linux-kernel - Re: question on read_barrier

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Date:	Fri, 18 Apr 2014 14:16:40 -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 Fri, 18 Apr 2014, Oliver Neukum wrote:

> 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()

I don't think so.

kfifo_put() uses the read of kfifo->out to determine whether to perform
the store; i.e., it won't store the data if the kfifo is already full.  
No architectures do speculative writes, so the data can never get
stored before kfifo->out is read, even without any memory barriers.  

smp_read_barrier_depends() is meant to prevent speculative _reads_, or
any similar mechanism.

Alan Stern

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