linux-imx

// SPDX-License-Identifier: GPL-2.0-or-later

/*

 *  Fast Userspace Mutexes (which I call "Futexes!").

 *  (C) Rusty Russell, IBM 2002

 *

 *  Generalized futexes, futex requeueing, misc fixes by Ingo Molnar

 *  (C) Copyright 2003 Red Hat Inc, All Rights Reserved

 *

 *  Removed page pinning, fix privately mapped COW pages and other cleanups

 *  (C) Copyright 2003, 2004 Jamie Lokier

 *

 *  Robust futex support started by Ingo Molnar

 *  (C) Copyright 2006 Red Hat Inc, All Rights Reserved

 *  Thanks to Thomas Gleixner for suggestions, analysis and fixes.

 *

 *  PI-futex support started by Ingo Molnar and Thomas Gleixner

 *  Copyright (C) 2006 Red Hat, Inc., Ingo Molnar <mingo@redhat.com>

 *  Copyright (C) 2006 Timesys Corp., Thomas Gleixner <tglx@timesys.com>

 *

 *  PRIVATE futexes by Eric Dumazet

 *  Copyright (C) 2007 Eric Dumazet <dada1@cosmosbay.com>

 *

 *  Requeue-PI support by Darren Hart <dvhltc@us.ibm.com>

 *  Copyright (C) IBM Corporation, 2009

 *  Thanks to Thomas Gleixner for conceptual design and careful reviews.

 *

 *  Thanks to Ben LaHaise for yelling "hashed waitqueues" loudly

 *  enough at me, Linus for the original (flawed) idea, Matthew

 *  Kirkwood for proof-of-concept implementation.

 *

 *  "The futexes are also cursed."

 *  "But they come in a choice of three flavours!"

 */

#include <linux/compat.h>

#include <linux/slab.h>

#include <linux/poll.h>

#include <linux/fs.h>

#include <linux/file.h>

#include <linux/jhash.h>

#include <linux/init.h>

#include <linux/futex.h>

#include <linux/mount.h>

#include <linux/pagemap.h>

#include <linux/syscalls.h>

#include <linux/signal.h>

#include <linux/export.h>

#include <linux/magic.h>

#include <linux/pid.h>

#include <linux/nsproxy.h>

#include <linux/ptrace.h>

#include <linux/sched/rt.h>

#include <linux/sched/wake_q.h>

#include <linux/sched/mm.h>

#include <linux/hugetlb.h>

#include <linux/freezer.h>

#include <linux/memblock.h>

#include <linux/fault-inject.h>

#include <linux/refcount.h>

​

#include <asm/futex.h>

​

#include "locking/rtmutex_common.h"

​

/*

 * READ this before attempting to hack on futexes!

 *

 * Basic futex operation and ordering guarantees

 * =============================================

 *

 * The waiter reads the futex value in user space and calls

 * futex_wait(). This function computes the hash bucket and acquires

 * the hash bucket lock. After that it reads the futex user space value

 * again and verifies that the data has not changed. If it has not changed

 * it enqueues itself into the hash bucket, releases the hash bucket lock

 * and schedules.

 *

 * The waker side modifies the user space value of the futex and calls

 * futex_wake(). This function computes the hash bucket and acquires the

 * hash bucket lock. Then it looks for waiters on that futex in the hash

 * bucket and wakes them.

 *

 * In futex wake up scenarios where no tasks are blocked on a futex, taking

 * the hb spinlock can be avoided and simply return. In order for this

 * optimization to work, ordering guarantees must exist so that the waiter

 * being added to the list is acknowledged when the list is concurrently being

 * checked by the waker, avoiding scenarios like the following:

 *

 * CPU 0                               CPU 1

 * val = *futex;

 * sys_futex(WAIT, futex, val);

 *   futex_wait(futex, val);

 *   uval = *futex;