From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752709Ab3GFGNK (ORCPT ); Sat, 6 Jul 2013 02:13:10 -0400 Received: from mail-pd0-f170.google.com ([209.85.192.170]:47627 "EHLO mail-pd0-f170.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752046Ab3GFGNI (ORCPT ); Sat, 6 Jul 2013 02:13:08 -0400 Date: Sat, 6 Jul 2013 14:12:05 +0800 From: Wang YanQing To: Preeti U Murthy Cc: xiaoguangrong@cn.fujitsu.com, mingo@elte.hu, paulmck@linux.vnet.ibm.com, linux-kernel@vger.kernel.org, a.p.zijlstra@chello.nl, npiggin@suse.de, deepthi@linux.vnet.ibm.com, peterz@infradead.org, rusty@rustcorp.com.au, heiko.carstens@de.ibm.com, rostedt@goodmis.org, miltonm@bga.com, srivatsa.bhat@linux.vnet.ibm.com, jens.axboe@oracle.com, tj@kernel.org, akpm@linux-foundation.org, svaidy@linux.vnet.ibm.com, shli@kernel.org, tglx@linutronix.de, lig.fnst@cn.fujitsu.com, anton@samba.org Subject: Re: [PATCH 2/3] smp/ipi:Clarify ambiguous comments around deadlock scenarios in smp_call_function variants. Message-ID: <20130706061205.GA3518@udknight> Mail-Followup-To: Wang YanQing , Preeti U Murthy , xiaoguangrong@cn.fujitsu.com, mingo@elte.hu, paulmck@linux.vnet.ibm.com, linux-kernel@vger.kernel.org, a.p.zijlstra@chello.nl, npiggin@suse.de, deepthi@linux.vnet.ibm.com, peterz@infradead.org, rusty@rustcorp.com.au, heiko.carstens@de.ibm.com, rostedt@goodmis.org, miltonm@bga.com, srivatsa.bhat@linux.vnet.ibm.com, jens.axboe@oracle.com, tj@kernel.org, akpm@linux-foundation.org, svaidy@linux.vnet.ibm.com, shli@kernel.org, tglx@linutronix.de, lig.fnst@cn.fujitsu.com, anton@samba.org References: <20130705162548.16888.18957.stgit@preeti.in.ibm.com> <20130705162711.16888.30274.stgit@preeti.in.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20130705162711.16888.30274.stgit@preeti.in.ibm.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jul 05, 2013 at 09:57:11PM +0530, Preeti U Murthy wrote: > Elaborate on when deadlocks can occur when a call is made to > smp_call_function_single() and its friends. This avoids ambiguity about > when to use these calls. > > Signed-off-by: Preeti U Murthy > Cc: Peter Zijlstra > Cc: Ingo Molnar > Cc: Xiao Guangrong > Cc: srivatsa.bhat@linux.vnet.ibm.com > Cc: Paul E. McKenney > Cc: Steven Rostedt > Cc: Rusty Russell --- > > kernel/smp.c | 46 ++++++++++++++++++++++++++++++++++++++++++++-- > 1 file changed, 44 insertions(+), 2 deletions(-) > > diff --git a/kernel/smp.c b/kernel/smp.c > index 89be6e6..b6981ae 100644 > --- a/kernel/smp.c > +++ b/kernel/smp.c > @@ -230,7 +230,23 @@ int smp_call_function_single(int cpu, smp_call_func_t func, void *info, > this_cpu = get_cpu(); > > /* > - * Can deadlock when called with interrupts disabled. > + * Can deadlock when called with interrupts disabled under two > + * different circumstances depending on the wait parameter. > + * > + * 1. wait = 1: Two CPUs execute smp_call_function_single(), send an > + * IPI to each other, and wait for func to finish on each other. > + * Since they are interrupt disabled, neither receives this IPI, > + * nor do they proceed forward,as they wait for each other to complete > + * execution of func. > + * Yes, we should avoid this situation, but I am not sure whether this is the meaning of "deadlock" in the original comment. > + * 2. wait = 0: This function could be called from an interrupt > + * context, and can get blocked on the csd_lock(csd) below in > + * "non wait cases". > + * This is because the percpu copy of csd of this_cpu is used > + * in non wait cases. Under such circumstances, if the previous caller > + * of this function who got preempted by this interrupt has already taken > + * the lock under non wait condition, it will result in deadlock. > + * No, it will not cause deadlock, it is not mutex lock, it is busy wait, so when the CSD_FLAG_LOCK be cleared, the code will go on running. After stare into the kernel/smp.c, I can't catch what the exactly meaning of the "DeadLock" in the original comment also. I hope someone can clarify it. Thanks.