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Subject: frlock and barrier discussion
From: Stephen Hemminger <shemminger@osdl.org>
To: Richard Henderson <rth@twiddle.net>
Cc: Andrea Arcangeli <andrea@suse.de>, Andrew Morton <akpm@digeo.com>,
       Andi Kleen <ak@suse.de>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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	 <20030128230639.A17385@twiddle.net>
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On Tue, 2003-01-28 at 23:06, Richard Henderson wrote:
> On Tue, Jan 28, 2003 at 03:42:21PM -0800, Stephen Hemminger wrote:
> > +static inline void fr_write_begin(frlock_t *rw)
> > +{
> > +	preempt_disable();
> > +	rw->pre_sequence++;
> > +	wmb();
> > +}
> > +
> > +static inline void fr_write_end(frlock_t *rw)
> > +{
> > +	wmb();
> > +	rw->post_sequence++;
> 
> These need to be mb(), not wmb(), if you want the bits in between
> to actually happen in between, as with your xtime example.  At
> present there's nothing stoping xtime from being *read* before
> your read from pre_sequence happens.


First, write_begin/end can only be safely used when there is separate
writer synchronization such as a spin_lock or semaphore.  
As far as I know, semaphore or spin_lock guarantees a barrier.
So xtime or anything else can not be read before the spin_lock.

Using mb() is more paranoid than necessary. 




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