Re: [PATCH 2/3] smp/ipi:Clarify ambiguous comments around deadlock scenarios in smp_call_function variants.

[Wang YanQing <udknight@gmail.com>]

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@linux.vnet.ibm.com>
> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Cc: Ingo Molnar <mingo@elte.hu>
> Cc: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
> Cc: srivatsa.bhat@linux.vnet.ibm.com
> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Rusty Russell <rusty@rustcorp.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.