From patchwork Wed Oct 18 22:28:32 2023 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: "Paul E. McKenney" X-Patchwork-Id: 13428057 X-Patchwork-Delegate: bpf@iogearbox.net Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 56AFC3C08B for ; Wed, 18 Oct 2023 22:28:32 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="hKer3voR" Received: by smtp.kernel.org (Postfix) with ESMTPSA id A32FFC433C7; Wed, 18 Oct 2023 22:28:32 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1697668112; bh=kOr6kBKVTsDGo/Ewz8kI8OgtxzzOHafK+KurBxJCfV4=; h=Date:From:To:Cc:Subject:Reply-To:From; b=hKer3voRhWWEcxhPtyOE+NLlGnF6jr3xISPAMNr4ybU/hA5S6CXivbUBZjcGHBxB6 wuYohN9nefPMJfk4aJhNP/cR4rzEzuztuez/hfZL0pqRkkvoh6fcNb1iF4iqJvTdJt 3+UjEiB3JaB1dpqP48RZrOT21dOxB1zJpi6sr5d/0sUKUGuNS4V0vh8vC8ouNFAaFN /awoxU/kLjfhQeT9YUfQ7pL2luEG0xvBJWFQGoNzQVjX9VZqrfnINzPbXxomD/Cenz Eh03ihOpiIAtcMo8YBvFv3yQVjW7F+zBXj3atJK8gHCmkpjxNv3DL+iezwHfjJ6B2D FnBUUduSXSBQw== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 3F028CE0BB0; Wed, 18 Oct 2023 15:28:32 -0700 (PDT) Date: Wed, 18 Oct 2023 15:28:32 -0700 From: "Paul E. McKenney" To: bpf@vger.kernel.org Cc: 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: [PATCH bpf] Fold smp_mb__before_atomic() into atomic_set_release() Message-ID: Reply-To: paulmck@kernel.org Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Disposition: inline X-Patchwork-Delegate: bpf@iogearbox.net 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 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);