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    SubjectRe: [PATCH 8/9] jrcu: fix broken rcu_barrier
    On Wed, Mar 23, 2011 at 09:46:28AM -0400, Joe Korty wrote:
    > On Wed, Mar 23, 2011 at 02:35:24AM -0400, Paul E. McKenney wrote:
    > > On Mon, Mar 14, 2011 at 06:09:28PM -0400, Joe Korty wrote:
    > > > On Mon, Mar 14, 2011 at 04:47:18AM -0400, Paul E. McKenney wrote:
    > > > > On Sun, Mar 13, 2011 at 11:50:44PM -0400, Joe Korty wrote:
    > > > > > +void rcu_barrier(void)
    > > > > > +{
    > > > > > + synchronize_sched();
    > > > > > + synchronize_sched();
    > > > > > +}
    > > > >
    > > > > This is still broken. You need to wait until all pre-existing RCU
    > > > > callbacks on all CPUs have been invoked. Although this does guarantee
    > > > > that all callbacks on the current CPU have been invoked, if one of the
    > > > > other CPUs is busy, it might not yet have processed the callbacks from
    > > > > earlier grace periods. The usual way to take care of this is to post
    > > > > a callback on each CPU, then wait for them all to be invoked.
    > > >
    > > > Hi Paul,
    > > > JRCU (eventually) chains together all callbacks from all
    > > > cpus into a single queue and then it expires that single
    > > > queue. So the double synchronize_sched() should work as
    > > > a rcu_barrier implementation.
    > >
    > > Hello, Joe,
    > >
    > > So the following grace period is not permitted to start until all of
    > > the callbacks from the prior grace period have been invoked?
    >
    > Yes, it is all synchronized. Every cpu's current batch is
    > closed off at the same time. We tolerate some fuzz in the
    > timing, but that is not the same as being unsynchronized.
    >
    > This happens as an automatic side effect of doing
    > everything (except for call_rcu) from a single CPU.

    Got it -- the rcu_delimit_batches() doesn't return until all callbacks
    have been invoked, and a new grace period cannot start until this function
    returns. I am not yet sure whether this is a good thing or a bad thing.
    Given your 50-millisecond grace periods, it is less of a bad thing than
    it would be for shorter grace periods, I suppose. ;-)

    Thanx, Paul


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