time.
Regards,
Hou
>
>>>> // 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.
>> Thanks. Will fix the patch.
> Very good!
>
> Thanx, Paul
>
>> Regards,
>> Hou
>>> Thanx, Paul
>>>
>>>> Regards,
>>>> Hou
>>>>
>>>> [1]:
>>>> https://lore.kernel.org/bpf/20231017125717.241101-2-houtao@huaweicloud.com/
>>>>
>>>>
>>>>> 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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