Re: [PATCH] lib/percpu_counter.c: disable local irq when updating percpu couter

[Andrew Morton <akpm@linux-foundation.org>]

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?