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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>,
	Juri Lelli <juri.lelli@redhat.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Daniel Bristot de Oliveira <bristot@redhat.com>,
	Will Deacon <will@kernel.org>, Waiman Long <longman@redhat.com>,
	Boqun Feng <boqun.feng@gmail.com>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	Davidlohr Bueso <dave@stgolabs.net>
Subject: [patch 45/50] futex: Clarify comment in futex_requeue()
Date: Tue, 13 Jul 2021 17:11:39 +0200	[thread overview]
Message-ID: <20210713160750.348812588@linutronix.de> (raw)
In-Reply-To: 20210713151054.700719949@linutronix.de

From: Thomas Gleixner <tglx@linutronix.de>

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>
---
 kernel/futex.c | 28 ++++++++++++++++++++--------
 1 file changed, 20 insertions(+), 8 deletions(-)
---
diff --git a/kernel/futex.c b/kernel/futex.c
index 8ffeb9871476..e92c871aa133 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1960,15 +1960,27 @@ 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;


  parent reply	other threads:[~2021-07-13 16:14 UTC|newest]

Thread overview: 77+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-13 15:10 [patch 00/50] locking, sched: The PREEMPT-RT locking infrastructure Thomas Gleixner
2021-07-13 15:10 ` [patch 01/50] sched: Split out the wakeup state check Thomas Gleixner
2021-07-13 15:10 ` [patch 02/50] sched: Introduce TASK_RTLOCK_WAIT Thomas Gleixner
2021-07-13 15:10 ` [patch 03/50] sched: Prepare for RT sleeping spin/rwlocks Thomas Gleixner
2021-07-13 19:52   ` Waiman Long
2021-07-14 23:20   ` Valentin Schneider
2021-07-15  9:27     ` Peter Zijlstra
2021-07-15 14:08       ` Valentin Schneider
2021-07-13 15:10 ` [patch 04/50] sched: Rework the __schedule() preempt argument Thomas Gleixner
2021-07-13 20:04   ` Waiman Long
2021-07-13 15:10 ` [patch 05/50] sched: Provide schedule point for RT locks Thomas Gleixner
2021-07-14  8:28   ` Peter Zijlstra
2021-07-14  9:49     ` Thomas Gleixner
2021-07-14 10:17       ` Peter Zijlstra
2021-07-14 11:32         ` Thomas Gleixner
2021-07-13 15:11 ` [patch 06/50] sched/wake_q: Provide WAKE_Q_HEAD_INITIALIZER Thomas Gleixner
2021-07-13 15:11 ` [patch 07/50] rtmutex: Convert macros to inlines Thomas Gleixner
2021-07-13 15:11 ` [patch 08/50] rtmutex: Switch to try_cmpxchg() Thomas Gleixner
2021-07-13 15:11 ` [patch 09/50] rtmutex: Split API and implementation Thomas Gleixner
2021-07-13 15:11 ` [patch 10/50] locking/rtmutex: Provide rt_mutex_slowlock_locked() Thomas Gleixner
2021-07-27 17:20   ` Valentin Schneider
2021-07-27 19:02     ` Thomas Gleixner
2021-07-13 15:11 ` [patch 11/50] locking/rtmutex: Provide lockdep less variants of rtmutex interfaces Thomas Gleixner
2021-07-13 15:11 ` [patch 12/50] locking: Add base code for RT rw_semaphore and rwlock Thomas Gleixner
2021-07-13 15:11 ` [patch 13/50] locking/rwsem: Add rtmutex based R/W semaphore implementation Thomas Gleixner
2021-07-13 15:11 ` [patch 14/50] locking/rtmutex: Add wake_state to rt_mutex_waiter Thomas Gleixner
2021-07-14 10:18   ` Peter Zijlstra
2021-07-14 11:33     ` Thomas Gleixner
2021-07-13 15:11 ` [patch 15/50] locking/rtmutex: Provide rt_mutex_wake_q and helpers Thomas Gleixner
2021-07-13 15:11 ` [patch 16/50] locking/rtmutex: Use rt_mutex_wake_q_head Thomas Gleixner
2021-07-14  8:55   ` Peter Zijlstra
2021-07-14  9:51     ` Thomas Gleixner
2021-07-13 15:11 ` [patch 17/50] locking/rtmutex: Prepare RT rt_mutex_wake_q for RT locks Thomas Gleixner
2021-07-14  8:55   ` Peter Zijlstra
2021-07-14  9:52     ` Thomas Gleixner
2021-07-13 15:11 ` [patch 18/50] locking/rtmutex: Guard regular sleeping locks specific functions Thomas Gleixner
2021-07-13 15:11 ` [patch 19/50] locking/spinlock: Split the lock types header Thomas Gleixner
2021-07-13 15:11 ` [patch 20/50] locking/rtmutex: Prevent future include recursion hell Thomas Gleixner
2021-07-13 15:11 ` [patch 21/50] locking/lockdep: Reduce includes in debug_locks.h Thomas Gleixner
2021-07-13 15:11 ` [patch 22/50] rbtree: Split out the rbtree type definitions Thomas Gleixner
2021-07-14  9:24   ` Peter Zijlstra
2021-07-14  9:31     ` Arnaldo
2021-07-13 15:11 ` [patch 23/50] locking/rtmutex: Include only rbtree types Thomas Gleixner
2021-07-13 15:11 ` [patch 24/50] locking/spinlock: Provide RT specific spinlock type Thomas Gleixner
2021-07-14  9:54   ` Peter Zijlstra
2021-07-14 10:07     ` [PATCH] media/atomisp: Use lockdep instead of *mutex_is_locked() Peter Zijlstra
2021-07-13 15:11 ` [patch 25/50] locking/spinlock: Provide RT variant header Thomas Gleixner
2021-07-13 15:11 ` [patch 26/50] locking/rtmutex: Provide the spin/rwlock core lock function Thomas Gleixner
2021-07-14 10:59   ` Peter Zijlstra
2021-07-13 15:11 ` [patch 27/50] locking/spinlock: Provide RT variant Thomas Gleixner
2021-07-14 11:25   ` Peter Zijlstra
2021-07-13 15:11 ` [patch 28/50] locking/rwlock: " Thomas Gleixner
2021-07-13 15:11 ` [patch 29/50] locking/mutex: Consolidate core headers Thomas Gleixner
2021-07-13 15:11 ` [patch 30/50] locking/mutex: Move waiter to core header Thomas Gleixner
2021-07-13 15:11 ` [patch 31/50] locking/ww_mutex: Move ww_mutex declarations into ww_mutex.h Thomas Gleixner
2021-07-13 15:11 ` [patch 32/50] locking/mutex: Make mutex::wait_lock raw Thomas Gleixner
2021-07-13 15:11 ` [patch 33/50] locking/mutex: Introduce _mutex_t Thomas Gleixner
2021-07-15 17:31   ` Peter Zijlstra
2021-07-13 15:11 ` [patch 34/50] locking/mutex: Rename the ww_mutex relevant functions Thomas Gleixner
2021-07-13 15:11 ` [patch 35/50] locking/ww_mutex: Switch to _mutex_t Thomas Gleixner
2021-07-13 15:11 ` [patch 36/50] locking/mutex: Replace struct mutex in core code Thomas Gleixner
2021-07-13 15:11 ` [patch 37/50] locking/mutex: Rearrange items in mutex.h Thomas Gleixner
2021-07-13 15:11 ` [patch 38/50] locking/mutex: Exclude non-ww_mutex API for RT Thomas Gleixner
2021-07-13 15:11 ` [patch 39/50] locking/rtmutex: Add mutex variant " Thomas Gleixner
2021-07-13 15:11 ` [patch 40/50] lib/test_lockup: Adapt to changed variables Thomas Gleixner
2021-07-13 15:11 ` [patch 41/50] futex: Validate waiter correctly in futex_proxy_trylock_atomic() Thomas Gleixner
2021-07-13 15:11 ` [patch 42/50] futex: Cleanup stale comments Thomas Gleixner
2021-07-28 23:28   ` André Almeida
2021-07-13 15:11 ` [patch 43/50] futex: Correct the number of requeued waiters for PI Thomas Gleixner
2021-07-13 15:11 ` [patch 44/50] futex: Restructure futex_requeue() Thomas Gleixner
2021-07-13 15:11 ` Thomas Gleixner [this message]
2021-07-13 15:11 ` [patch 46/50] futex: Prevent requeue_pi() lock nesting issue on RT Thomas Gleixner
2021-07-13 15:11 ` [patch 47/50] rtmutex: Prevent lockdep false positive with PI futexes Thomas Gleixner
2021-07-13 15:11 ` [patch 48/50] preempt: Adjust PREEMPT_LOCK_OFFSET for RT Thomas Gleixner
2021-07-13 15:11 ` [patch 49/50] locking/rtmutex: Implement equal priority lock stealing Thomas Gleixner
2021-07-13 15:11 ` [patch 50/50] locking/rtmutex: Add adaptive spinwait mechanism Thomas Gleixner
2021-07-27 17:19 ` [patch 00/50] locking, sched: The PREEMPT-RT locking infrastructure Valentin Schneider

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