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Commit f511fc62 authored by Paul E. McKenney's avatar Paul E. McKenney Committed by Paul E. McKenney
Browse files

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: default avatarDipankar Sarma <dipankar@in.ibm.com>
Signed-off-by: default avatarPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
parent 79b9a75f

kernel/rcutree_plugin.h

+8 −5
Original line number Original line Diff line number Diff line
@@ -2110,6 +2110,8 @@ static void rcu_cleanup_after_idle(int cpu) 
 */

 */

static void rcu_prepare_for_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

	 * 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

	 * 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) &&

	if (!per_cpu(rcu_idle_first_pass, cpu) &&

	    (per_cpu(rcu_nonlazy_posted, cpu) ==

	    (per_cpu(rcu_nonlazy_posted, cpu) ==

	     per_cpu(rcu_nonlazy_posted_snap, cpu))) {

	     per_cpu(rcu_nonlazy_posted_snap, cpu))) {

		if (rcu_cpu_has_callbacks(cpu))

		if (rcu_cpu_has_callbacks(cpu)) {

			mod_timer(&per_cpu(rcu_idle_gp_timer, cpu),

			tp = &per_cpu(rcu_idle_gp_timer, cpu);

				  per_cpu(rcu_idle_gp_timer_expires, cpu));

			mod_timer_pinned(tp, per_cpu(rcu_idle_gp_timer_expires, cpu));

		}

		return;

		return;

	}

	}

	per_cpu(rcu_idle_first_pass, cpu) = 0;

	per_cpu(rcu_idle_first_pass, cpu) = 0;
@@ -2167,8 +2170,8 @@ static void rcu_prepare_for_idle(int cpu) 
		else

		else

			per_cpu(rcu_idle_gp_timer_expires, cpu) =

			per_cpu(rcu_idle_gp_timer_expires, cpu) =

					   jiffies + RCU_IDLE_LAZY_GP_DELAY;

					   jiffies + RCU_IDLE_LAZY_GP_DELAY;

		mod_timer(&per_cpu(rcu_idle_gp_timer, cpu),

		tp = &per_cpu(rcu_idle_gp_timer, cpu);

			  per_cpu(rcu_idle_gp_timer_expires, 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_snap, cpu) =

			per_cpu(rcu_nonlazy_posted, cpu);

			per_cpu(rcu_nonlazy_posted, cpu);

		return; /* Nothing more to do immediately. */

		return; /* Nothing more to do immediately. */