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From: Thomas Gleixner <tglx@linutronix.de>
To: LKML <linux-kernel@vger.kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@kernel.org>,
	Davidlohr Bueso <dave@stgolabs.net>,
	Darren Hart <dvhart@infradead.org>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Subject: [patch 2/3] futex: Clarify comment for requeue_pi_wake_futex()
Date: Thu,  2 Sep 2021 11:48:50 +0200 (CEST)	[thread overview]
Message-ID: <20210902094414.618613025@linutronix.de> (raw)
In-Reply-To: <20210902093755.087908334@linutronix.de>

It's slightly confusing.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
 kernel/futex.c |   26 ++++++++++++++++++++------
 1 file changed, 20 insertions(+), 6 deletions(-)

--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1954,12 +1954,26 @@ static inline int futex_requeue_pi_wakeu
  * @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.
  */
 static inline
 void requeue_pi_wake_futex(struct futex_q *q, union futex_key *key,


  parent reply	other threads:[~2021-09-02  9:49 UTC|newest]

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-02  9:48 [patch 0/3] futex: Prevent inconsistent state and exit race Thomas Gleixner
2021-09-02  9:48 ` [patch 1/3] " Thomas Gleixner
2021-09-02 19:55   ` [tip: locking/urgent] " tip-bot2 for Thomas Gleixner
2021-09-02 20:14   ` tip-bot2 for Thomas Gleixner
2021-09-02  9:48 ` Thomas Gleixner [this message]
2021-09-02 19:55   ` [tip: locking/urgent] futex: Clarify comment for requeue_pi_wake_futex() tip-bot2 for Thomas Gleixner
2021-09-02 20:14   ` tip-bot2 for Thomas Gleixner
2021-09-02  9:48 ` [patch 3/3] futex: Avoid redundant task lookup Thomas Gleixner
2021-09-02 19:55   ` [tip: locking/urgent] " tip-bot2 for Thomas Gleixner
2021-09-02 20:14   ` tip-bot2 for Thomas Gleixner
2021-09-02 11:06 ` [patch 0/3] futex: Prevent inconsistent state and exit race Peter Zijlstra

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