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Date: Fri, 15 Aug 2014 09:28:44 -0700
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: linux-kernel@...r.kernel.org
Cc: mingo@...nel.org, laijs@...fujitsu.com, dipankar@...ibm.com,
akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
dvhart@...ux.intel.com, fweisbec@...il.com, oleg@...hat.com,
bobby.prani@...il.com,
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH v6 tip/core/rcu 16/17] rcu: Remove local_irq_disable() in rcu_preempt_note_context_switch()
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
The rcu_preempt_note_context_switch() function is on a scheduling fast
path, so it would be good to avoid disabling irqs. The reason that irqs
are disabled is to synchronize process-level and irq-handler access to
the task_struct ->rcu_read_unlock_special bitmask. This commit therefore
makes ->rcu_read_unlock_special instead be a union of bools with a short
allowing single-access checks in RCU's __rcu_read_unlock(). This results
in the process-level and irq-handler accesses being simple loads and
stores, so that irqs need no longer be disabled. This commit therefore
removes the irq disabling from rcu_preempt_note_context_switch().
Reported-by: Peter Zijlstra <peterz@...radead.org>
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---
include/linux/init_task.h | 2 +-
include/linux/sched.h | 16 +++++++++-------
kernel/rcu/tree_plugin.h | 32 +++++++++++++++-----------------
kernel/rcu/update.c | 2 +-
4 files changed, 26 insertions(+), 26 deletions(-)
diff --git a/include/linux/init_task.h b/include/linux/init_task.h
index 642828009324..44ebf358dc6b 100644
--- a/include/linux/init_task.h
+++ b/include/linux/init_task.h
@@ -117,7 +117,7 @@ extern struct group_info init_groups;
#ifdef CONFIG_PREEMPT_RCU
#define INIT_TASK_RCU_PREEMPT(tsk) \
.rcu_read_lock_nesting = 0, \
- .rcu_read_unlock_special = 0, \
+ .rcu_read_unlock_special.s = 0, \
.rcu_node_entry = LIST_HEAD_INIT(tsk.rcu_node_entry), \
INIT_TASK_RCU_TREE_PREEMPT() \
INIT_TASK_RCU_BOOST()
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 389af9de6781..344f38d627ed 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1212,6 +1212,13 @@ struct sched_dl_entity {
struct hrtimer dl_timer;
};
+union rcu_special {
+ struct {
+ bool blocked;
+ bool need_qs;
+ } b;
+ short s;
+};
struct rcu_node;
enum perf_event_task_context {
@@ -1264,7 +1271,7 @@ struct task_struct {
#ifdef CONFIG_PREEMPT_RCU
int rcu_read_lock_nesting;
- char rcu_read_unlock_special;
+ union rcu_special rcu_read_unlock_special;
struct list_head rcu_node_entry;
#endif /* #ifdef CONFIG_PREEMPT_RCU */
#ifdef CONFIG_TREE_PREEMPT_RCU
@@ -2003,16 +2010,11 @@ extern void task_clear_jobctl_trapping(struct task_struct *task);
extern void task_clear_jobctl_pending(struct task_struct *task,
unsigned int mask);
-#ifdef CONFIG_PREEMPT_RCU
-#define RCU_READ_UNLOCK_BLOCKED (1 << 0) /* blocked while in RCU read-side. */
-#define RCU_READ_UNLOCK_NEED_QS (1 << 1) /* RCU core needs CPU response. */
-#endif /* #ifdef CONFIG_PREEMPT_RCU */
-
static inline void rcu_copy_process(struct task_struct *p)
{
#ifdef CONFIG_PREEMPT_RCU
p->rcu_read_lock_nesting = 0;
- p->rcu_read_unlock_special = 0;
+ p->rcu_read_unlock_special.s = 0;
p->rcu_blocked_node = NULL;
#ifdef CONFIG_RCU_BOOST
p->rcu_boost_mutex = NULL;
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 0d8ef5cb1976..2a6509581f40 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -154,9 +154,8 @@ EXPORT_SYMBOL_GPL(rcu_batches_completed);
* not in a quiescent state. There might be any number of tasks blocked
* while in an RCU read-side critical section.
*
- * Unlike the other rcu_*_qs() functions, callers to this function
- * must disable irqs in order to protect the assignment to
- * ->rcu_read_unlock_special.
+ * As with the other rcu_*_qs() functions, callers to this function
+ * must disable preemption.
*/
static void rcu_preempt_qs(int cpu)
{
@@ -165,7 +164,7 @@ static void rcu_preempt_qs(int cpu)
if (rdp->passed_quiesce == 0)
trace_rcu_grace_period(TPS("rcu_preempt"), rdp->gpnum, TPS("cpuqs"));
rdp->passed_quiesce = 1;
- current->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_NEED_QS;
+ current->rcu_read_unlock_special.b.need_qs = false;
}
/*
@@ -189,14 +188,14 @@ static void rcu_preempt_note_context_switch(int cpu)
struct rcu_node *rnp;
if (t->rcu_read_lock_nesting > 0 &&
- (t->rcu_read_unlock_special & RCU_READ_UNLOCK_BLOCKED) == 0) {
+ !t->rcu_read_unlock_special.b.blocked) {
/* Possibly blocking in an RCU read-side critical section. */
rdp = per_cpu_ptr(rcu_preempt_state.rda, cpu);
rnp = rdp->mynode;
raw_spin_lock_irqsave(&rnp->lock, flags);
smp_mb__after_unlock_lock();
- t->rcu_read_unlock_special |= RCU_READ_UNLOCK_BLOCKED;
+ t->rcu_read_unlock_special.b.blocked = true;
t->rcu_blocked_node = rnp;
/*
@@ -238,7 +237,7 @@ static void rcu_preempt_note_context_switch(int cpu)
: rnp->gpnum + 1);
raw_spin_unlock_irqrestore(&rnp->lock, flags);
} else if (t->rcu_read_lock_nesting < 0 &&
- t->rcu_read_unlock_special) {
+ t->rcu_read_unlock_special.s) {
/*
* Complete exit from RCU read-side critical section on
@@ -256,9 +255,7 @@ static void rcu_preempt_note_context_switch(int cpu)
* grace period, then the fact that the task has been enqueued
* means that we continue to block the current grace period.
*/
- local_irq_save(flags);
rcu_preempt_qs(cpu);
- local_irq_restore(flags);
}
/*
@@ -339,7 +336,7 @@ void rcu_read_unlock_special(struct task_struct *t)
struct rt_mutex *rbmp = NULL;
#endif /* #ifdef CONFIG_RCU_BOOST */
struct rcu_node *rnp;
- int special;
+ union rcu_special special;
/* NMI handlers cannot block and cannot safely manipulate state. */
if (in_nmi())
@@ -349,12 +346,13 @@ void rcu_read_unlock_special(struct task_struct *t)
/*
* If RCU core is waiting for this CPU to exit critical section,
- * let it know that we have done so.
+ * let it know that we have done so. Because irqs are disabled,
+ * t->rcu_read_unlock_special cannot change.
*/
special = t->rcu_read_unlock_special;
- if (special & RCU_READ_UNLOCK_NEED_QS) {
+ if (special.b.need_qs) {
rcu_preempt_qs(smp_processor_id());
- if (!t->rcu_read_unlock_special) {
+ if (!t->rcu_read_unlock_special.s) {
local_irq_restore(flags);
return;
}
@@ -367,8 +365,8 @@ void rcu_read_unlock_special(struct task_struct *t)
}
/* Clean up if blocked during RCU read-side critical section. */
- if (special & RCU_READ_UNLOCK_BLOCKED) {
- t->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_BLOCKED;
+ if (special.b.blocked) {
+ t->rcu_read_unlock_special.b.blocked = false;
/*
* Remove this task from the list it blocked on. The
@@ -658,7 +656,7 @@ static void rcu_preempt_check_callbacks(int cpu)
}
if (t->rcu_read_lock_nesting > 0 &&
per_cpu(rcu_preempt_data, cpu).qs_pending)
- t->rcu_read_unlock_special |= RCU_READ_UNLOCK_NEED_QS;
+ t->rcu_read_unlock_special.b.need_qs = true;
}
#ifdef CONFIG_RCU_BOOST
@@ -941,7 +939,7 @@ void exit_rcu(void)
return;
t->rcu_read_lock_nesting = 1;
barrier();
- t->rcu_read_unlock_special = RCU_READ_UNLOCK_BLOCKED;
+ t->rcu_read_unlock_special.b.blocked = true;
__rcu_read_unlock();
}
diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index ede5dbe77bcf..06ae98b6ad03 100644
--- a/kernel/rcu/update.c
+++ b/kernel/rcu/update.c
@@ -96,7 +96,7 @@ void __rcu_read_unlock(void)
udelay(10); /* Make preemption more probable. */
#endif /* #ifdef CONFIG_PROVE_RCU_DELAY */
barrier(); /* assign before ->rcu_read_unlock_special load */
- if (unlikely(ACCESS_ONCE(t->rcu_read_unlock_special)))
+ if (unlikely(ACCESS_ONCE(t->rcu_read_unlock_special.s)))
rcu_read_unlock_special(t);
barrier(); /* ->rcu_read_unlock_special load before assign */
t->rcu_read_lock_nesting = 0;
--
1.8.1.5
--
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