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authorPeter Zijlstra <peterz@infradead.org>2015-10-16 14:39:38 +0200
committerIngo Molnar <mingo@kernel.org>2015-12-04 10:33:41 +0100
commitb3e0b1b6d841a4b2f64fc09ea728913da8218424 (patch)
tree93df412814210f3e4d36dae4e39487aec17a431e /kernel/sched
parent829cf31751aa1c50b1527c1e0c821c99246b5639 (diff)
downloadlinux-b3e0b1b6d841a4b2f64fc09ea728913da8218424.tar.bz2
locking, sched: Introduce smp_cond_acquire() and use it
Introduce smp_cond_acquire() which combines a control dependency and a read barrier to form acquire semantics. This primitive has two benefits: - it documents control dependencies, - its typically cheaper than using smp_load_acquire() in a loop. Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Mike Galbraith <efault@gmx.de> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Signed-off-by: Ingo Molnar <mingo@kernel.org>
Diffstat (limited to 'kernel/sched')
-rw-r--r--kernel/sched/core.c8
-rw-r--r--kernel/sched/sched.h2
2 files changed, 2 insertions, 8 deletions
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