From 5b72f9643b52a5148bb8ced126e20563adfa3466 Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Wed, 12 Apr 2017 15:29:14 -0700 Subject: [PATCH] rcu: Complain if blocking in preemptible RCU read-side critical section Although preemptible RCU allows its read-side critical sections to be preempted, general blocking is forbidden. The reason for this is that excessive preemption times can be handled by CONFIG_RCU_BOOST=y, but a voluntarily blocked task doesn't care how high you boost its priority. Because preemptible RCU is a global mechanism, one ill-behaved reader hurts everyone. Hence the prohibition against general blocking in RCU-preempt read-side critical sections. Preemption yes, blocking no. This commit enforces this prohibition. There is a special exception for the -rt patchset (which they kindly volunteered to implement): It is OK to block (as opposed to merely being preempted) within an RCU-preempt read-side critical section, but only if the blocking is subject to priority inheritance. This exception permits CONFIG_RCU_BOOST=y to get -rt RCU readers out of trouble. Why doesn't this exception also apply to mainline's rt_mutex? Because of the possibility that someone does general blocking while holding an rt_mutex. Yes, the priority boosting will affect the rt_mutex, but it won't help with the task doing general blocking while holding that rt_mutex. Reported-by: Thomas Gleixner Signed-off-by: Paul E. McKenney --- kernel/rcu/tree.c | 2 +- kernel/rcu/tree.h | 2 +- kernel/rcu/tree_plugin.h | 5 +++-- 3 files changed, 5 insertions(+), 4 deletions(-) diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 657056c3e0cd..9ce682242e99 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -478,7 +478,7 @@ void rcu_note_context_switch(bool preempt) barrier(); /* Avoid RCU read-side critical sections leaking down. */ trace_rcu_utilization(TPS("Start context switch")); rcu_sched_qs(); - rcu_preempt_note_context_switch(); + rcu_preempt_note_context_switch(preempt); /* Load rcu_urgent_qs before other flags. */ if (!smp_load_acquire(this_cpu_ptr(&rcu_dynticks.rcu_urgent_qs))) goto out; diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h index ba38262c3554..0fa7aee9ef55 100644 --- a/kernel/rcu/tree.h +++ b/kernel/rcu/tree.h @@ -477,7 +477,7 @@ DECLARE_PER_CPU(char, rcu_cpu_has_work); /* Forward declarations for rcutree_plugin.h */ static void rcu_bootup_announce(void); -static void rcu_preempt_note_context_switch(void); +static void rcu_preempt_note_context_switch(bool preempt); static int rcu_preempt_blocked_readers_cgp(struct rcu_node *rnp); #ifdef CONFIG_HOTPLUG_CPU static bool rcu_preempt_has_tasks(struct rcu_node *rnp); diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h index c9a48657512a..a421753e8e9c 100644 --- a/kernel/rcu/tree_plugin.h +++ b/kernel/rcu/tree_plugin.h @@ -286,12 +286,13 @@ static void rcu_preempt_qs(void) * * Caller must disable interrupts. */ -static void rcu_preempt_note_context_switch(void) +static void rcu_preempt_note_context_switch(bool preempt) { struct task_struct *t = current; struct rcu_data *rdp; struct rcu_node *rnp; + WARN_ON_ONCE(!preempt && t->rcu_read_lock_nesting > 0); if (t->rcu_read_lock_nesting > 0 && !t->rcu_read_unlock_special.b.blocked) { @@ -738,7 +739,7 @@ static void __init rcu_bootup_announce(void) * Because preemptible RCU does not exist, we never have to check for * CPUs being in quiescent states. */ -static void rcu_preempt_note_context_switch(void) +static void rcu_preempt_note_context_switch(bool preempt) { }