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Date: Fri, 5 Jul 2013 16:37:14 +0200 (CEST)
From: Thomas Gleixner <tglx@linutronix.de>
To: Chuansheng Liu <chuansheng.liu@intel.com>
cc: mingo@kernel.org, peterz@infradead.org, jbeulich@suse.com,
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Subject: Re: [PATCH V2] smp: Give WARN()ing when calling smp_call_function_many()/single()
 in serving irq
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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