linux-kernel - Re: question on read_barrier

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Date:	Wed, 16 Apr 2014 11:26:53 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Oliver Neukum <oneukum@...e.de>
cc:	linux-kernel@...r.kernel.org
Subject: Re: question on read_barrier_depends

On Wed, 16 Apr 2014, Oliver Neukum wrote:

> Hi,
> 
> I am looking at memory ordering and a question hit me.
> I was looking at the kfifo code. kfifo_put() has a barrier:
> 
> 			)[__kfifo->in & __tmp->kfifo.mask] = \
> 				(typeof(*__tmp->type))__val; \
> 			smp_wmb(); \
> 			__kfifo->in++; \
> 
> Looking at kfifo_get() 
> 
> 		__ret = !kfifo_is_empty(__tmp); \
> 		if (__ret) { \
> 			*(typeof(__tmp->type))__val = \
> 				(__is_kfifo_ptr(__tmp) ? \
> 
> A thought struck me. There is no corresponding barrier. I cannot
> help myself, but I think there needs to be a smp_read_barrier_depends()
> between reading kfifo->in (in kfifo_is empty) and reading val.
> What do you think?

I think you are right.

In addition, the following code in kfifo_get() does this:

			*(typeof(__tmp->type))__val = \
				(__is_kfifo_ptr(__tmp) ? \
				((typeof(__tmp->type))__kfifo->data) : \
				(__tmp->buf) \
				)[__kfifo->out & __tmp->kfifo.mask]; \
			smp_wmb(); \
			__kfifo->out++; \

It looks like the smp_wmb() should really be smp_mb(), because it 
separates the _read_ for val from the _write_ of kfifo->out.

Alan Stern

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