Messages in this thread Patch in this message | | | Date | Tue, 7 Jan 2014 14:27:42 -0800 | From | Andrew Morton <> | Subject | Re: [PATCH] lib/percpu_counter.c: disable local irq when updating percpu couter |
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On Tue, 7 Jan 2014 18:29:27 +0800 Ming Lei <tom.leiming@gmail.com> wrote:
> __percpu_counter_add() may be called in softirq/hardirq handler > (such as, blk_mq_queue_exit() is typically called in hardirq/softirq > handler), so we need to disable local irq when updating the percpu > counter, otherwise counts may be lost.
OK.
> The patch fixes problem that 'rmmod null_blk' may hang in blk_cleanup_queue() > because of miscounting of request_queue->mq_usage_counter. > > ... > > --- a/lib/percpu_counter.c > +++ b/lib/percpu_counter.c > @@ -75,19 +75,19 @@ EXPORT_SYMBOL(percpu_counter_set); > void __percpu_counter_add(struct percpu_counter *fbc, s64 amount, s32 batch) > { > s64 count; > + unsigned long flags; > > - preempt_disable(); > + raw_local_irq_save(flags); > count = __this_cpu_read(*fbc->counters) + amount; > if (count >= batch || count <= -batch) { > - unsigned long flags; > - raw_spin_lock_irqsave(&fbc->lock, flags); > + raw_spin_lock(&fbc->lock); > fbc->count += count; > - raw_spin_unlock_irqrestore(&fbc->lock, flags); > + raw_spin_unlock(&fbc->lock); > __this_cpu_write(*fbc->counters, 0); > } else { > __this_cpu_write(*fbc->counters, count); > } > - preempt_enable(); > + raw_local_irq_restore(flags); > } > EXPORT_SYMBOL(__percpu_counter_add);
Can this be made more efficient?
The this_cpu_foo() documentation is fairly dreadful, but way down at the end of Documentation/this_cpu_ops.txt we find "this_cpu ops are interrupt safe". So I think this is a more efficient fix:
--- a/lib/percpu_counter.c~a +++ a/lib/percpu_counter.c @@ -82,10 +82,10 @@ void __percpu_counter_add(struct percpu_ unsigned long flags; raw_spin_lock_irqsave(&fbc->lock, flags); fbc->count += count; + __this_cpu_sub(*fbc->counters, count); raw_spin_unlock_irqrestore(&fbc->lock, flags); - __this_cpu_write(*fbc->counters, 0); } else { - __this_cpu_write(*fbc->counters, count); + this_cpu_add(*fbc->counters, amount); } preempt_enable(); } It avoids the local_irq_disable() in the common case, when the CPU supports efficient this_cpu_add(). It will in rare race situations permit the cpu-local counter to exceed `batch', but that should be harmless.
What do you think?
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