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Subject[PATCH 1/6] sched/wait: Add swq_has_sleepers()
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Which is the equivalent of what we have in regular waitqueues.
I'm not crazy about the name, but this also helps us get both
apis closer -- which iirc comes originally from the -net folks.

We also duplicate the comments for the lockless swait_active(),
from wait.h. Future users will make use of this interface.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
---
include/linux/swait.h | 57 +++++++++++++++++++++++++++++++++++++++++++++++++--
1 file changed, 55 insertions(+), 2 deletions(-)

diff --git a/include/linux/swait.h b/include/linux/swait.h
index 4a4e180d0a35..ce4c2582fd42 100644
--- a/include/linux/swait.h
+++ b/include/linux/swait.h
@@ -79,9 +79,62 @@ extern void __init_swait_queue_head(struct swait_queue_head *q, const char *name
DECLARE_SWAIT_QUEUE_HEAD(name)
#endif

-static inline int swait_active(struct swait_queue_head *q)
+/**
+ * swait_active -- locklessly test for waiters on the queue
+ * @wq: the waitqueue to test for waiters
+ *
+ * returns true if the wait list is not empty
+ *
+ * NOTE: this function is lockless and requires care, incorrect usage _will_
+ * lead to sporadic and non-obvious failure.
+ *
+ * NOTE2: this function has the same above implications as regular waitqueues.
+ *
+ * Use either while holding swait_queue_head::lock or when used for wakeups
+ * with an extra smp_mb() like:
+ *
+ * CPU0 - waker CPU1 - waiter
+ *
+ * for (;;) {
+ * @cond = true; prepare_to_wait(&wq_head, &wait, state);
+ * smp_mb(); // smp_mb() from set_current_state()
+ * if (swait_active(wq_head)) if (@cond)
+ * wake_up(wq_head); break;
+ * schedule();
+ * }
+ * finish_wait(&wq_head, &wait);
+ *
+ * Because without the explicit smp_mb() it's possible for the
+ * swait_active() load to get hoisted over the @cond store such that we'll
+ * observe an empty wait list while the waiter might not observe @cond.
+ *
+ * Also note that this 'optimization' trades a spin_lock() for an smp_mb(),
+ * which (when the lock is uncontended) are of roughly equal cost.
+ */
+static inline int swait_active(struct swait_queue_head *wq)
+{
+ return !list_empty(&wq->task_list);
+}
+
+/**
+ * swq_has_sleeper - check if there are any waiting processes
+ * @wq: wait queue head
+ *
+ * Returns true if @wq has waiting processes
+ *
+ * Please refer to the comment for swait_active.
+ */
+static inline bool swq_has_sleeper(struct swait_queue_head *wq)
{
- return !list_empty(&q->task_list);
+ /*
+ * We need to be sure we are in sync with the list_add()
+ * modifications to the wait queue (task_list).
+ *
+ * This memory barrier should be paired with one on the
+ * waiting side.
+ */
+ smp_mb();
+ return swait_active(wq);
}

extern void swake_up(struct swait_queue_head *q);
--
2.12.0
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