Messages in this thread | | | Date | Mon, 8 Nov 2010 11:50:19 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH] a local-timer-free version of RCU |
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On Mon, Nov 08, 2010 at 10:18:29AM -0500, Joe Korty wrote: > On Mon, Nov 08, 2010 at 10:06:47AM -0500, Frederic Weisbecker wrote: > > On Sat, Nov 06, 2010 at 12:28:12PM -0700, Paul E. McKenney wrote: > >> OK, so your approach treats preempt_disable code sequences as RCU > >> read-side critical sections by relying on the fact that the per-CPU > >> ->krcud task cannot run until such code sequences complete, correct? > >> > >> This seems to require that each CPU's ->krcud task be awakened at > >> least once per grace period, but I might well be missing something. > > > > I understood it differently, but I might also be wrong as well. krcud > > executes the callbacks, but it is only woken up for CPUs that want to > > execute callbacks, not for those that only signal a quiescent state, > > which is only determined in two ways through rcu_poll_other_cpus(): > > > > - if the CPU is in an rcu_read_lock() critical section, it has the > > IN_RCU_READ_LOCK flag. If so then we set up its DO_RCU_COMPLETION flag so > > that it signals its quiescent state on rcu_read_unlock(). > > > > - otherwise it's in a quiescent state. > > > > This works for rcu and rcu bh critical sections. > > But this works in rcu sched critical sections only if rcu_read_lock_sched() has > > been called explicitly, otherwise that doesn't work (in preempt_disable(), > > local_irq_save(), etc...). I think this is what is not complete when > > Joe said it's not yet a complete rcu implementation. > > > > This is also the part that scaries me most :) > > Mostly, I meant that the new RCU API interfaces that have come into > existance since 2004 were only hastily wrapped or NOPed by me to get > things going.
Ah, understood.
> Jim's method only works with explicit rcu_read_lock..unlock sequences, > implicit sequences via preempt_disable..enable and the like are not > handled. I had thought all such sequences were converted to rcu_read_lock > but maybe that is not yet correct.
Not yet, unfortunately. Having them all marked, for lockdep if nothing else, could be a big benefit.
> Jim will have to comment on the full history. He is incommunicado > at the moment; hopefully he will be able to participate sometime in > the next few days.
Sounds good!
Thanx, Paul
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