commit 92aa39e9dc77 upstream.

The per-CPU rcu_dynticks.rcu_urgent_qs variable communicates an urgent need for an RCU quiescent state from the force-quiescent-state processing within the grace-period kthread to context switches and to cond_resched(). Unfortunately, such urgent needs are not communicated to need_resched(), which is sometimes used to decide when to invoke cond_resched(), for but one example, within the KVM vcpu_run() function. As of v4.15, this can result in synchronize_sched() being delayed by up to ten seconds, which can be problematic, to say nothing of annoying.

This commit therefore checks rcu_dynticks.rcu_urgent_qs from within rcu_check_callbacks(), which is invoked from the scheduling-clock interrupt handler. If the current task is not an idle task and is not executing in usermode, a context switch is forced, and either way, the rcu_dynticks.rcu_urgent_qs variable is set to false. If the current task is an idle task, then RCU's dyntick-idle code will detect the quiescent state, so no further action is required. Similarly, if the task is executing in usermode, other code in rcu_check_callbacks() and its called functions will report the corresponding quiescent state.

Reported-by: Marius Hillenbrand mhillenb@amazon.de Reported-by: David Woodhouse dwmw2@infradead.org Suggested-by: Peter Zijlstra peterz@infradead.org Signed-off-by: Paul E. McKenney paulmck@linux.vnet.ibm.com [ paulmck: Backported to make patch apply cleanly on older versions. ] Tested-by: Marius Hillenbrand mhillenb@amazon.de Cc: stable@vger.kernel.org # 4.12.x - 4.19.x

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 0b760c1369f7..15301ed19da6 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -2662,6 +2662,15 @@ void rcu_check_callbacks(int user) rcu_bh_qs(); } rcu_preempt_check_callbacks(); + /* The load-acquire pairs with the store-release setting to true. */ + if (smp_load_acquire(this_cpu_ptr(&rcu_dynticks.rcu_urgent_qs))) { + /* Idle and userspace execution already are quiescent states. */ + if (!rcu_is_cpu_rrupt_from_idle() && !user) { + set_tsk_need_resched(current); + set_preempt_need_resched(); + } + __this_cpu_write(rcu_dynticks.rcu_urgent_qs, false); + } if (rcu_pending()) invoke_rcu_core();