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authorPaul E. McKenney <paulmck@linux.vnet.ibm.com>2014-08-14 23:01:53 (GMT)
committerPaul E. McKenney <paulmck@linux.vnet.ibm.com>2014-09-07 23:27:34 (GMT)
commit1d082fd061884a587c490c4fc8a2056ce1e47624 (patch)
treea8f715a674a036b1b7500a16ea11381d40659e42 /kernel
parent4ff475ed4cf61a7f56bbfbc424147189d0022b38 (diff)
downloadlinux-1d082fd061884a587c490c4fc8a2056ce1e47624.tar.xz
rcu: Remove local_irq_disable() in rcu_preempt_note_context_switch()
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@infradead.org> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Diffstat (limited to 'kernel')
-rw-r--r--kernel/rcu/tree_plugin.h32
-rw-r--r--kernel/rcu/update.c2
2 files changed, 16 insertions, 18 deletions
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index e466b40..0981c0c 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -155,9 +155,8 @@ 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)
{
@@ -166,7 +165,7 @@ 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;
}
/*
@@ -190,14 +189,14 @@ 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;
/*
@@ -239,7 +238,7 @@ 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
@@ -257,9 +256,7 @@ 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);
}
/*
@@ -340,7 +337,7 @@ void rcu_read_unlock_special(struct task_struct *t)
bool drop_boost_mutex = false;
#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())
@@ -350,12 +347,13 @@ 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;
}
@@ -368,8 +366,8 @@ 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 9487b48..6fb9115 100644
--- a/kernel/rcu/update.c
+++ b/kernel/rcu/update.c
@@ -93,7 +93,7 @@ void __rcu_read_unlock(void)
barrier(); /* critical section before exit code. */
t->rcu_read_lock_nesting = INT_MIN;
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;