Messages in this thread | | | Subject | Re: [PATCH 1/11] Add generic helpers for arch IPI function calls | From | Peter Zijlstra <> | Date | Wed, 23 Apr 2008 08:32:00 +0200 |
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On Wed, 2008-04-23 at 08:07 +0200, Jens Axboe wrote: > On Tue, Apr 22 2008, Peter Zijlstra wrote: > > On Tue, 2008-04-22 at 20:50 +0200, Jens Axboe wrote: > > > > > +int smp_call_function_single(int cpu, void (*func) (void *info), void *info, > > > + int retry, int wait) > > > +{ > > > + unsigned long flags; > > > + /* prevent preemption and reschedule on another processor */ > > > + int me = get_cpu(); > > > + > > > + /* Can deadlock when called with interrupts disabled */ > > > + WARN_ON(wait && irqs_disabled()); > > > > With this fallback to wait the above condition isn't sufficient. > > What deadlock are you concerned with here? Would making cfd_fallback > per-cpu make you feel better?
CPU0 CPU1
local_irq_disable() local_irq_disable()
smp_call_function_single(0,..,0) test_and_set_bit_lock() send IPI smp_call_function_single(1,..,0) while(test_and_set_bit_lock()) cpu_relax();
This will spin forever, because it needs to handle the IPI in order to free the cfd_fallback thingy, but can't for its waiting for it.
That particular deadlock can indeed be solved by making cfd_fallback per-cpu.
But if you were to use multiple smp_call_function*() calls under a single IRQ disabled, then that would not be sufficient. Now I can't directly come up with a good reason to need to do that, but still.
You'd need somethine like:
local_irq_disable()
smp_call_function_single(n, func_a,..,0) smp_call_function_single(m, func_b,..,0)
local_irq_enable()
And invite 3 cpus to the party while under memory pressure and you get deadlock potential.
[ if it were both the same function, you'd want to use smp_call_function() and provide a mask; if it were the same cpu you'd want to call a function doing both ]
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