From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753928AbZAZXQ0 (ORCPT ); Mon, 26 Jan 2009 18:16:26 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752210AbZAZXQQ (ORCPT ); Mon, 26 Jan 2009 18:16:16 -0500 Received: from mx3.mail.elte.hu ([157.181.1.138]:52832 "EHLO mx3.mail.elte.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751818AbZAZXQQ (ORCPT ); Mon, 26 Jan 2009 18:16:16 -0500 Date: Tue, 27 Jan 2009 00:15:55 +0100 From: Ingo Molnar To: Ed Swierk , Oleg Nesterov Cc: rml@tech9.net, linux-kernel@vger.kernel.org, kpreempt-tech@lists.sourceforge.net Subject: Re: [PATCH] Fix BUG: using smp_processor_id() in preemptible code in print_fatal_signal() Message-ID: <20090126231555.GB6556@elte.hu> References: <1233010818.14510.11.camel@localhost.localdomain> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1233010818.14510.11.camel@localhost.localdomain> User-Agent: Mutt/1.5.18 (2008-05-17) X-ELTE-VirusStatus: clean X-ELTE-SpamScore: -1.5 X-ELTE-SpamLevel: X-ELTE-SpamCheck: no X-ELTE-SpamVersion: ELTE 2.0 X-ELTE-SpamCheck-Details: score=-1.5 required=5.9 tests=BAYES_00 autolearn=no SpamAssassin version=3.2.3 -1.5 BAYES_00 BODY: Bayesian spam probability is 0 to 1% [score: 0.0000] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Ed Swierk wrote: > With print-fatal-signals=1 on a kernel with CONFIG_PREEMPT=y, sending an > unexpected signal to a process causes a BUG: using smp_processor_id() in > preemptible code. > > get_signal_to_deliver() releases the siglock before calling > print_fatal_signal(), which calls show_regs(), which calls > smp_processor_id(), which is not supposed to be called from a > preemptible thread. > > Calling print_fatal_signal() before releasing the siglock avoids this > problem. > > Signed-off-by: Ed Swierk > > --- > Index: linux-2.6.27.4/kernel/signal.c > =================================================================== > --- linux-2.6.27.4.orig/kernel/signal.c > +++ linux-2.6.27.4/kernel/signal.c > @@ -1848,6 +1848,11 @@ relock: > continue; > } > > + if (sig_kernel_coredump(signr) && print_fatal_signals) { > + /* hold lock for smp_processor_id() */ > + print_fatal_signal(regs, info->si_signo); > + } > + > spin_unlock_irq(&sighand->siglock); This trades a (harmless) debug warning against a potential deadlock or even a crash, because print_fatal_signal() can do this: __get_user(insn, (unsigned char *)(regs->ip + i)); which will work without a fault most of the time but might also generate a pagefault and schedule away from atomic context. So please add preempt_disable()+preempt_enable() calls around the show_regs() call instead. Ingo