From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) (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 A9B8D3AD52F for ; Mon, 14 Sep 2026 20:22:51 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.133.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789417373; cv=none; b=T/nZR6TUv2DM0yfHwr+OwpEpZI1QezTECP0BRfBE0g/24D042B+RxSXMtW6a+4UUCZ7GrlTgoBB/uSqJ5ohLNvM/oFBGsBLGcu/t1DH9FcV3Bce2cWAPnkKJ12qJMsVWciRMSlxqolQt3t9K3JHIXu/a5cISqHej/xpoBo5XEtE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789417373; c=relaxed/simple; bh=zvlpMyfSUrjRz9Di3IssgzDebEemfiRCuSfb7GSOGvk=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=j4Syyb/acPQYBEmyw+KNhO+GIf7XrMAQqRRryTnfG6dsZJpPubLjkF2M6rflD4ldL/gZUBf5X97Pgh+X+3hMVEttLK8+JQQ7wWsSjrxgL5WP7nxXTHp4ojqybOqFy1nX6g4rwM64vAcpZwJvbUyCyeI5K3ayMXTaKFrnXyoXTFc= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=WdNU8n8J; arc=none smtp.client-ip=170.10.133.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="WdNU8n8J" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1789417370; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=E8upkNH2O8IYnyasulTlrzDdS2mPY5U+7GMoB4t9ZbM=; b=WdNU8n8J3Cr/K9rfGrVnpLGWLcZkah0xZx2QEGeHns9mkR18zH0lZjfFDhBlBTLLYo4t+/ BJu0zKmy+alhbXCuFjtdMLTIGtV2Ye9zFe1xO4IdAxbLrA7Hi/MGuk20p4lUYQNYiI2lRs 37zvvO15GSsKoXlUjS6lUneQfU4xMHw= Received: from mx-prod-mc-03.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-418-1NwwZKnzPtqg0x3B70M1UA-1; Mon, 14 Sep 2026 16:22:47 -0400 X-MC-Unique: 1NwwZKnzPtqg0x3B70M1UA-1 X-Mimecast-MFC-AGG-ID: 1NwwZKnzPtqg0x3B70M1UA_1789417365 Received: from mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.12]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 2AF2C19C95C9; Mon, 14 Sep 2026 20:22:45 +0000 (UTC) Received: from [100.91.18.181] (headnet05.pony-001.prod.iad2.dc.redhat.com [10.2.32.117]) by mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id 567591956087; Mon, 14 Sep 2026 20:22:43 +0000 (UTC) Message-ID: <2e389029-5abe-40fa-9dba-40ca66189957@redhat.com> Date: Mon, 14 Sep 2026 16:22:42 -0400 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] locking/osq_lock: Ensure proper locking semantics for osq_lock/osq_unlock() To: David Laight Cc: Peter Zijlstra , Ingo Molnar , Will Deacon , Boqun Feng , linux-kernel@vger.kernel.org, Davidlohr Bueso , Haakon Bugge , Linus Torvalds , Yafang Shao , Steven Rostedt References: <20260910141908.592414-1-longman@redhat.com> <20260914105337.576bab34@pumpkin> Content-Language: en-US From: Waiman Long In-Reply-To: <20260914105337.576bab34@pumpkin> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 3.0 on 10.30.177.12 On 9/14/26 5:53 AM, David Laight wrote: > On Thu, 10 Sep 2026 10:19:08 -0400 > Waiman Long wrote: > >> The osq_lock is special in the sense that lock transfer from one CPU to >> the next can happen either over the common optimistic_spin_queue.tail >> value with uncontended lock or over a lock waiter's own percpu >> optimistic_spin_node.locked flag when the lock is contended. >> >> To ensure proper lock synchronization, we need to provide >> the acquire/release semantics for the osq_lock/osq_unlock() >> functions in both cases. This is currently the case for the >> common optimistic_spin_queue.tail value, but not for the percpu >> optimistic_spin_node.locked flag as the proper barriers are missing in >> some places. Fix that by adding the needed barriers in those places. >> >> Note that the two percpu optimistic_spin_node.locked setting in >> osq_unlock() are proceeded by a full barrier xchg() call, but the >> contended cachelines are different. This should probably work in most >> cases except in some exotic architectures where the barrier semantics >> may be cacheline specific. Nevertheless a release barrier is still added >> for safety reason as we may opt to relax the xchg() calls in the future. >> >> The "node->locked" read in osq_lock() was relaxed by commit 036cc30c6b6a >> ("locking/osq: No need for load/acquire when acquire-polling") a while >> ago as the smp_load_acquire() loop was causing a performance hit due to >> the repeated acquire barriers in the loop and it argued that an earlier >> atomic_xchg() call could provide the needed barrier. That may not be >> enough especially if we have to loop for a while before the lock is >> released. Now with the new smp_cond_load_acquire() helper, only one >> acquire barrier is added at the end of the loop. So it shouldn't have >> the performance hit noted in that commit. >> >> Currently osq_lock is used only by mutex and rw_semaphore code for queuing >> purpose. As a result, the imperfect lock synchronization support does >> not cause harmful consequence as the new osq_lock owner of a contended >> osq_lock will still have to wait for the real mutex and rwsem lock to >> be released by the pervious osq_lock owner before it can acquire it and >> go into its critical section. For correctness, we still have to fix it >> in case it is used elsewhere which doesn't have this inherent protection. >> >> Fixes: 036cc30c6b6a ("locking/osq: No need for load/acquire when acquire-polling") >> Tested-by: Håkon Bugge >> Signed-off-by: Waiman Long >> --- >> kernel/locking/osq_lock.c | 11 +++++++---- >> 1 file changed, 7 insertions(+), 4 deletions(-) >> >> diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c >> index b4233dc2c2b0..ef1bbd914917 100644 >> --- a/kernel/locking/osq_lock.c >> +++ b/kernel/locking/osq_lock.c >> @@ -143,7 +143,7 @@ bool osq_lock(struct optimistic_spin_queue *lock) >> * is implemented with a monitor-wait. vcpu_is_preempted() relies on >> * polling, be careful. >> */ >> - if (smp_cond_load_relaxed(&node->locked, VAL || need_resched() || >> + if (smp_cond_load_acquire(&node->locked, VAL || need_resched() || >> vcpu_is_preempted(node_cpu(node->prev)))) > The comment above needs changing to match. > > David Thank for noticing that. I update the comment in v3 patch. Cheers, Longman