From 5b72f9643b52a5148bb8ced126e20563adfa3466 Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Wed, 12 Apr 2017 15:29:14 -0700 Subject: 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_plugin.h | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-) (limited to 'kernel/rcu/tree_plugin.h') 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) { } -- cgit