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SubjectRe: [PATCH] Remove WARN_ON_ONCE in __smp_call_function_single and smp_call_function_single
On Sat, Jan 5, 2013 at 4:31 PM, Li Zhong <zhong@linux.vnet.ibm.com> wrote:
> On Sat, 2013-01-05 at 12:26 +0800, Hui Zhu wrote:
>> The comments of these WARN_ON_ONCE said "Can deadlock when called with
>> interrupts disabled".
>> But I didn't find anything that will block this function.
>
> It seems that if two cpus try to IPI each other with wait equals true,
> they will deadlock if interrupts are disabled, because neither of them
> can take the interrupt.
>
> If wait equals false, it seems deadlock is still possible, for example,
> if smp_call_function_single() is called from interrupt context, then one
> cpu might
> -->call smp_call_function_single()
> -->csd_lock()
> -->be interrupted before sending the ipi out
> -->in interrupt context, call smp_call_function_single() again
> -->csd_lock_wait() forever in csd_lock()

Thanks for your mail. It make me more clear about these functions.

Best,
Hui

>
> I'm not sure whether it is possible that irqs be enabled in interrupt
> context? If so, maybe we also need check that if wait equals false?
>
> Thanks, Zhong
>
>> So I think maybe this WARN_ON_ONCE is not need now. So I post a patch for that.
>>
>> Signed-off-by: Hui Zhu <teawater@gmail.com>
>> Signed-off-by: Zhu Yanhai <gaoyang.zyh@taobao.com>
>>
>> --- a/kernel/smp.c
>> +++ b/kernel/smp.c
>> @@ -303,25 +303,13 @@ int smp_call_function_single(int cpu, sm
>> .flags = 0,
>> };
>> unsigned long flags;
>> - int this_cpu;
>> int err = 0;
>>
>> /*
>> * prevent preemption and reschedule on another processor,
>> * as well as CPU removal
>> */
>> - this_cpu = get_cpu();
>> -
>> - /*
>> - * Can deadlock when called with interrupts disabled.
>> - * We allow cpu's that are not yet online though, as no one else can
>> - * send smp call function interrupt to this cpu and as such deadlocks
>> - * can't happen.
>> - */
>> - WARN_ON_ONCE(cpu_online(this_cpu) && irqs_disabled()
>> - && !oops_in_progress);
>> -
>> - if (cpu == this_cpu) {
>> + if (cpu == get_cpu()) {
>> local_irq_save(flags);
>> func(info);
>> local_irq_restore(flags);
>> @@ -409,17 +397,7 @@ void __smp_call_function_single(int cpu,
>> unsigned int this_cpu;
>> unsigned long flags;
>>
>> - this_cpu = get_cpu();
>> - /*
>> - * Can deadlock when called with interrupts disabled.
>> - * We allow cpu's that are not yet online though, as no one else can
>> - * send smp call function interrupt to this cpu and as such deadlocks
>> - * can't happen.
>> - */
>> - WARN_ON_ONCE(cpu_online(smp_processor_id()) && wait && irqs_disabled()
>> - && !oops_in_progress);
>> -
>> - if (cpu == this_cpu) {
>> + if (cpu == get_cpu()) {
>> local_irq_save(flags);
>> data->func(data->info);
>> local_irq_restore(flags);
>> --
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>
>


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