[tip: locking/urgent] futex: Clarify comment for requeue_pi_wake_futex()
From: tip-bot2 for Thomas Gleixner
Date: Thu Sep 02 2021 - 16:14:44 EST
The following commit has been merged into the locking/urgent branch of tip:
Author: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
AuthorDate: Thu, 02 Sep 2021 11:48:50 +02:00
Committer: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitterDate: Thu, 02 Sep 2021 22:07:18 +02:00
futex: Clarify comment for requeue_pi_wake_futex()
It's slightly confusing.
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
kernel/futex.c | 26 ++++++++++++++++++++------
1 file changed, 20 insertions(+), 6 deletions(-)
diff --git a/kernel/futex.c b/kernel/futex.c
index 04164d4..82cd270 100644
@@ -1954,12 +1954,26 @@ static inline int futex_requeue_pi_wakeup_sync(struct futex_q *q)
* @hb: the hash_bucket of the requeue target futex
* During futex_requeue, with requeue_pi=1, it is possible to acquire the
- * target futex if it is uncontended or via a lock steal. Set the futex_q key
- * to the requeue target futex so the waiter can detect the wakeup on the right
- * futex, but remove it from the hb and NULL the rt_waiter so it can detect
- * atomic lock acquisition. Set the q->lock_ptr to the requeue target hb->lock
- * to protect access to the pi_state to fixup the owner later. Must be called
- * with both q->lock_ptr and hb->lock held.
+ * target futex if it is uncontended or via a lock steal.
+ * 1) Set @q::key to the requeue target futex key so the waiter can detect
+ * the wakeup on the right futex.
+ * 2) Dequeue @q from the hash bucket.
+ * 3) Set @q::rt_waiter to NULL so the woken up task can detect atomic lock
+ * acquisition.
+ * 4) Set the q->lock_ptr to the requeue target hb->lock for the case that
+ * the waiter has to fixup the pi state.
+ * 5) Complete the requeue state so the waiter can make progress. After
+ * this point the waiter task can return from the syscall immediately in
+ * case that the pi state does not have to be fixed up.
+ * 6) Wake the waiter task.
+ * Must be called with both q->lock_ptr and hb->lock held.
void requeue_pi_wake_futex(struct futex_q *q, union futex_key *key,