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From: Thomas Gleixner <tglx@linutronix.de>
Date: Sun, 15 Aug 2021 23:29:14 +0200
Subject: [PATCH 63/72] 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@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20210815211305.524990421@linutronix.de
---
kernel/futex.c | 28 ++++++++++++++++++++--------
1 file changed, 20 insertions(+), 8 deletions(-)
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1939,15 +1939,27 @@ static int futex_requeue(u32 __user *uad
*/
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;
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