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rcu: Eliminate RCU_FAST_NO_HZ grace-period hang

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With the new implementation of RCU_FAST_NO_HZ, it was possible to hang
RCU grace periods as follows:

o	CPU 0 attempts to go idle, cycles several times through the
	rcu_prepare_for_idle() loop, then goes dyntick-idle when
	RCU needs nothing more from it, while still having at least
	on RCU callback pending.

o	CPU 1 goes idle with no callbacks.

Both CPUs can then stay in dyntick-idle mode indefinitely, preventing
the RCU grace period from ever completing, possibly hanging the system.

This commit therefore prevents CPUs that have RCU callbacks from entering
dyntick-idle mode.  This approach also eliminates the need for the
end-of-grace-period IPIs used previously.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit is contained in:
Paul E. McKenney
2011-11-22 20:43:02 -08:00
committed by Paul E. McKenney
parent 84ad00cb61
commit f535a607c1
4 changed files with 2 additions and 82 deletions
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2
kernel/rcutree.c
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@@ -1086,7 +1086,6 @@ static void rcu_report_qs_rsp(struct rcu_state *rsp, unsigned long flags)
* callbacks are waiting on the grace period that just now
* completed.
*/
rcu_schedule_wake_gp_end();
if (*rdp->nxttail[RCU_WAIT_TAIL] == NULL) {
raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
@@ -1672,7 +1671,6 @@ static void rcu_process_callbacks(struct softirq_action *unused)
&__get_cpu_var(rcu_sched_data));
__rcu_process_callbacks(&rcu_bh_state, &__get_cpu_var(rcu_bh_data));
rcu_preempt_process_callbacks();
rcu_wake_cpus_for_gp_end();
trace_rcu_utilization("End RCU core");
}