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Date: Tue, 15 Nov 2016 09:16:55 +0100 From: Peter Zijlstra <peterz@...radead.org> To: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> Cc: Josh Triplett <josh@...htriplett.org>, 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 10:12:37AM -0800, Paul E. McKenney wrote: > On Mon, Nov 14, 2016 at 06:37:33PM +0100, Peter Zijlstra wrote: > > 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. > > It is weird, and checking to see if it is safe to convert it and its > friends to something with less overhead is on my list. This starts > with a patch series I will post soon that consolidates all these > atomic_add_return() calls into a single function, which will ease testing > and other verification. > > All that aside, please keep in mind that much is required from this load. > It is part of a network of ordered operations that guarantee that any > operation from any CPU preceding a given grace period is seen to precede > any other operation from any CPU following that same grace period. > And each and every CPU must agree on the order of those two operations, > otherwise, RCU is broken. OK, so something similar to: smp_mb(); atomic_read(); then? That would order, with global transitivity, against prior operations. > In addition, please note also that these operations are nowhere near > any fastpaths. My concern is mostly that it reads very weird. I appreciate this not being fast path code, but confusing code is bad in any form.
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