patch-2.4.5 linux/arch/ppc/kernel/semaphore.c

• Lines: 237
• Date: Mon May 21 17:04:47 2001
• Orig file: v2.4.4/linux/arch/ppc/kernel/semaphore.c
• Orig date: Tue Apr 17 17:19:25 2001

diff -u --recursive --new-file v2.4.4/linux/arch/ppc/kernel/semaphore.c linux/arch/ppc/kernel/semaphore.c
@@ -1,6 +1,7 @@
 /*
- * $Id: semaphore.c,v 1.1 1999/08/31 15:11:44 cort Exp $
- *
+ * BK Id: SCCS/s.semaphore.c 1.12 05/17/01 18:14:22 cort
+ */
+/*
  * PowerPC-specific semaphore code.
  *
  * Copyright (C) 1999 Cort Dougan <cort@cs.nmt.edu>
@@ -9,131 +10,122 @@
  * 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.
+ *
+ * April 2001 - Reworked by Paul Mackerras <paulus@samba.org>
+ * to eliminate the SMP races in the old version between the updates
+ * of `count' and `waking'.  Now we use negative `count' values to
+ * indicate that some process(es) are waiting for the semaphore.
  */
 
 #include <linux/sched.h>
-
+#include <asm/atomic.h>
 #include <asm/semaphore.h>
-#include <asm/semaphore-helper.h>
 
 /*
- * Semaphores are implemented using a two-way counter:
- * The "count" variable is decremented for each process
- * that tries to sleep, while the "waking" variable is
- * incremented when the "up()" code goes to wake up waiting
- * processes.
- *
- * 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.
- *
- * waking_non_zero() (from asm/semaphore.h) must execute
- * atomically.
+ * Atomically update sem->count.
+ * This does the equivalent of the following:
  *
- * When __up() is called, the count was negative before
- * incrementing it, and we need to wake up somebody.
- *
- * This routine adds one to the count of processes that need to
- * wake up and exit.  ALL waiting processes actually wake up but
- * only the one that gets to the "waking" field first will gate
- * through and acquire the semaphore.  The others will go back
- * to sleep.
- *
- * 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.
+ *	old_count = sem->count;
+ *	tmp = MAX(old_count, 0) + incr;
+ *	sem->count = tmp;
+ *	return old_count;
  */
+static inline int __sem_update_count(struct semaphore *sem, int incr)
+{
+	int old_count, tmp;
+
+	__asm__ __volatile__("\n"
+"1:	lwarx	%0,0,%3\n"
+"	srawi	%1,%0,31\n"
+"	andc	%1,%0,%1\n"
+"	add	%1,%1,%4\n"
+"	stwcx.	%1,0,%3\n"
+"	bne	1b"
+	: "=&r" (old_count), "=&r" (tmp), "=m" (sem->count)
+	: "r" (&sem->count), "r" (incr), "m" (sem->count)
+	: "cc");
+
+	return old_count;
+}
+
 void __up(struct semaphore *sem)
 {
-	wake_one_more(sem);
+	/*
+	 * Note that we incremented count in up() before we came here,
+	 * but that was ineffective since the result was <= 0, and
+	 * any negative value of count is equivalent to 0.
+	 * This ends up setting count to 1, unless count is now > 0
+	 * (i.e. because some other cpu has called up() in the meantime),
+	 * in which case we just increment count.
+	 */
+	__sem_update_count(sem, 1);
 	wake_up(&sem->wait);
 }
 
 /*
- * Perform the "down" function.  Return zero for semaphore acquired,
- * return negative for signalled out of the function.
- *
- * If called from __down, the return is ignored and the wait loop is
- * not interruptible.  This means that a task waiting on a semaphore
- * using "down()" cannot be killed until someone does an "up()" on
- * the semaphore.
- *
- * If called from __down_interruptible, the return value gets checked
- * upon return.  If the return value is negative then the task continues
- * with the negative value in the return register (it can be tested by
- * the caller).
- *
- * Either form may be used in conjunction with "up()".
- *
+ * Note that when we come in to __down or __down_interruptible,
+ * we have already decremented count, but that decrement was
+ * ineffective since the result was < 0, and any negative value
+ * of count is equivalent to 0.
+ * Thus it is only when we decrement count from some value > 0
+ * that we have actually got the semaphore.
  */
+void __down(struct semaphore *sem)
+{
+	struct task_struct *tsk = current;
+	DECLARE_WAITQUEUE(wait, tsk);
 
-#define DOWN_VAR				\
-	struct task_struct *tsk = current;	\
-	wait_queue_t wait;			\
-	init_waitqueue_entry(&wait, tsk);
-
-#define DOWN_HEAD(task_state)						\
-									\
-									\
-	tsk->state = (task_state);					\
-	add_wait_queue(&sem->wait, &wait);				\
-									\
-	/*								\
-	 * Ok, we're set up.  sem->count is known to be less than zero	\
-	 * so we must wait.						\
-	 *								\
-	 * We can let go the lock for purposes of waiting.		\
-	 * We re-acquire it after awaking so as to protect		\
-	 * all semaphore operations.					\
-	 *								\
-	 * If "up()" is called before we call waking_non_zero() then	\
-	 * we will catch it right away.  If it is called later then	\
-	 * we will have to go through a wakeup cycle to catch it.	\
-	 *								\
-	 * Multiple waiters contend for the semaphore lock to see	\
-	 * who gets to gate through and who has to wait some more.	\
-	 */								\
-	for (;;) {
-
-#define DOWN_TAIL(task_state)			\
-		tsk->state = (task_state);	\
-	}					\
-	tsk->state = TASK_RUNNING;		\
+	tsk->state = TASK_UNINTERRUPTIBLE;
+	add_wait_queue_exclusive(&sem->wait, &wait);
+	smp_wmb();
+
+	/*
+	 * Try to get the semaphore.  If the count is > 0, then we've
+	 * got the semaphore; we decrement count and exit the loop.
+	 * If the count is 0 or negative, we set it to -1, indicating
+	 * that we are asleep, and then sleep.
+	 */
+	while (__sem_update_count(sem, -1) <= 0) {
+		schedule();
+		tsk->state = TASK_UNINTERRUPTIBLE;
+	}
 	remove_wait_queue(&sem->wait, &wait);
+	tsk->state = TASK_RUNNING;
 
-void __down(struct semaphore * sem)
-{
-	DOWN_VAR
-	DOWN_HEAD(TASK_UNINTERRUPTIBLE)
-	if (waking_non_zero(sem))
-		break;
-	schedule();
-	DOWN_TAIL(TASK_UNINTERRUPTIBLE)
+	/*
+	 * If there are any more sleepers, wake one of them up so
+	 * that it can either get the semaphore, or set count to -1
+	 * indicating that there are still processes sleeping.
+	 */
+	wake_up(&sem->wait);
 }
 
 int __down_interruptible(struct semaphore * sem)
 {
-	int ret = 0;
-	DOWN_VAR
-	DOWN_HEAD(TASK_INTERRUPTIBLE)
-
-	ret = waking_non_zero_interruptible(sem, tsk);
-	if (ret)
-	{
-		if (ret == 1)
-			/* ret != 0 only if we get interrupted -arca */
-			ret = 0;
-		break;
+	int retval = 0;
+	struct task_struct *tsk = current;
+	DECLARE_WAITQUEUE(wait, tsk);
+
+	tsk->state = TASK_INTERRUPTIBLE;
+	add_wait_queue_exclusive(&sem->wait, &wait);
+	smp_wmb();
+
+	while (__sem_update_count(sem, -1) <= 0) {
+		if (signal_pending(current)) {
+			/*
+			 * A signal is pending - give up trying.
+			 * Set sem->count to 0 if it is negative,
+			 * since we are no longer sleeping.
+			 */
+			__sem_update_count(sem, 0);
+			retval = -EINTR;
+			break;
+		}
+		schedule();
+		tsk->state = TASK_INTERRUPTIBLE;
 	}
-	schedule();
-	DOWN_TAIL(TASK_INTERRUPTIBLE)
-	return ret;
-}
-
-int __down_trylock(struct semaphore * sem)
-{
-	return waking_non_zero_trylock(sem);
+	tsk->state = TASK_RUNNING;
+	remove_wait_queue(&sem->wait, &wait);
+	wake_up(&sem->wait);
+	return retval;
 }