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Commit 5b72f964 authored by Paul E. McKenney's avatar Paul E. McKenney
Browse files

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: default avatarThomas Gleixner <tglx@linutronix.de>
Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
parent 881ec9d2

kernel/rcu/tree.c

+1 −1
Original line number Diff line number Diff line
@@ -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;

kernel/rcu/tree.h

+1 −1
Original line number Diff line number Diff line
@@ -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);

kernel/rcu/tree_plugin.h

+3 −2
Original line number Diff line number Diff line
@@ -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)

{

}