From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754857Ab1KYLsW (ORCPT ); Fri, 25 Nov 2011 06:48:22 -0500 Received: from casper.infradead.org ([85.118.1.10]:53540 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754417Ab1KYLsV (ORCPT ); Fri, 25 Nov 2011 06:48:21 -0500 Subject: Re: [PATCH] printk: fix recursion of logbuf_lock spinlock From: Peter Zijlstra To: Andrea Arcangeli Cc: linux-kernel@vger.kernel.org, Andrew Morton In-Reply-To: <1322161455-21242-1-git-send-email-aarcange@redhat.com> References: <1322161455-21242-1-git-send-email-aarcange@redhat.com> Content-Type: text/plain; charset="UTF-8" Date: Fri, 25 Nov 2011 12:49:06 +0100 Message-ID: <1322221746.2535.3.camel@laptop> Mime-Version: 1.0 X-Mailer: Evolution 2.32.2 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 2011-11-24 at 20:04 +0100, Andrea Arcangeli wrote: > kernel/printk.c | 2 +- > 1 files changed, 1 insertions(+), 1 deletions(-) > > diff --git a/kernel/printk.c b/kernel/printk.c > index 1455a0d..fc3c831 100644 > --- a/kernel/printk.c > +++ b/kernel/printk.c > @@ -1259,9 +1259,9 @@ void console_unlock(void) > > console_may_schedule = 0; > > -again: > for ( ; ; ) { > raw_spin_lock_irqsave(&logbuf_lock, flags); > +again: > wake_klogd |= log_start - log_end; > if (con_start == log_end) > break; /* Nothing to print */ The better change would be the below one, since that avoid doing console_trylock() while holding logbuf_lock. The problem is that console_trylock() can do up() and thus result in a wakeup while holding logbuf_lock and we're trying to get rid of locks nested under logbug_lock. --- Subject: printk: Avoid double lock acquire Commit 4f2a8d3cf5e ("printk: Fix console_sem vs logbuf_lock unlock race") introduced another silly bug where we would want to acquire an already held lock. Avoid this. Reported-by: Andrea Arcangeli Signed-off-by: Peter Zijlstra --- kernel/printk.c | 3 ++- 1 files changed, 2 insertions(+), 1 deletions(-) diff --git a/kernel/printk.c b/kernel/printk.c index dfd8f73..187662f 100644 --- a/kernel/printk.c +++ b/kernel/printk.c @@ -1292,10 +1292,11 @@ void console_unlock(void) raw_spin_lock(&logbuf_lock); if (con_start != log_end) retry = 1; + raw_spin_unlock_irqrestore(&logbuf_lock, flags); + if (retry && console_trylock()) goto again; - raw_spin_unlock_irqrestore(&logbuf_lock, flags); if (wake_klogd) wake_up_klogd(); }