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2 * i386 and x86-64 semaphore implementation.
4 * (C) Copyright 1999 Linus Torvalds
6 * Portions Copyright 1999 Red Hat, Inc.
8 * This program is free software; you can redistribute it and/or
9 * modify it under the terms of the GNU General Public License
10 * as published by the Free Software Foundation; either version
11 * 2 of the License, or (at your option) any later version.
13 * rw semaphores implemented November 1999 by Benjamin LaHaise <bcrl@kvack.org>
15 #include <linux/config.h>
16 #include <linux/sched.h>
17 #include <linux/err.h>
18 #include <linux/init.h>
19 #include <asm/semaphore.h>
22 * Semaphores are implemented using a two-way counter:
23 * The "count" variable is decremented for each process
24 * that tries to acquire the semaphore, while the "sleeping"
25 * variable is a count of such acquires.
27 * Notably, the inline "up()" and "down()" functions can
28 * efficiently test if they need to do any extra work (up
29 * needs to do something only if count was negative before
30 * the increment operation.
32 * "sleeping" and the contention routine ordering is protected
33 * by the spinlock in the semaphore's waitqueue head.
35 * Note that these functions are only called when there is
36 * contention on the lock, and as such all this is the
37 * "non-critical" part of the whole semaphore business. The
38 * critical part is the inline stuff in <asm/semaphore.h>
39 * where we want to avoid any extra jumps and calls.
44 * - only on a boundary condition do we need to care. When we go
45 * from a negative count to a non-negative, we wake people up.
46 * - when we go from a non-negative count to a negative do we
47 * (a) synchronize with the "sleeper" count and (b) make sure
48 * that we're on the wakeup list before we synchronize so that
49 * we cannot lose wakeup events.
52 fastcall
void __up(struct semaphore
*sem
)
57 fastcall
void __sched
__down(struct semaphore
* sem
)
59 struct task_struct
*tsk
= current
;
60 DECLARE_WAITQUEUE(wait
, tsk
);
63 tsk
->state
= TASK_UNINTERRUPTIBLE
;
64 spin_lock_irqsave(&sem
->wait
.lock
, flags
);
65 add_wait_queue_exclusive_locked(&sem
->wait
, &wait
);
69 int sleepers
= sem
->sleepers
;
72 * Add "everybody else" into it. They aren't
73 * playing, because we own the spinlock in
74 * the wait_queue_head.
76 if (!atomic_add_negative(sleepers
- 1, &sem
->count
)) {
80 sem
->sleepers
= 1; /* us - see -1 above */
81 spin_unlock_irqrestore(&sem
->wait
.lock
, flags
);
85 spin_lock_irqsave(&sem
->wait
.lock
, flags
);
86 tsk
->state
= TASK_UNINTERRUPTIBLE
;
88 remove_wait_queue_locked(&sem
->wait
, &wait
);
89 wake_up_locked(&sem
->wait
);
90 spin_unlock_irqrestore(&sem
->wait
.lock
, flags
);
91 tsk
->state
= TASK_RUNNING
;
94 fastcall
int __sched
__down_interruptible(struct semaphore
* sem
)
97 struct task_struct
*tsk
= current
;
98 DECLARE_WAITQUEUE(wait
, tsk
);
101 tsk
->state
= TASK_INTERRUPTIBLE
;
102 spin_lock_irqsave(&sem
->wait
.lock
, flags
);
103 add_wait_queue_exclusive_locked(&sem
->wait
, &wait
);
107 int sleepers
= sem
->sleepers
;
110 * With signals pending, this turns into
111 * the trylock failure case - we won't be
112 * sleeping, and we* can't get the lock as
113 * it has contention. Just correct the count
116 if (signal_pending(current
)) {
119 atomic_add(sleepers
, &sem
->count
);
124 * Add "everybody else" into it. They aren't
125 * playing, because we own the spinlock in
126 * wait_queue_head. The "-1" is because we're
127 * still hoping to get the semaphore.
129 if (!atomic_add_negative(sleepers
- 1, &sem
->count
)) {
133 sem
->sleepers
= 1; /* us - see -1 above */
134 spin_unlock_irqrestore(&sem
->wait
.lock
, flags
);
138 spin_lock_irqsave(&sem
->wait
.lock
, flags
);
139 tsk
->state
= TASK_INTERRUPTIBLE
;
141 remove_wait_queue_locked(&sem
->wait
, &wait
);
142 wake_up_locked(&sem
->wait
);
143 spin_unlock_irqrestore(&sem
->wait
.lock
, flags
);
145 tsk
->state
= TASK_RUNNING
;
150 * Trylock failed - make sure we correct for
151 * having decremented the count.
153 * We could have done the trylock with a
154 * single "cmpxchg" without failure cases,
155 * but then it wouldn't work on a 386.
157 fastcall
int __down_trylock(struct semaphore
* sem
)
162 spin_lock_irqsave(&sem
->wait
.lock
, flags
);
163 sleepers
= sem
->sleepers
+ 1;
167 * Add "everybody else" and us into it. They aren't
168 * playing, because we own the spinlock in the
171 if (!atomic_add_negative(sleepers
, &sem
->count
)) {
172 wake_up_locked(&sem
->wait
);
175 spin_unlock_irqrestore(&sem
->wait
.lock
, flags
);