On Wed, 2008-04-23 at 09:49 +0200, Jens Axboe wrote: > On Wed, Apr 23 2008, Peter Zijlstra wrote: > > 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. > > Right, that is the case I was thinking of. I added per-cpu fallbacks to > cover that case. Great, thanks! > > 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 ] > > I think that is plenty far off into theoretical country that we can get > by with just documenting this limitation. Nobody is doing that currently > in the kernel, and I see no practical use case for it. Agreed. -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html