linux-kernel - Re: [PATCH 2/3] smp/ipi:Clarify ambiguous comments around deadlock scenarios in smp_call

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Date:	Sat, 6 Jul 2013 21:48:51 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
cc:	Wang YanQing <udknight@...il.com>, xiaoguangrong@...fujitsu.com,
	mingo@...e.hu, paulmck@...ux.vnet.ibm.com,
	linux-kernel@...r.kernel.org, a.p.zijlstra@...llo.nl,
	npiggin@...e.de, deepthi@...ux.vnet.ibm.com, peterz@...radead.org,
	rusty@...tcorp.com.au, heiko.carstens@...ibm.com,
	rostedt@...dmis.org, miltonm@....com,
	srivatsa.bhat@...ux.vnet.ibm.com, jens.axboe@...cle.com,
	tj@...nel.org, akpm@...ux-foundation.org,
	svaidy@...ux.vnet.ibm.com, shli@...nel.org,
	lig.fnst@...fujitsu.com, anton@...ba.org
Subject: Re: [PATCH 2/3] smp/ipi:Clarify ambiguous comments around deadlock
 scenarios in smp_call_function variants.

On Sat, 6 Jul 2013, Preeti U Murthy wrote:

> Hi Wang,
> 
> On 07/06/2013 11:42 AM, Wang YanQing wrote:
> > 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.
> >>
> >> +	 * 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.
> 
> A deadlock might not result, but a potential long wait in an interrupt
> context could result if the source cpu got preempted by an interrupt
> between  csd_lock(csd) and generic_exec_single(), where it actually
> sends an ipi to the target cpu.

See https://lkml.org/lkml/2013/7/5/183 and the related thread for real
deadlock scenarios.
 
Thanks,

	tglx
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