[tip: locking/core] futex: Clarify comment in futex_requeue()

[tip-bot2 for Thomas Gleixner]

The following commit has been merged into the locking/core branch of tip:

Commit-ID:     c18eaa3aca43688a3aee199d85ce4227686a29b6
Gitweb:        https://git.kernel.org/tip/c18eaa3aca43688a3aee199d85ce4227686a29b6
Author:        Thomas Gleixner <tglx@xxxxxxxxxxxxx>
AuthorDate:    Sun, 15 Aug 2021 23:29:14 +02:00
Committer:     Ingo Molnar <mingo@xxxxxxxxxx>
CommitterDate: Tue, 17 Aug 2021 19:05:51 +02:00

futex: Clarify comment in futex_requeue()

The comment about the restriction of the number of waiters to wake for the
REQUEUE_PI case is confusing at best. Rewrite it.

Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20210815211305.524990421@xxxxxxxxxxxxx
---
 kernel/futex.c | 28 ++++++++++++++++++++--------
 1 file changed, 20 insertions(+), 8 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index 6cb6b5d..8d8bad5 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1939,15 +1939,27 @@ static int futex_requeue(u32 __user *uaddr1, unsigned int flags,
 		 */
 		if (refill_pi_state_cache())
 			return -ENOMEM;
+
 		/*
-		 * requeue_pi must wake as many tasks as it can, up to nr_wake
-		 * + nr_requeue, since it acquires the rt_mutex prior to
-		 * returning to userspace, so as to not leave the rt_mutex with
-		 * waiters and no owner.  However, second and third wake-ups
-		 * cannot be predicted as they involve race conditions with the
-		 * first wake and a fault while looking up the pi_state.  Both
-		 * pthread_cond_signal() and pthread_cond_broadcast() should
-		 * use nr_wake=1.
+		 * futex_requeue() allows the caller to define the number
+		 * of waiters to wake up via the @nr_wake argument. With
+		 * REQUEUE_PI, waking up more than one waiter is creating
+		 * more problems than it solves. Waking up a waiter makes
+		 * only sense if the PI futex @uaddr2 is uncontended as
+		 * this allows the requeue code to acquire the futex
+		 * @uaddr2 before waking the waiter. The waiter can then
+		 * return to user space without further action. A secondary
+		 * wakeup would just make the futex_wait_requeue_pi()
+		 * handling more complex, because that code would have to
+		 * look up pi_state and do more or less all the handling
+		 * which the requeue code has to do for the to be requeued
+		 * waiters. So restrict the number of waiters to wake to
+		 * one, and only wake it up when the PI futex is
+		 * uncontended. Otherwise requeue it and let the unlock of
+		 * the PI futex handle the wakeup.
+		 *
+		 * All REQUEUE_PI users, e.g. pthread_cond_signal() and
+		 * pthread_cond_broadcast() must use nr_wake=1.
 		 */
 		if (nr_wake != 1)
 			return -EINVAL;