linux-kernel - Re: question on read_barrier

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Message-ID: <Pine.LNX.4.44L0.1404171149001.881-100000@iolanthe.rowland.org>
Date:	Thu, 17 Apr 2014 11:50:47 -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 Thu, 17 Apr 2014, Oliver Neukum wrote:

> On Wed, 2014-04-16 at 11:26 -0400, Alan Stern wrote:
> 
> > 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.
> 
> But where is kfifo->out read at all?

It is read in kfifo_put(), inside the call to kfifo_is_full() -> 
kfifo_len().

Alan Stern

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