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 20C9DC4167B for ; Thu, 30 Nov 2023 23:57:01 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1377330AbjK3X4x (ORCPT ); Thu, 30 Nov 2023 18:56:53 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:36406 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1377294AbjK3X4v (ORCPT ); Thu, 30 Nov 2023 18:56:51 -0500 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 03A9810DF for ; Thu, 30 Nov 2023 15:56:57 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1701388616; 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=isRSmHfA7wl0UpgP4f3nZ8cWSqDYWRF8DVcNYkEZCpE=; b=DUswoKqRA+fdxyQTBJ/ORhk9v1ExBy374ToOi+b+a9lxmA2omzcUmZ2Ohdp47QcUZuRIL8 /qPeMLcXDepsr5ZG8qS6gW0babgVM4QhNhDi1+tgRM2jCO3mu+TWds1MdHHh1ZBoZMcK47 9rHTT3yONOyofKtM8yKrdPFqQLySNCg= Received: from mimecast-mx02.redhat.com (mx-ext.redhat.com [66.187.233.73]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-333-y6qr_XQ5O52A4CV1P-MZsQ-1; Thu, 30 Nov 2023 18:56:54 -0500 X-MC-Unique: y6qr_XQ5O52A4CV1P-MZsQ-1 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.rdu2.redhat.com [10.11.54.4]) (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 mimecast-mx02.redhat.com (Postfix) with ESMTPS id 87785380450D; Thu, 30 Nov 2023 23:56:53 +0000 (UTC) Received: from [10.22.9.192] (unknown [10.22.9.192]) by smtp.corp.redhat.com (Postfix) with ESMTP id 234422026D4C; Thu, 30 Nov 2023 23:56:53 +0000 (UTC) Message-ID: <151f2b05-bcb7-4e69-866a-c76286383175@redhat.com> Date: Thu, 30 Nov 2023 18:56:52 -0500 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] locking: Document that mutex_unlock() is non-atomic Content-Language: en-US To: Jann Horn Cc: Peter Zijlstra , Ingo Molnar , Will Deacon , Jonathan Corbet , linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org References: <20231130204817.2031407-1-jannh@google.com> <2f17a9a6-5781-43ef-a09b-f39310843fe6@redhat.com> From: Waiman Long In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 3.4.1 on 10.11.54.4 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 11/30/23 17:24, Jann Horn wrote: > On Thu, Nov 30, 2023 at 10:53 PM Waiman Long wrote: >> On 11/30/23 15:48, Jann Horn wrote: >>> I have seen several cases of attempts to use mutex_unlock() to release an >>> object such that the object can then be freed by another task. >>> My understanding is that this is not safe because mutex_unlock(), in the >>> MUTEX_FLAG_WAITERS && !MUTEX_FLAG_HANDOFF case, accesses the mutex >>> structure after having marked it as unlocked; so mutex_unlock() requires >>> its caller to ensure that the mutex stays alive until mutex_unlock() >>> returns. >>> >>> If MUTEX_FLAG_WAITERS is set and there are real waiters, those waiters >>> have to keep the mutex alive, I think; but we could have a spurious >>> MUTEX_FLAG_WAITERS left if an interruptible/killable waiter bailed >>> between the points where __mutex_unlock_slowpath() did the cmpxchg >>> reading the flags and where it acquired the wait_lock. >> Could you clarify under what condition a concurrent task can decide to >> free the object holding the mutex? Is it !mutex_is_locked() or after a >> mutex_lock()/mutex_unlock sequence? > I mean a mutex_lock()+mutex_unlock() sequence. Because of optimistic spinning, a mutex_lock()/mutex_unlock() can succeed even if there are still waiters waiting for the lock. > >> mutex_is_locked() will return true if the mutex has waiter even if it >> is currently free. > I don't understand your point, and maybe I also don't understand what > you mean by "free". Isn't mutex_is_locked() defined such that it only > looks at whether a mutex has an owner, and doesn't look at the waiter > list? What I mean is that the mutex is in an unlocked state ready to be acquired by another locker. mutex_is_locked() considers the state of the mutex as locked if any of the owner flags is set. Beside the mutex_lock()/mutex_unlock() sequence, I will suggest adding a mutex_is_locked() check just to be sure. Cheers, Longman