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-rw-r--r--include/linux/compiler.h17
-rw-r--r--kernel/locking/qspinlock.c3
-rw-r--r--kernel/sched/core.c8
-rw-r--r--kernel/sched/sched.h2
4 files changed, 20 insertions, 10 deletions
diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 4dac1036594f..00b042c49ccd 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -299,6 +299,23 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
__u.__val; \
})
+/**
+ * smp_cond_acquire() - Spin wait for cond with ACQUIRE ordering
+ * @cond: boolean expression to wait for
+ *
+ * Equivalent to using smp_load_acquire() on the condition variable but employs
+ * the control dependency of the wait to reduce the barrier on many platforms.
+ *
+ * The control dependency provides a LOAD->STORE order, the additional RMB
+ * provides LOAD->LOAD order, together they provide LOAD->{LOAD,STORE} order,
+ * aka. ACQUIRE.
+ */
+#define smp_cond_acquire(cond) do { \
+ while (!(cond)) \
+ cpu_relax(); \
+ smp_rmb(); /* ctrl + rmb := acquire */ \
+} while (0)
+
#endif /* __KERNEL__ */
#endif /* __ASSEMBLY__ */
diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
index 986207887def..ed9d96708f93 100644
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -433,8 +433,7 @@ queue:
*
*/
pv_wait_head(lock, node);
- while ((val = smp_load_acquire(&lock->val.counter)) & _Q_LOCKED_PENDING_MASK)
- cpu_relax();
+ smp_cond_acquire(!((val = atomic_read(&lock->val)) & _Q_LOCKED_PENDING_MASK));
/*
* claim the lock:
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 7063c6a07440..9f7862da2cd1 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1968,19 +1968,13 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
/*
* If the owning (remote) cpu is still in the middle of schedule() with
* this task as prev, wait until its done referencing the task.
- */
- while (p->on_cpu)
- cpu_relax();
- /*
- * Combined with the control dependency above, we have an effective
- * smp_load_acquire() without the need for full barriers.
*
* Pairs with the smp_store_release() in finish_lock_switch().
*
* This ensures that tasks getting woken will be fully ordered against
* their previous state and preserve Program Order.
*/
- smp_rmb();
+ smp_cond_acquire(!p->on_cpu);
p->sched_contributes_to_load = !!task_contributes_to_load(p);
p->state = TASK_WAKING;
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index b242775bf670..1e0bb4afe3fd 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1076,7 +1076,7 @@ static inline void finish_lock_switch(struct rq *rq, struct task_struct *prev)
* In particular, the load of prev->state in finish_task_switch() must
* happen before this.
*
- * Pairs with the control dependency and rmb in try_to_wake_up().
+ * Pairs with the smp_cond_acquire() in try_to_wake_up().
*/
smp_store_release(&prev->on_cpu, 0);
#endif