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-rw-r--r--include/linux/wait.h51
1 files changed, 51 insertions, 0 deletions
diff --git a/include/linux/wait.h b/include/linux/wait.h
index 513b36f04dfd..ae71a769b89e 100644
--- a/include/linux/wait.h
+++ b/include/linux/wait.h
@@ -102,11 +102,62 @@ init_waitqueue_func_entry(wait_queue_t *q, wait_queue_func_t func)
q->func = func;
}
+/**
+ * waitqueue_active -- locklessly test for waiters on the queue
+ * @q: 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.
+ *
+ * Use either while holding wait_queue_head_t::lock or when used for wakeups
+ * with an extra smp_mb() like:
+ *
+ * CPU0 - waker CPU1 - waiter
+ *
+ * for (;;) {
+ * @cond = true; prepare_to_wait(&wq, &wait, state);
+ * smp_mb(); // smp_mb() from set_current_state()
+ * if (waitqueue_active(wq)) if (@cond)
+ * wake_up(wq); break;
+ * schedule();
+ * }
+ * finish_wait(&wq, &wait);
+ *
+ * Because without the explicit smp_mb() it's possible for the
+ * waitqueue_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 waitqueue_active(wait_queue_head_t *q)
{
return !list_empty(&q->task_list);
}
+/**
+ * wq_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 waitqueue_active.
+ */
+static inline bool wq_has_sleeper(wait_queue_head_t *wq)
+{
+ /*
+ * We need to be sure we are in sync with the
+ * add_wait_queue modifications to the wait queue.
+ *
+ * This memory barrier should be paired with one on the
+ * waiting side.
+ */
+ smp_mb();
+ return waitqueue_active(wq);
+}
+
extern void add_wait_queue(wait_queue_head_t *q, wait_queue_t *wait);
extern void add_wait_queue_exclusive(wait_queue_head_t *q, wait_queue_t *wait);
extern void remove_wait_queue(wait_queue_head_t *q, wait_queue_t *wait);