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

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Message-ID: <20161114173733.GJ3142@twins.programming.kicks-ass.net>
Date:   Mon, 14 Nov 2016 18:37:33 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Triplett <josh@...htriplett.org>
Cc:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        tglx@...utronix.de, rostedt@...dmis.org, dhowells@...hat.com,
        edumazet@...gle.com, dvhart@...ux.intel.com, fweisbec@...il.com,
        oleg@...hat.com, bobby.prani@...il.com
Subject: Re: [PATCH tip/core/rcu 6/7] rcu: Make expedited grace periods
 recheck dyntick idle state

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@...hat.com>
> > Signed-off-by: Paul E. McKenney <paulmck@...ux.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.