patch-2.2.2 linux/include/asm-i386/semaphore.h

• Lines: 206
• Date: Mon Feb 22 14:50:23 1999
• Orig file: v2.2.1/linux/include/asm-i386/semaphore.h
• Orig date: Tue Jan 19 11:32:53 1999

diff -u --recursive --new-file v2.2.1/linux/include/asm-i386/semaphore.h linux/include/asm-i386/semaphore.h
@@ -12,6 +12,11 @@
  *                     the original code and to make semaphore waits
  *                     interruptible so that processes waiting on
  *                     semaphores can be killed.
+ * Modified 1999-02-14 by Andrea Arcangeli, split the sched.c helper
+ *		       functions in asm/sempahore-helper.h while fixing a
+ *		       potential and subtle race discovered by Ulrich Schmid
+ *		       in down_interruptible(). Since I started to play here I
+ *		       also implemented the `trylock' semaphore operation.
  *
  * If you would like to see an analysis of this implementation, please
  * ftp to gcom.com and download the file
@@ -23,56 +28,23 @@
 #include <asm/atomic.h>
 #include <asm/spinlock.h>
 
-/*
- * Semaphores are recursive: we allow the holder process
- * to recursively do down() operations on a semaphore that
- * the process already owns. In order to do that, we need
- * to keep a semaphore-local copy of the owner and the
- * "depth of ownership".
- *
- * NOTE! Nasty memory ordering rules:
- *  - "owner" and "owner_count" may only be modified once you hold the
- *    lock. 
- *  - "owner_count" must be written _after_ modifying owner, and
- *    must be read _before_ reading owner. There must be appropriate
- *    write and read barriers to enforce this.
- *
- * On an x86, writes are always ordered, so the only enformcement
- * necessary is to make sure that the owner_depth is written after
- * the owner value in program order.
- *
- * For read ordering guarantees, the semaphore wake_lock spinlock
- * is already giving us ordering guarantees.
- *
- * Other (saner) architectures would use "wmb()" and "rmb()" to
- * do this in a more obvious manner.
- */
 struct semaphore {
 	atomic_t count;
-	unsigned long owner, owner_depth;
 	int waking;
 	struct wait_queue * wait;
 };
 
-/*
- * Because we want the non-contention case to be
- * fast, we save the stack pointer into the "owner"
- * field, and to get the true task pointer we have
- * to do the bit masking. That moves the masking
- * operation into the slow path.
- */
-#define semaphore_owner(sem) \
-	((struct task_struct *)((2*PAGE_MASK) & (sem)->owner))
-
-#define MUTEX ((struct semaphore) { ATOMIC_INIT(1), 0, 0, 0, NULL })
-#define MUTEX_LOCKED ((struct semaphore) { ATOMIC_INIT(0), 0, 1, 0, NULL })
+#define MUTEX ((struct semaphore) { ATOMIC_INIT(1), 0, NULL })
+#define MUTEX_LOCKED ((struct semaphore) { ATOMIC_INIT(0), 0, NULL })
 
 asmlinkage void __down_failed(void /* special register calling convention */);
 asmlinkage int  __down_failed_interruptible(void  /* params in registers */);
+asmlinkage int  __down_failed_trylock(void  /* params in registers */);
 asmlinkage void __up_wakeup(void /* special register calling convention */);
 
 asmlinkage void __down(struct semaphore * sem);
 asmlinkage int  __down_interruptible(struct semaphore * sem);
+asmlinkage int  __down_trylock(struct semaphore * sem);
 asmlinkage void __up(struct semaphore * sem);
 
 extern spinlock_t semaphore_wake_lock;
@@ -80,75 +52,6 @@
 #define sema_init(sem, val)	atomic_set(&((sem)->count), (val))
 
 /*
- * These two _must_ execute atomically wrt each other.
- *
- * This is trivially done with load_locked/store_cond,
- * but on the x86 we need an external synchronizer.
- */
-static inline void wake_one_more(struct semaphore * sem)
-{
-	unsigned long flags;
-
-	spin_lock_irqsave(&semaphore_wake_lock, flags);
-	sem->waking++;
-	spin_unlock_irqrestore(&semaphore_wake_lock, flags);
-}
-
-/*
- * NOTE NOTE NOTE!
- *
- * We read owner-count _before_ getting the semaphore. This
- * is important, because the semaphore also acts as a memory
- * ordering point between reading owner_depth and reading
- * the owner.
- *
- * Why is this necessary? The "owner_depth" essentially protects
- * us from using stale owner information - in the case that this
- * process was the previous owner but somebody else is racing to
- * aquire the semaphore, the only way we can see ourselves as an
- * owner is with "owner_depth" of zero (so that we know to avoid
- * the stale value).
- *
- * In the non-race case (where we really _are_ the owner), there
- * is not going to be any question about what owner_depth is.
- *
- * In the race case, the race winner will not even get here, because
- * it will have successfully gotten the semaphore with the locked
- * decrement operation.
- *
- * Basically, we have two values, and we cannot guarantee that either
- * is really up-to-date until we have aquired the semaphore. But we
- * _can_ depend on a ordering between the two values, so we can use
- * one of them to determine whether we can trust the other:
- *
- * Cases:
- *  - owner_depth == zero: ignore the semaphore owner, because it
- *    cannot possibly be us. Somebody else may be in the process
- *    of modifying it and the zero may be "stale", but it sure isn't
- *    going to say that "we" are the owner anyway, so who cares?
- *  - owner_depth is non-zero. That means that even if somebody
- *    else wrote the non-zero count value, the write ordering requriement
- *    means that they will have written themselves as the owner, so
- *    if we now see ourselves as an owner we can trust it to be true.
- */
-static inline int waking_non_zero(struct semaphore *sem, struct task_struct *tsk)
-{
-	unsigned long flags;
-	unsigned long owner_depth = sem->owner_depth;
-	int ret = 0;
-
-	spin_lock_irqsave(&semaphore_wake_lock, flags);
-	if (sem->waking > 0 || (owner_depth && semaphore_owner(sem) == tsk)) {
-		sem->owner = (unsigned long) tsk;
-		sem->owner_depth++;	/* Don't use the possibly stale value */
-		sem->waking--;
-		ret = 1;
-	}
-	spin_unlock_irqrestore(&semaphore_wake_lock, flags);
-	return ret;
-}
-
-/*
  * This is ugly, but we want the default case to fall through.
  * "down_failed" is a special asm handler that calls the C
  * routine that actually waits. See arch/i386/lib/semaphore.S
@@ -161,9 +64,7 @@
 		"lock ; "
 #endif
 		"decl 0(%0)\n\t"
-		"js 2f\n\t"
-		"movl %%esp,4(%0)\n"
-		"movl $1,8(%0)\n\t"
+		"js 2f\n"
 		"1:\n"
 		".section .text.lock,\"ax\"\n"
 		"2:\tpushl $1b\n\t"
@@ -185,8 +86,6 @@
 #endif
 		"decl 0(%1)\n\t"
 		"js 2f\n\t"
-		"movl %%esp,4(%1)\n\t"
-		"movl $1,8(%1)\n\t"
 		"xorl %0,%0\n"
 		"1:\n"
 		".section .text.lock,\"ax\"\n"
@@ -199,6 +98,28 @@
 	return result;
 }
 
+extern inline int down_trylock(struct semaphore * sem)
+{
+	int result;
+
+	__asm__ __volatile__(
+		"# atomic interruptible down operation\n\t"
+#ifdef __SMP__
+		"lock ; "
+#endif
+		"decl 0(%1)\n\t"
+		"js 2f\n\t"
+		"xorl %0,%0\n"
+		"1:\n"
+		".section .text.lock,\"ax\"\n"
+		"2:\tpushl $1b\n\t"
+		"jmp __down_failed_trylock\n"
+		".previous"
+		:"=a" (result)
+		:"c" (sem)
+		:"memory");
+	return result;
+}
 
 /*
  * Note! This is subtle. We jump to wake people up only if
@@ -210,7 +131,6 @@
 {
 	__asm__ __volatile__(
 		"# atomic up operation\n\t"
-		"decl 8(%0)\n\t"
 #ifdef __SMP__
 		"lock ; "
 #endif