On Fri, 5 Jul 2013, Thomas Gleixner wrote:
> On Sat, 16 Feb 2013, Chuansheng Liu wrote:
> > Currently the functions smp_call_function_many()/single() will
> > give a WARN()ing only in the case of irqs_disabled(), but that
> > check is not enough to guarantee execution of the SMP
> > cross-calls.
> >
> > In many other cases such as softirq handling/interrupt handling,
> > the two APIs still can not be called, just as the
> > smp_call_function_many() comments say:
> >
> > * You must not call this function with disabled interrupts or from a
> > * hardware interrupt handler or from a bottom half handler. Preemption
> > * must be disabled when calling this function.
> >
> > There is a real case for softirq DEADLOCK case:
> >
> > CPUA CPUB
> > spin_lock(&spinlock)
> > Any irq coming, call the irq handler
> > irq_exit()
> > spin_lock_irq(&spinlock)
> > <== Blocking here due to
> > CPUB hold it
> > __do_softirq()
> > run_timer_softirq()
> > timer_cb()
> > call smp_call_function_many()
> > send IPI interrupt to CPUA
> > wait_csd()
> >
> > Then both CPUA and CPUB will be deadlocked here.
>
> That's not true if called with wait = 0 as we won't wait for the csd
> in that case. The function will be invoked on cpuA after it reenables
> interrupt. So for callers who don't care about synchronous execution
> it should not warn in softirq context.
Hmm, even there it matters, because of the following scenario:
CPU 0
smp_call_function_single(CPU 1)
csd_lock(CPU 1)
irq_enter()
irq_exit()
__do_softirq()
smp_call_function_many()
setup csd (CPU 1)
csd_lock(CPU 1) ==> CPU 0 deadlocked itself.
And this is even more likely to happen than the lock issue.
Thanks,
tglx
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