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Subject: Re: [PATCHv11 2.6.36-rc2-tip 3/15]  3: uprobes: Slot allocation
 for Execution out of line(XOL)
From: Peter Zijlstra <peterz@infradead.org>
To: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
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Date: Fri, 03 Sep 2010 18:51:54 +0200
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On Fri, 2010-09-03 at 22:10 +0530, Srikar Dronamraju wrote:
> > > +   mb();
> > 
> > Where is the matching barrier?
> 
> I dont want the compiler to reorder the instructions and do the
> assignment for user_bkpt to be done before we complete the copy above.
> 
> If the assignment happens before we copy the content into the slot,
> someother thread that might hit the same probe actually things the slot
> is ready and tries to jump to that slot even before the slot is
> initialized.
> 
> Please let me know if I could have done it differently. 


If you want a compiler barrier, use barrier(), but here you seem to
describe a multi-threaded situation, in which case the observer thread
needs at least a rmb() in order for that mb() to mean anything other
than the compiler barrier it implies.

Also, use smp_* barriers.