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authorLinus Torvalds <torvalds@ppc970.osdl.org>2005-04-16 15:20:36 -0700
committerLinus Torvalds <torvalds@ppc970.osdl.org>2005-04-16 15:20:36 -0700
commit1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 (patch)
tree0bba044c4ce775e45a88a51686b5d9f90697ea9d /arch/i386/kernel/semaphore.c
downloadlinux-stericsson-1da177e4c3f41524e886b7f1b8a0c1fc7321cac2.tar.gz
Linux-2.6.12-rc2v2.6.12-rc2
Initial git repository build. I'm not bothering with the full history, even though we have it. We can create a separate "historical" git archive of that later if we want to, and in the meantime it's about 3.2GB when imported into git - space that would just make the early git days unnecessarily complicated, when we don't have a lot of good infrastructure for it. Let it rip!
Diffstat (limited to 'arch/i386/kernel/semaphore.c')
-rw-r--r--arch/i386/kernel/semaphore.c297
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diff --git a/arch/i386/kernel/semaphore.c b/arch/i386/kernel/semaphore.c
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+/*
+ * i386 semaphore implementation.
+ *
+ * (C) Copyright 1999 Linus Torvalds
+ *
+ * Portions Copyright 1999 Red Hat, Inc.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version
+ * 2 of the License, or (at your option) any later version.
+ *
+ * rw semaphores implemented November 1999 by Benjamin LaHaise <bcrl@kvack.org>
+ */
+#include <linux/config.h>
+#include <linux/sched.h>
+#include <linux/err.h>
+#include <linux/init.h>
+#include <asm/semaphore.h>
+
+/*
+ * Semaphores are implemented using a two-way counter:
+ * The "count" variable is decremented for each process
+ * that tries to acquire the semaphore, while the "sleeping"
+ * variable is a count of such acquires.
+ *
+ * Notably, the inline "up()" and "down()" functions can
+ * efficiently test if they need to do any extra work (up
+ * needs to do something only if count was negative before
+ * the increment operation.
+ *
+ * "sleeping" and the contention routine ordering is protected
+ * by the spinlock in the semaphore's waitqueue head.
+ *
+ * Note that these functions are only called when there is
+ * contention on the lock, and as such all this is the
+ * "non-critical" part of the whole semaphore business. The
+ * critical part is the inline stuff in <asm/semaphore.h>
+ * where we want to avoid any extra jumps and calls.
+ */
+
+/*
+ * Logic:
+ * - only on a boundary condition do we need to care. When we go
+ * from a negative count to a non-negative, we wake people up.
+ * - when we go from a non-negative count to a negative do we
+ * (a) synchronize with the "sleeper" count and (b) make sure
+ * that we're on the wakeup list before we synchronize so that
+ * we cannot lose wakeup events.
+ */
+
+static fastcall void __attribute_used__ __up(struct semaphore *sem)
+{
+ wake_up(&sem->wait);
+}
+
+static fastcall void __attribute_used__ __sched __down(struct semaphore * sem)
+{
+ struct task_struct *tsk = current;
+ DECLARE_WAITQUEUE(wait, tsk);
+ unsigned long flags;
+
+ tsk->state = TASK_UNINTERRUPTIBLE;
+ spin_lock_irqsave(&sem->wait.lock, flags);
+ add_wait_queue_exclusive_locked(&sem->wait, &wait);
+
+ sem->sleepers++;
+ for (;;) {
+ int sleepers = sem->sleepers;
+
+ /*
+ * Add "everybody else" into it. They aren't
+ * playing, because we own the spinlock in
+ * the wait_queue_head.
+ */
+ if (!atomic_add_negative(sleepers - 1, &sem->count)) {
+ sem->sleepers = 0;
+ break;
+ }
+ sem->sleepers = 1; /* us - see -1 above */
+ spin_unlock_irqrestore(&sem->wait.lock, flags);
+
+ schedule();
+
+ spin_lock_irqsave(&sem->wait.lock, flags);
+ tsk->state = TASK_UNINTERRUPTIBLE;
+ }
+ remove_wait_queue_locked(&sem->wait, &wait);
+ wake_up_locked(&sem->wait);
+ spin_unlock_irqrestore(&sem->wait.lock, flags);
+ tsk->state = TASK_RUNNING;
+}
+
+static fastcall int __attribute_used__ __sched __down_interruptible(struct semaphore * sem)
+{
+ int retval = 0;
+ struct task_struct *tsk = current;
+ DECLARE_WAITQUEUE(wait, tsk);
+ unsigned long flags;
+
+ tsk->state = TASK_INTERRUPTIBLE;
+ spin_lock_irqsave(&sem->wait.lock, flags);
+ add_wait_queue_exclusive_locked(&sem->wait, &wait);
+
+ sem->sleepers++;
+ for (;;) {
+ int sleepers = sem->sleepers;
+
+ /*
+ * With signals pending, this turns into
+ * the trylock failure case - we won't be
+ * sleeping, and we* can't get the lock as
+ * it has contention. Just correct the count
+ * and exit.
+ */
+ if (signal_pending(current)) {
+ retval = -EINTR;
+ sem->sleepers = 0;
+ atomic_add(sleepers, &sem->count);
+ break;
+ }
+
+ /*
+ * Add "everybody else" into it. They aren't
+ * playing, because we own the spinlock in
+ * wait_queue_head. The "-1" is because we're
+ * still hoping to get the semaphore.
+ */
+ if (!atomic_add_negative(sleepers - 1, &sem->count)) {
+ sem->sleepers = 0;
+ break;
+ }
+ sem->sleepers = 1; /* us - see -1 above */
+ spin_unlock_irqrestore(&sem->wait.lock, flags);
+
+ schedule();
+
+ spin_lock_irqsave(&sem->wait.lock, flags);
+ tsk->state = TASK_INTERRUPTIBLE;
+ }
+ remove_wait_queue_locked(&sem->wait, &wait);
+ wake_up_locked(&sem->wait);
+ spin_unlock_irqrestore(&sem->wait.lock, flags);
+
+ tsk->state = TASK_RUNNING;
+ return retval;
+}
+
+/*
+ * Trylock failed - make sure we correct for
+ * having decremented the count.
+ *
+ * We could have done the trylock with a
+ * single "cmpxchg" without failure cases,
+ * but then it wouldn't work on a 386.
+ */
+static fastcall int __attribute_used__ __down_trylock(struct semaphore * sem)
+{
+ int sleepers;
+ unsigned long flags;
+
+ spin_lock_irqsave(&sem->wait.lock, flags);
+ sleepers = sem->sleepers + 1;
+ sem->sleepers = 0;
+
+ /*
+ * Add "everybody else" and us into it. They aren't
+ * playing, because we own the spinlock in the
+ * wait_queue_head.
+ */
+ if (!atomic_add_negative(sleepers, &sem->count)) {
+ wake_up_locked(&sem->wait);
+ }
+
+ spin_unlock_irqrestore(&sem->wait.lock, flags);
+ return 1;
+}
+
+
+/*
+ * The semaphore operations have a special calling sequence that
+ * allow us to do a simpler in-line version of them. These routines
+ * need to convert that sequence back into the C sequence when
+ * there is contention on the semaphore.
+ *
+ * %eax contains the semaphore pointer on entry. Save the C-clobbered
+ * registers (%eax, %edx and %ecx) except %eax whish is either a return
+ * value or just clobbered..
+ */
+asm(
+".section .sched.text\n"
+".align 4\n"
+".globl __down_failed\n"
+"__down_failed:\n\t"
+#if defined(CONFIG_FRAME_POINTER)
+ "pushl %ebp\n\t"
+ "movl %esp,%ebp\n\t"
+#endif
+ "pushl %edx\n\t"
+ "pushl %ecx\n\t"
+ "call __down\n\t"
+ "popl %ecx\n\t"
+ "popl %edx\n\t"
+#if defined(CONFIG_FRAME_POINTER)
+ "movl %ebp,%esp\n\t"
+ "popl %ebp\n\t"
+#endif
+ "ret"
+);
+
+asm(
+".section .sched.text\n"
+".align 4\n"
+".globl __down_failed_interruptible\n"
+"__down_failed_interruptible:\n\t"
+#if defined(CONFIG_FRAME_POINTER)
+ "pushl %ebp\n\t"
+ "movl %esp,%ebp\n\t"
+#endif
+ "pushl %edx\n\t"
+ "pushl %ecx\n\t"
+ "call __down_interruptible\n\t"
+ "popl %ecx\n\t"
+ "popl %edx\n\t"
+#if defined(CONFIG_FRAME_POINTER)
+ "movl %ebp,%esp\n\t"
+ "popl %ebp\n\t"
+#endif
+ "ret"
+);
+
+asm(
+".section .sched.text\n"
+".align 4\n"
+".globl __down_failed_trylock\n"
+"__down_failed_trylock:\n\t"
+#if defined(CONFIG_FRAME_POINTER)
+ "pushl %ebp\n\t"
+ "movl %esp,%ebp\n\t"
+#endif
+ "pushl %edx\n\t"
+ "pushl %ecx\n\t"
+ "call __down_trylock\n\t"
+ "popl %ecx\n\t"
+ "popl %edx\n\t"
+#if defined(CONFIG_FRAME_POINTER)
+ "movl %ebp,%esp\n\t"
+ "popl %ebp\n\t"
+#endif
+ "ret"
+);
+
+asm(
+".section .sched.text\n"
+".align 4\n"
+".globl __up_wakeup\n"
+"__up_wakeup:\n\t"
+ "pushl %edx\n\t"
+ "pushl %ecx\n\t"
+ "call __up\n\t"
+ "popl %ecx\n\t"
+ "popl %edx\n\t"
+ "ret"
+);
+
+/*
+ * rw spinlock fallbacks
+ */
+#if defined(CONFIG_SMP)
+asm(
+".section .sched.text\n"
+".align 4\n"
+".globl __write_lock_failed\n"
+"__write_lock_failed:\n\t"
+ LOCK "addl $" RW_LOCK_BIAS_STR ",(%eax)\n"
+"1: rep; nop\n\t"
+ "cmpl $" RW_LOCK_BIAS_STR ",(%eax)\n\t"
+ "jne 1b\n\t"
+ LOCK "subl $" RW_LOCK_BIAS_STR ",(%eax)\n\t"
+ "jnz __write_lock_failed\n\t"
+ "ret"
+);
+
+asm(
+".section .sched.text\n"
+".align 4\n"
+".globl __read_lock_failed\n"
+"__read_lock_failed:\n\t"
+ LOCK "incl (%eax)\n"
+"1: rep; nop\n\t"
+ "cmpl $1,(%eax)\n\t"
+ "js 1b\n\t"
+ LOCK "decl (%eax)\n\t"
+ "js __read_lock_failed\n\t"
+ "ret"
+);
+#endif