From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 70375CDB465 for ; Thu, 19 Oct 2023 04:54:46 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232592AbjJSEyp (ORCPT ); Thu, 19 Oct 2023 00:54:45 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:38856 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229894AbjJSEyo (ORCPT ); Thu, 19 Oct 2023 00:54:44 -0400 Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 19D4B9F for ; Wed, 18 Oct 2023 21:54:42 -0700 (PDT) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 646F4C433C7; Thu, 19 Oct 2023 04:54:41 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1697691281; bh=3lWzFUlU/97b/dHwAa2C06N+ewC9NXooDEcnsNFfjJo=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=GwbdbaWanBnKpKWHI+R6KDatN9lE/1ktRQZf8n7y6cfIiUf9sVuGcti1EQrRuuXQn arhNQyhIYyHxw2NgiKDTa6S8K8vac2V+qxrSlpBsCXArusAemLg+rS+FLKAi7fzdvj qZLb4Q9JodZBQWZbIuEtDokTW1Gptt6mpFk3xiEvLGsqs+3KGlwgz3ise84xdFoIxu dS/LYDeCDP19ZVJOH4NrzYA7Ybge4NUWMKthkK3VwQjFHMbsYa0/MfnZkxZMxgWYj2 GU9vqyD7c41CEERak5SLGddt7gxtNFHEyWsm1Hb40coRt9cbydjXn/1S/xv/38A0VW QFoviAAZPdBDg== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id E308FCE0F15; Wed, 18 Oct 2023 21:54:40 -0700 (PDT) Date: Wed, 18 Oct 2023 21:54:40 -0700 From: "Paul E. McKenney" To: Hou Tao Cc: bpf@vger.kernel.org, David Vernet , Andrii Nakryiko , Alexei Starovoitov , Daniel Borkmann , Martin KaFai Lau , Song Liu , Yonghong Song , John Fastabend , KP Singh , Stanislav Fomichev , Hao Luo , Jiri Olsa , linux-kernel@vger.kernel.org Subject: Re: [PATCH bpf] Fold smp_mb__before_atomic() into atomic_set_release() Message-ID: Reply-To: paulmck@kernel.org References: <722b64d7-281b-b4ab-4d4d-403abc41a36b@huaweicloud.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <722b64d7-281b-b4ab-4d4d-403abc41a36b@huaweicloud.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 > > Acked-by: David Vernet > > Cc: Andrii Nakryiko > > Cc: Alexei Starovoitov > > Cc: Daniel Borkmann > > Cc: Martin KaFai Lau > > Cc: Song Liu > > Cc: Yonghong Song > > Cc: John Fastabend > > Cc: KP Singh > > Cc: Stanislav Fomichev > > Cc: Hao Luo > > Cc: Jiri Olsa > > Cc: > > > > 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); > > > > . >