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-rw-r--r--lib/Makefile1
-rw-r--r--lib/semaphore-sleepers.c177
2 files changed, 178 insertions, 0 deletions
diff --git a/lib/Makefile b/lib/Makefile
index 52f83380f704..3e2bd0df23bb 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -18,6 +18,7 @@ endif
lib-$(CONFIG_RWSEM_GENERIC_SPINLOCK) += rwsem-spinlock.o
lib-$(CONFIG_RWSEM_XCHGADD_ALGORITHM) += rwsem.o
+lib-$(CONFIG_SEMAPHORE_SLEEPERS) += semaphore-sleepers.o
lib-$(CONFIG_GENERIC_FIND_NEXT_BIT) += find_next_bit.o
obj-$(CONFIG_LOCK_KERNEL) += kernel_lock.o
obj-$(CONFIG_DEBUG_PREEMPT) += smp_processor_id.o
diff --git a/lib/semaphore-sleepers.c b/lib/semaphore-sleepers.c
new file mode 100644
index 000000000000..4d5f18889fa5
--- /dev/null
+++ b/lib/semaphore-sleepers.c
@@ -0,0 +1,177 @@
+/*
+ * i386 and x86-64 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.
+ */
+
+fastcall void __up(struct semaphore *sem)
+{
+ wake_up(&sem->wait);
+}
+
+fastcall void __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;
+}
+
+fastcall int __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.
+ */
+fastcall int __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;
+}

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