linux-kernel - Re: question on read_barrier

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Message-ID: <Pine.LNX.4.44L0.1404211002080.1201-100000@iolanthe.rowland.org>
Date:	Mon, 21 Apr 2014 10:03:16 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Oliver Neukum <oliver@...kum.org>
cc:	linux-kernel@...r.kernel.org
Subject: Re: question on read_barrier_depends

On Mon, 21 Apr 2014, Oliver Neukum 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.
> 
> On the third hand, I now think wmb() is sufficient, because
> there's also a write to __val. It does depend on the read
> of buf[out & mask], but if no CPU does speculative writes
> it must be correct.

You are right; I missed that.  Good point.

Alan Stern

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