Date: Mon, 14 Nov 2016 18:37:33 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Josh Triplett <josh@joshtriplett.org>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        linux-kernel@vger.kernel.org, mingo@kernel.org, jiangshanlai@gmail.com,
        dipankar@in.ibm.com, akpm@linux-foundation.org,
        mathieu.desnoyers@efficios.com, tglx@linutronix.de,
        rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com,
        dvhart@linux.intel.com, fweisbec@gmail.com, oleg@redhat.com,
        bobby.prani@gmail.com
Subject: Re: [PATCH tip/core/rcu 6/7] rcu: Make expedited grace periods
 recheck dyntick idle state
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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@redhat.com>
> > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> 
> 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.