linux-kernel - Re: question on read_barrier

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Date:	Fri, 18 Apr 2014 19:17:43 +0200
From:	Oliver Neukum <oneukum@...e.de>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: question on read_barrier_depends


On Thu, 2014-04-17 at 11:50 -0400, Alan Stern wrote:
> 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().

I had overlooked that. In that case kfifo_put() also needs
smp_read_barrier_depends()

	Regards
		Oliver




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