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=-8.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_2 autolearn=ham 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 19C7BC2D0A3 for ; Mon, 9 Nov 2020 17:18:33 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id C234F20E65 for ; Mon, 9 Nov 2020 17:18:32 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730189AbgKIRSc (ORCPT ); Mon, 9 Nov 2020 12:18:32 -0500 Received: from mail.kernel.org ([198.145.29.99]:33978 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729432AbgKIRSc (ORCPT ); Mon, 9 Nov 2020 12:18:32 -0500 Received: from oasis.local.home (cpe-66-24-58-225.stny.res.rr.com [66.24.58.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 81D8120E65; Mon, 9 Nov 2020 17:18:30 +0000 (UTC) Date: Mon, 9 Nov 2020 12:18:28 -0500 From: Steven Rostedt To: Cc: Subject: Re: FAILED: patch "[PATCH] ring-buffer: Fix recursion protection transitions between" failed to apply to 4.14-stable tree Message-ID: <20201109121828.4bf8bd2e@oasis.local.home> In-Reply-To: <1604763982232117@kroah.com> References: <1604763982232117@kroah.com> X-Mailer: Claws Mail 3.17.3 (GTK+ 2.24.32; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org On Sat, 07 Nov 2020 16:46:22 +0100 wrote: > The patch below does not apply to the 4.14-stable tree. > If someone wants it applied there, or to any other stable or longterm > tree, then please email the backport, including the original git commit > id to . > This should work for 4.14, 4.9 and 4.4: -- Steve >>From b02414c8f045ab3b9afc816c3735bc98c5c3d262 Mon Sep 17 00:00:00 2001 From: "Steven Rostedt (VMware)" Date: Mon, 2 Nov 2020 15:31:27 -0500 Subject: [PATCH] ring-buffer: Fix recursion protection transitions between interrupt context The recursion protection of the ring buffer depends on preempt_count() to be correct. But it is possible that the ring buffer gets called after an interrupt comes in but before it updates the preempt_count(). This will trigger a false positive in the recursion code. Use the same trick from the ftrace function callback recursion code which uses a "transition" bit that gets set, to allow for a single recursion for to handle transitions between contexts. Cc: stable@vger.kernel.org Fixes: 567cd4da54ff4 ("ring-buffer: User context bit recursion checking") Signed-off-by: Steven Rostedt (VMware) Index: linux-test.git/kernel/trace/ring_buffer.c =================================================================== --- linux-test.git.orig/kernel/trace/ring_buffer.c +++ linux-test.git/kernel/trace/ring_buffer.c @@ -416,14 +416,16 @@ struct rb_event_info { /* * Used for which event context the event is in. - * NMI = 0 - * IRQ = 1 - * SOFTIRQ = 2 - * NORMAL = 3 + * TRANSITION = 0 + * NMI = 1 + * IRQ = 2 + * SOFTIRQ = 3 + * NORMAL = 4 * * See trace_recursive_lock() comment below for more details. */ enum { + RB_CTX_TRANSITION, RB_CTX_NMI, RB_CTX_IRQ, RB_CTX_SOFTIRQ, @@ -2553,10 +2555,10 @@ rb_wakeups(struct ring_buffer *buffer, s * a bit of overhead in something as critical as function tracing, * we use a bitmask trick. * - * bit 0 = NMI context - * bit 1 = IRQ context - * bit 2 = SoftIRQ context - * bit 3 = normal context. + * bit 1 = NMI context + * bit 2 = IRQ context + * bit 3 = SoftIRQ context + * bit 4 = normal context. * * This works because this is the order of contexts that can * preempt other contexts. A SoftIRQ never preempts an IRQ @@ -2579,6 +2581,30 @@ rb_wakeups(struct ring_buffer *buffer, s * The least significant bit can be cleared this way, and it * just so happens that it is the same bit corresponding to * the current context. + * + * Now the TRANSITION bit breaks the above slightly. The TRANSITION bit + * is set when a recursion is detected at the current context, and if + * the TRANSITION bit is already set, it will fail the recursion. + * This is needed because there's a lag between the changing of + * interrupt context and updating the preempt count. In this case, + * a false positive will be found. To handle this, one extra recursion + * is allowed, and this is done by the TRANSITION bit. If the TRANSITION + * bit is already set, then it is considered a recursion and the function + * ends. Otherwise, the TRANSITION bit is set, and that bit is returned. + * + * On the trace_recursive_unlock(), the TRANSITION bit will be the first + * to be cleared. Even if it wasn't the context that set it. That is, + * if an interrupt comes in while NORMAL bit is set and the ring buffer + * is called before preempt_count() is updated, since the check will + * be on the NORMAL bit, the TRANSITION bit will then be set. If an + * NMI then comes in, it will set the NMI bit, but when the NMI code + * does the trace_recursive_unlock() it will clear the TRANSTION bit + * and leave the NMI bit set. But this is fine, because the interrupt + * code that set the TRANSITION bit will then clear the NMI bit when it + * calls trace_recursive_unlock(). If another NMI comes in, it will + * set the TRANSITION bit and continue. + * + * Note: The TRANSITION bit only handles a single transition between context. */ static __always_inline int @@ -2597,8 +2623,16 @@ trace_recursive_lock(struct ring_buffer_ } else bit = RB_CTX_NORMAL; - if (unlikely(val & (1 << bit))) - return 1; + if (unlikely(val & (1 << bit))) { + /* + * It is possible that this was called by transitioning + * between interrupt context, and preempt_count() has not + * been updated yet. In this case, use the TRANSITION bit. + */ + bit = RB_CTX_TRANSITION; + if (val & (1 << bit)) + return 1; + } val |= (1 << bit); cpu_buffer->current_context = val;