From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752624AbbIONPQ (ORCPT ); Tue, 15 Sep 2015 09:15:16 -0400 Received: from mail-wi0-f170.google.com ([209.85.212.170]:37396 "EHLO mail-wi0-f170.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752043AbbIONPO (ORCPT ); Tue, 15 Sep 2015 09:15:14 -0400 Subject: Re: [PATCH v2] arm: Fix backtrace generation when IPI is masked To: Russell King - ARM Linux References: <1442222294-2756-1-git-send-email-daniel.thompson@linaro.org> <1442315112-14039-1-git-send-email-daniel.thompson@linaro.org> <20150915113003.GS21084@n2100.arm.linux.org.uk> Cc: linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, patches@linaro.org, linaro-kernel@lists.linaro.org, Thomas Gleixner , Hillf Danton From: Daniel Thompson Message-ID: <55F819DE.9040704@linaro.org> Date: Tue, 15 Sep 2015 14:15:10 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.2.0 MIME-Version: 1.0 In-Reply-To: <20150915113003.GS21084@n2100.arm.linux.org.uk> Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 15/09/15 12:30, Russell King - ARM Linux wrote: > On Tue, Sep 15, 2015 at 12:05:12PM +0100, Daniel Thompson wrote: >> Currently on ARM when is triggered from an interrupt handler >> (e.g. a SysRq issued using UART or kbd) the main CPU will wedge for ten >> seconds with interrupts masked before issuing a backtrace for every CPU >> except itself. >> >> The new backtrace code introduced by commit 96f0e00378d4 ("ARM: add >> basic support for on-demand backtrace of other CPUs") does not work >> correctly when run from an interrupt handler because IPI_CPU_BACKTRACE >> is used to generate the backtrace on all CPUs but cannot preempt the >> current calling context. > > This patch needs a little more work - what happens to the IPI_CPU_BACKTRACE > we've sent to ourselves? (It fires after the interrupt handler for the > UART/kbd has finished.) It ought to be masked out if we're going to > handle it a different way. Actually it already gets masked out. The argument to raise_nmi() points to a data structure owned by the backtrace library functions and this structure if altered during the execution of nmi_cpu_backtrace() to clear the calling CPU. I had originally planned to use cpumask_test_and_clear_cpu() for the conditional branch but that would be broken because nmi_cpu_backtrace() would become a nop if we clear anything from the mask before calling it! I guess I should add a comment about this to save us from broken but "obviously correct" cleanups in the future... Daniel.