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 X-Spam-Level: X-Spam-Status: No, score=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id DF744C04AB6 for ; Fri, 31 May 2019 12:02:31 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id B8D4124E61 for ; Fri, 31 May 2019 12:02:31 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726518AbfEaMCb (ORCPT ); Fri, 31 May 2019 08:02:31 -0400 Received: from mx1.redhat.com ([209.132.183.28]:34542 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726330AbfEaMCa (ORCPT ); Fri, 31 May 2019 08:02:30 -0400 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.phx2.redhat.com [10.5.11.15]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id AD5363001789; Fri, 31 May 2019 12:02:29 +0000 (UTC) Received: from warthog.procyon.org.uk (ovpn-120-173.rdu2.redhat.com [10.10.120.173]) by smtp.corp.redhat.com (Postfix) with ESMTP id 1F522176B4; Fri, 31 May 2019 12:02:17 +0000 (UTC) Organization: Red Hat UK Ltd. Registered Address: Red Hat UK Ltd, Amberley Place, 107-111 Peascod Street, Windsor, Berkshire, SI4 1TE, United Kingdom. Registered in England and Wales under Company Registration No. 3798903 From: David Howells In-Reply-To: <20190531111445.GO2677@hirez.programming.kicks-ass.net> References: <20190531111445.GO2677@hirez.programming.kicks-ass.net> <20190528162603.GA24097@kroah.com> <155905930702.7587.7100265859075976147.stgit@warthog.procyon.org.uk> <155905931502.7587.11705449537368497489.stgit@warthog.procyon.org.uk> <4031.1559064620@warthog.procyon.org.uk> <20190528231218.GA28384@kroah.com> <31936.1559146000@warthog.procyon.org.uk> <16193.1559163763@warthog.procyon.org.uk> To: Peter Zijlstra Cc: dhowells@redhat.com, Jann Horn , Greg KH , Al Viro , raven@themaw.net, linux-fsdevel , Linux API , linux-block@vger.kernel.org, keyrings@vger.kernel.org, linux-security-module , kernel list , Kees Cook , Kernel Hardening Subject: Re: [PATCH 1/7] General notification queue with user mmap()'able ring buffer MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-ID: <21941.1559304135.1@warthog.procyon.org.uk> Date: Fri, 31 May 2019 13:02:15 +0100 Message-ID: <21942.1559304135@warthog.procyon.org.uk> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.44]); Fri, 31 May 2019 12:02:30 +0000 (UTC) Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org Peter Zijlstra wrote: > Can you re-iterate the exact problem? I konw we talked about this in the > past, but I seem to have misplaced those memories :/ Take this for example: void afs_put_call(struct afs_call *call) { struct afs_net *net = call->net; int n = atomic_dec_return(&call->usage); int o = atomic_read(&net->nr_outstanding_calls); trace_afs_call(call, afs_call_trace_put, n + 1, o, __builtin_return_address(0)); ASSERTCMP(n, >=, 0); if (n == 0) { ... } } I am printing the usage count in the afs_call tracepoint so that I can use it to debug refcount bugs. If I do it like this: void afs_put_call(struct afs_call *call) { int n = refcount_read(&call->usage); int o = atomic_read(&net->nr_outstanding_calls); trace_afs_call(call, afs_call_trace_put, n, o, __builtin_return_address(0)); if (refcount_dec_and_test(&call->usage)) { ... } } then there's a temporal gap between the usage count being read and the actual atomic decrement in which another CPU can alter the count. This can be exacerbated by an interrupt occurring, a softirq occurring or someone enabling the tracepoint. I can't do the tracepoint after the decrement if refcount_dec_and_test() returns false unless I save all the values from the object that I might need as the object could be destroyed any time from that point on. In this particular case, that's just call->debug_id, but it could be other things in other cases. Note that I also can't touch the afs_net object in that situation either, and the outstanding calls count that I record will potentially be out of date - but there's not a lot I can do about that. David