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rcu: Ensure that RCU_FAST_NO_HZ timers expire on correct CPU 

Timers are subject to migration, which can lead to the following
system-hang scenario when CONFIG_RCU_FAST_NO_HZ=y:

1.	CPU 0 executes synchronize_rcu(), which posts an RCU callback.

2.	CPU 0 then goes idle.  It cannot immediately invoke the callback,
	but there is nothing RCU needs from ti, so it enters dyntick-idle
	mode after posting a timer.

3.	The timer gets migrated to CPU 1.

4.	CPU 0 never wakes up, so the synchronize_rcu() never returns, so
	the system hangs.

This commit fixes this problem by using mod_timer_pinned(), as suggested
by Peter Zijlstra, to ensure that the timer is actually posted on the
running CPU.

Reported-by: Dipankar Sarma <dipankar@in.ibm.com>
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 2012-03-15 12:16:26 -07:00 committed by Paul E. McKenney
parent 79b9a75fb7
commit f511fc6246
1 changed files with 8 additions and 5 deletions
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13
kernel/rcutree_plugin.h
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@ -2110,6 +2110,8 @@ static void rcu_cleanup_after_idle(int cpu)
*/
static void rcu_prepare_for_idle(int cpu)
{
struct timer_list *tp;
/*
* If this is an idle re-entry, for example, due to use of
* RCU_NONIDLE() or the new idle-loop tracing API within the idle
@ -2121,9 +2123,10 @@ static void rcu_prepare_for_idle(int cpu)
if (!per_cpu(rcu_idle_first_pass, cpu) &&
(per_cpu(rcu_nonlazy_posted, cpu) ==
per_cpu(rcu_nonlazy_posted_snap, cpu))) {
if (rcu_cpu_has_callbacks(cpu))
mod_timer(&per_cpu(rcu_idle_gp_timer, cpu),
per_cpu(rcu_idle_gp_timer_expires, cpu));
if (rcu_cpu_has_callbacks(cpu)) {
tp = &per_cpu(rcu_idle_gp_timer, cpu);
mod_timer_pinned(tp, per_cpu(rcu_idle_gp_timer_expires, cpu));
}
return;
}
per_cpu(rcu_idle_first_pass, cpu) = 0;
@ -2167,8 +2170,8 @@ static void rcu_prepare_for_idle(int cpu)
else
per_cpu(rcu_idle_gp_timer_expires, cpu) =
jiffies + RCU_IDLE_LAZY_GP_DELAY;
mod_timer(&per_cpu(rcu_idle_gp_timer, cpu),
per_cpu(rcu_idle_gp_timer_expires, cpu));
tp = &per_cpu(rcu_idle_gp_timer, cpu);
mod_timer_pinned(tp, per_cpu(rcu_idle_gp_timer_expires, cpu));
per_cpu(rcu_nonlazy_posted_snap, cpu) =
per_cpu(rcu_nonlazy_posted, cpu);
return; /* Nothing more to do immediately. */