rcu: Ensure that RCU_FAST_NO_HZ timers expire on correct CPU (f511fc62) · Commits · 戴 / test

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>

kernel/rcutree_plugin.h

+8 −5

Original line number	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)
		{
		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. */

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