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SubjectRe: [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@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.



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