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 14:12:05 +0800
From:	Wang YanQing <udknight@...il.com>
To:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
Cc:	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, tglx@...utronix.de,
	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 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 <preeti@...ux.vnet.ibm.com>
> Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> Cc: Ingo Molnar <mingo@...e.hu>
> Cc: Xiao Guangrong <xiaoguangrong@...fujitsu.com>
> Cc: srivatsa.bhat@...ux.vnet.ibm.com
> Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Rusty Russell <rusty@...tcorp.com.au
> ---
> 
>  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.

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