linux-kernel - question on read_barrier

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Message-ID: <1397634429.21891.13.camel@linux-fkkt.site>
Date:	Wed, 16 Apr 2014 09:47:09 +0200
From:	Oliver Neukum <oneukum@...e.de>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	linux-kernel@...r.kernel.org
Subject: question on read_barrier_depends

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?

	Regards
		Oliver


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