Re: [PATCH tip/core/rcu 1/6] rcu: Abstract the dynticks momentary-idle operation

[Josh Triplett <josh@joshtriplett.org>]

On Mon, Jan 16, 2017 at 03:22:39AM -0800, Paul E. McKenney wrote:
> On Sun, Jan 15, 2017 at 11:39:51PM -0800, Josh Triplett wrote:
> > On Sat, Jan 14, 2017 at 12:54:40AM -0800, Paul E. McKenney wrote:
> > > This commit is the first step towards full abstraction of all accesses to
> > > the ->dynticks counter, implementing the previously open-coded atomic add
> > > of two in a new rcu_dynticks_momentary_idle() function.  This abstraction
> > > will ease changes to the ->dynticks counter operation.
> > > 
> > > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > 
> > This change has an additional effect not documented in the commit
> > message: it eliminates the smp_mb__before_atomic and
> > smp_mb__after_atomic calls.  Can you please document that in the commit
> > message, and explain why that doesn't cause a problem?
> 
> The trick is that the old code used the non-value-returning atomic_add(),
> which does not imply ordering, hence the smp_mb__before_atomic() and
> smp_mb__after_atomic() calls.  The new code uses atomic_add_return(),
> which does return a value, and therefore implies full ordering in and
> of itself.
> 
> How would you like me to proceed?

With the above explanation added to the commit message:

Reviewed-by: Josh Triplett <josh@joshtriplett.org>