patch-2.2.0-pre8 linux/include/asm-alpha/semaphore.h

• Lines: 250
• Date: Sat Jan 16 17:02:51 1999
• Orig file: v2.2.0-pre7/linux/include/asm-alpha/semaphore.h
• Orig date: Tue May 13 22:41:15 1997

diff -u --recursive --new-file v2.2.0-pre7/linux/include/asm-alpha/semaphore.h linux/include/asm-alpha/semaphore.h
@@ -7,28 +7,80 @@
  * (C) Copyright 1996 Linus Torvalds
  */
 
+#include <asm/current.h>
+#include <asm/system.h>
 #include <asm/atomic.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.
+ */
+
 struct semaphore {
 	atomic_t count;
 	atomic_t waking;
+	struct task_struct *owner;
+	long owner_depth;
 	struct wait_queue * wait;
 };
 
-#define MUTEX ((struct semaphore) { ATOMIC_INIT(1), ATOMIC_INIT(0), NULL })
-#define MUTEX_LOCKED ((struct semaphore) { ATOMIC_INIT(0), ATOMIC_INIT(0), NULL })
+#define MUTEX ((struct semaphore) \
+ { ATOMIC_INIT(1), ATOMIC_INIT(0), NULL, 0, NULL })
+#define MUTEX_LOCKED ((struct semaphore) \
+ { ATOMIC_INIT(0), ATOMIC_INIT(0), NULL, 1, NULL })
+
+#define semaphore_owner(sem)	((sem)->owner)
+#define sema_init(sem, val)	atomic_set(&((sem)->count), val)
 
 extern void __down(struct semaphore * sem);
 extern int  __down_interruptible(struct semaphore * sem);
 extern void __up(struct semaphore * sem);
 
-#define sema_init(sem, val)	atomic_set(&((sem)->count), val)
+/* All three have custom assembly linkages.  */
+extern void __down_failed(struct semaphore * sem);
+extern void __down_failed_interruptible(struct semaphore * sem);
+extern void __up_wakeup(struct semaphore * sem);
+
 
 /*
  * These two _must_ execute atomically wrt each other.
  *
  * This is trivially done with load_locked/store_cond,
  * which we have.  Let the rest of the losers suck eggs.
+ *
+ * Tricky bits --
+ *
+ * (1) One task does two downs, no other contention
+ *	initial state:
+ *		count = 1, waking = 0, depth = undef;
+ *	down(&sem)
+ *		count = 0, waking = 0, depth = 1;
+ *	down(&sem)
+ *		atomic dec and test sends us to waking_non_zero via __down
+ *			count = -1, waking = 0;
+ *		conditional atomic dec on waking discovers no free slots
+ *			count = -1, waking = 0;
+ *		test for owner succeeeds and we return ok.
+ *			count = -1, waking = 0, depth = 2;
+ *	up(&sem)
+ *		dec depth
+ *			count = -1, waking = 0, depth = 0;
+ *		atomic inc and test sends us to slow path
+ *			count = 0, waking = 0, depth = 0;
+ *		notice !(depth < 0) and don't call __up.
+ *	up(&sem)
+ *		dec depth
+ *			count = 0, waking = 0, depth = -1;
+ *		atomic inc and test succeeds.
+ *			count = 1, waking = 0, depth = 0;
  */
 
 static inline void wake_one_more(struct semaphore * sem)
@@ -36,48 +88,153 @@
 	atomic_inc(&sem->waking);
 }
 
-static inline int waking_non_zero(struct semaphore *sem)
+static inline int waking_non_zero(struct semaphore *sem,
+				  struct task_struct *tsk)
 {
+	long owner_depth;
 	int ret, tmp;
 
+	owner_depth = sem->owner_depth;
+	
+	/* Atomic decrement, iff the value is > 0.  */
 	__asm__ __volatile__(
 	"1:	ldl_l	%1,%2\n"
 	"	ble	%1,2f\n"
 	"	subl	%1,1,%0\n"
 	"	stl_c	%0,%2\n"
 	"	beq	%0,3f\n"
-	"2:\n"
+	"2:	mb\n"
 	".section .text2,\"ax\"\n"
 	"3:	br	1b\n"
 	".previous"
 	: "=r"(ret), "=r"(tmp), "=m"(__atomic_fool_gcc(&sem->waking))
 	: "0"(0));
 
+	ret |= ((owner_depth != 0) & (sem->owner == tsk));
+	if (ret) {
+		sem->owner = tsk;
+		wmb();
+		/* Don't use the old value, which is stale in the
+		   !owner case.  */
+		sem->owner_depth++;
+	}
+
 	return ret;
 }
 
 /*
- * This isn't quite as clever as the x86 side, but the gp register
- * makes things a bit more complicated on the alpha..
+ * Whee.  Hidden out of line code is fun.  The contention cases are
+ * handled out of line in kernel/sched.c; arch/alpha/lib/semaphore.S
+ * takes care of making sure we can call it without clobbering regs.
  */
+
 extern inline void down(struct semaphore * sem)
 {
-	if (atomic_dec_return(&sem->count) < 0)
-		__down(sem);
+	/* Given that we have to use particular hard registers to 
+	   communicate with __down_failed anyway, reuse them in 
+	   the atomic operation as well. 
+
+	   __down_failed takes the semaphore address in $24, and
+	   it's return address in $28.  The pv is loaded as usual.
+	   The gp is clobbered (in the module case) as usual.  */
+
+	__asm__ __volatile__ (
+		"/* semaphore down operation */\n"
+		"1:	ldl_l	$27,%3\n"
+		"	subl	$27,1,$27\n"
+		"	mov	$27,$28\n"
+		"	stl_c	$28,%0\n"
+		"	beq	$28,2f\n"
+		"	blt	$27,3f\n"
+		/* Got the semaphore no contention.  Set owner and depth.  */
+		"	stq	$8,%1\n"
+		"	lda	$28,1\n"
+		"	wmb\n"
+		"	stq	$28,%2\n"
+		"4:	mb\n"
+		".section .text2,\"ax\"\n"
+		"2:	br	1b\n"
+		"3:	lda	$24,%3\n"
+		"	jsr	$28,__down_failed\n"
+		"	ldgp	$29,0($28)\n"
+		"	br	4b\n"
+		".previous"
+		: "=m"(sem->count), "=m"(sem->owner), "=m"(sem->owner_depth)
+		: "m"(sem->count)
+		: "$24", "$27", "$28", "memory");
 }
 
 extern inline int down_interruptible(struct semaphore * sem)
 {
-	int ret = 0;
-	if (atomic_dec_return(&sem->count) < 0)
-		ret = __down_interruptible(sem);
+	/* __down_failed_interruptible takes the semaphore address in $24,
+	   and it's return address in $28.  The pv is loaded as usual.
+	   The gp is clobbered (in the module case) as usual.  The return
+	   value is in $24.  */
+
+	register int ret __asm__("$24");
+
+	__asm__ __volatile__ (
+		"/* semaphore down interruptible operation */\n"
+		"1:	ldl_l	$27,%4\n"
+		"	subl	$27,1,$27\n"
+		"	mov	$27,$28\n"
+		"	stl_c	$28,%1\n"
+		"	beq	$28,2f\n"
+		"	blt	$27,3f\n"
+		/* Got the semaphore no contention.  Set owner and depth.  */
+		"	stq	$8,%2\n"
+		"	lda	$28,1\n"
+		"	wmb\n"
+		"	stq	$28,%3\n"
+		"	mov	$31,$24\n"
+		"4:	mb\n"
+		".section .text2,\"ax\"\n"
+		"2:	br	1b\n"
+		"3:	lda	$24,%4\n"
+		"	jsr	$28,__down_failed_interruptible\n"
+		"	ldgp	$29,0($28)\n"
+		"	br	4b\n"
+		".previous"
+		: "=r"(ret), "=m"(sem->count), "=m"(sem->owner),
+		  "=m"(sem->owner_depth)
+		: "m"(sem->count)
+		: "$27", "$28", "memory");
+
 	return ret;
 }
 
 extern inline void up(struct semaphore * sem)
 {
-	if (atomic_inc_return(&sem->count) <= 0)
-		__up(sem);
-}	
+	/* Given that we have to use particular hard registers to 
+	   communicate with __up_wakeup anyway, reuse them in 
+	   the atomic operation as well. 
+
+	   __up_wakeup takes the semaphore address in $24, and
+	   it's return address in $28.  The pv is loaded as usual.
+	   The gp is clobbered (in the module case) as usual.  */
+
+	__asm__ __volatile__ (
+		"/* semaphore up operation */\n"
+		"	mb\n"
+		"1:	ldl_l	$27,%1\n"
+		"	addl	$27,1,$27\n"
+		"	mov	$27,$28\n"
+		"	stl_c	$28,%0\n"
+		"	beq	$28,2f\n"
+		"	mb\n"
+		"	ble	$27,3f\n"
+		"4:\n"
+		".section .text2,\"ax\"\n"
+		"2:	br	1b\n"
+		"3:	lda	$24,%1\n"
+		"	bge	%2,4b\n"
+		"	jsr	$28,__up_wakeup\n"
+		"	ldgp	$29,0($28)\n"
+		"	br	4b\n"
+		".previous"
+		: "=m"(sem->count)
+		: "m"(sem->count), "r"(--sem->owner_depth)
+		: "$24", "$27", "$28", "memory");
+}
 
 #endif