Re: [PATCH tip/core/rcu 6/7] rcu: Make expedited grace periods recheck dyntick idle state

[Peter Zijlstra]

On Mon, Nov 14, 2016 at 09:25:12AM -0800, Josh Triplett wrote:
> On Mon, Nov 14, 2016 at 08:57:12AM -0800, Paul E. McKenney wrote:
> > Expedited grace periods check dyntick-idle state, and avoid sending
> > IPIs to idle CPUs, including those running guest OSes, and, on NOHZ_FULL
> > kernels, nohz_full CPUs.  However, the kernel has been observed checking
> > a CPU while it was non-idle, but sending the IPI after it has gone
> > idle.  This commit therefore rechecks idle state immediately before
> > sending the IPI, refraining from IPIing CPUs that have since gone idle.
> > 
> > Reported-by: Rik van Riel <riel@xxxxxxxxxx>
> > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> 
> atomic_add_return(0, ...) seems odd.  Do you actually want that, rather
> than atomic_read(...)?  If so, can you please document exactly why?

Yes that is weird. The only effective difference is that it would do a
load-exclusive instead of a regular load.