From: Paul E. McKenney Date: Thu, 15 Mar 2012 19:16:26 +0000 (-0700) Subject: rcu: Ensure that RCU_FAST_NO_HZ timers expire on correct CPU X-Git-Url: https://openfabrics.org/gitweb/?a=commitdiff_plain;h=f511fc624642f0bb8cf65aaa28979737514d4746;p=~shefty%2Frdma-dev.git 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 Signed-off-by: Paul E. McKenney Signed-off-by: Paul E. McKenney --- diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h index ad61da79b31..d01e26df55a 100644 --- a/kernel/rcutree_plugin.h +++ b/kernel/rcutree_plugin.h @@ -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. */