patch-2.4.4 linux/include/asm-i386/rwsem.h

• Lines: 227
• Date: Fri Apr 27 15:48:24 2001
• Orig file: v2.4.3/linux/include/asm-i386/rwsem.h
• Orig date: Wed Dec 31 16:00:00 1969

diff -u --recursive --new-file v2.4.3/linux/include/asm-i386/rwsem.h linux/include/asm-i386/rwsem.h
@@ -0,0 +1,226 @@
+/* rwsem.h: R/W semaphores implemented using XADD/CMPXCHG for i486+
+ *
+ * Written by David Howells (dhowells@redhat.com).
+ *
+ * Derived from asm-i386/semaphore.h
+ *
+ *
+ * The MSW of the count is the negated number of active writers and waiting
+ * lockers, and the LSW is the total number of active locks
+ *
+ * The lock count is initialized to 0 (no active and no waiting lockers).
+ *
+ * When a writer subtracts WRITE_BIAS, it'll get 0xffff0001 for the case of an
+ * uncontended lock. This can be determined because XADD returns the old value.
+ * Readers increment by 1 and see a positive value when uncontended, negative
+ * if there are writers (and maybe) readers waiting (in which case it goes to
+ * sleep).
+ *
+ * The value of WAITING_BIAS supports up to 32766 waiting processes. This can
+ * be extended to 65534 by manually checking the whole MSW rather than relying
+ * on the S flag.
+ *
+ * The value of ACTIVE_BIAS supports up to 65535 active processes.
+ *
+ * This should be totally fair - if anything is waiting, a process that wants a
+ * lock will go to the back of the queue. When the currently active lock is
+ * released, if there's a writer at the front of the queue, then that and only
+ * that will be woken up; if there's a bunch of consequtive readers at the
+ * front, then they'll all be woken up, but no other readers will be.
+ */
+
+#ifndef _I386_RWSEM_H
+#define _I386_RWSEM_H
+
+#ifndef _LINUX_RWSEM_H
+#error please dont include asm/rwsem.h directly, use linux/rwsem.h instead
+#endif
+
+#ifdef __KERNEL__
+
+#include <linux/list.h>
+#include <linux/spinlock.h>
+
+struct rwsem_waiter;
+
+extern struct rw_semaphore *FASTCALL(rwsem_down_read_failed(struct rw_semaphore *sem));
+extern struct rw_semaphore *FASTCALL(rwsem_down_write_failed(struct rw_semaphore *sem));
+extern struct rw_semaphore *FASTCALL(rwsem_wake(struct rw_semaphore *));
+
+/*
+ * the semaphore definition
+ */
+struct rw_semaphore {
+	signed long		count;
+#define RWSEM_UNLOCKED_VALUE		0x00000000
+#define RWSEM_ACTIVE_BIAS		0x00000001
+#define RWSEM_ACTIVE_MASK		0x0000ffff
+#define RWSEM_WAITING_BIAS		(-0x00010000)
+#define RWSEM_ACTIVE_READ_BIAS		RWSEM_ACTIVE_BIAS
+#define RWSEM_ACTIVE_WRITE_BIAS		(RWSEM_WAITING_BIAS + RWSEM_ACTIVE_BIAS)
+	spinlock_t		wait_lock;
+	struct list_head	wait_list;
+#if RWSEM_DEBUG
+	int			debug;
+#endif
+};
+
+/*
+ * initialisation
+ */
+#if RWSEM_DEBUG
+#define __RWSEM_DEBUG_INIT      , 0
+#else
+#define __RWSEM_DEBUG_INIT	/* */
+#endif
+
+#define __RWSEM_INITIALIZER(name) \
+{ RWSEM_UNLOCKED_VALUE, SPIN_LOCK_UNLOCKED, LIST_HEAD_INIT((name).wait_list) \
+	__RWSEM_DEBUG_INIT }
+
+#define DECLARE_RWSEM(name) \
+	struct rw_semaphore name = __RWSEM_INITIALIZER(name)
+
+static inline void init_rwsem(struct rw_semaphore *sem)
+{
+	sem->count = RWSEM_UNLOCKED_VALUE;
+	spin_lock_init(&sem->wait_lock);
+	INIT_LIST_HEAD(&sem->wait_list);
+#if RWSEM_DEBUG
+	sem->debug = 0;
+#endif
+}
+
+/*
+ * lock for reading
+ */
+static inline void __down_read(struct rw_semaphore *sem)
+{
+	__asm__ __volatile__(
+		"# beginning down_read\n\t"
+LOCK_PREFIX	"  incl      (%%eax)\n\t" /* adds 0x00000001, returns the old value */
+		"  js        2f\n\t" /* jump if we weren't granted the lock */
+		"1:\n\t"
+		".section .text.lock,\"ax\"\n"
+		"2:\n\t"
+		"  pushl     %%ecx\n\t"
+		"  pushl     %%edx\n\t"
+		"  call      rwsem_down_read_failed\n\t"
+		"  popl      %%edx\n\t"
+		"  popl      %%ecx\n\t"
+		"  jmp       1b\n"
+		".previous"
+		"# ending down_read\n\t"
+		: "+m"(sem->count)
+		: "a"(sem)
+		: "memory", "cc");
+}
+
+/*
+ * lock for writing
+ */
+static inline void __down_write(struct rw_semaphore *sem)
+{
+	int tmp;
+
+	tmp = RWSEM_ACTIVE_WRITE_BIAS;
+	__asm__ __volatile__(
+		"# beginning down_write\n\t"
+LOCK_PREFIX	"  xadd      %0,(%%eax)\n\t" /* subtract 0x0000ffff, returns the old value */
+		"  testl     %0,%0\n\t" /* was the count 0 before? */
+		"  jnz       2f\n\t" /* jump if we weren't granted the lock */
+		"1:\n\t"
+		".section .text.lock,\"ax\"\n"
+		"2:\n\t"
+		"  pushl     %%ecx\n\t"
+		"  call      rwsem_down_write_failed\n\t"
+		"  popl      %%ecx\n\t"
+		"  jmp       1b\n"
+		".previous\n"
+		"# ending down_write"
+		: "+d"(tmp), "+m"(sem->count)
+		: "a"(sem)
+		: "memory", "cc");
+}
+
+/*
+ * unlock after reading
+ */
+static inline void __up_read(struct rw_semaphore *sem)
+{
+	__s32 tmp = -RWSEM_ACTIVE_READ_BIAS;
+	__asm__ __volatile__(
+		"# beginning __up_read\n\t"
+LOCK_PREFIX	"  xadd      %%edx,(%%eax)\n\t" /* subtracts 1, returns the old value */
+		"  js        2f\n\t" /* jump if the lock is being waited upon */
+		"1:\n\t"
+		".section .text.lock,\"ax\"\n"
+		"2:\n\t"
+		"  decw      %%dx\n\t" /* do nothing if still outstanding active readers */
+		"  jnz       1b\n\t"
+		"  pushl     %%ecx\n\t"
+		"  call      rwsem_wake\n\t"
+		"  popl      %%ecx\n\t"
+		"  jmp       1b\n"
+		".previous\n"
+		"# ending __up_read\n"
+		: "+m"(sem->count), "+d"(tmp)
+		: "a"(sem)
+		: "memory", "cc");
+}
+
+/*
+ * unlock after writing
+ */
+static inline void __up_write(struct rw_semaphore *sem)
+{
+	__asm__ __volatile__(
+		"# beginning __up_write\n\t"
+		"  movl      %2,%%edx\n\t"
+LOCK_PREFIX	"  xaddl     %%edx,(%%eax)\n\t" /* tries to transition 0xffff0001 -> 0x00000000 */
+		"  jnz       2f\n\t" /* jump if the lock is being waited upon */
+		"1:\n\t"
+		".section .text.lock,\"ax\"\n"
+		"2:\n\t"
+		"  decw      %%dx\n\t" /* did the active count reduce to 0? */
+		"  jnz       1b\n\t" /* jump back if not */
+		"  pushl     %%ecx\n\t"
+		"  call      rwsem_wake\n\t"
+		"  popl      %%ecx\n\t"
+		"  jmp       1b\n"
+		".previous\n"
+		"# ending __up_write\n"
+		: "+m"(sem->count)
+		: "a"(sem), "i"(-RWSEM_ACTIVE_WRITE_BIAS)
+		: "memory", "cc", "edx");
+}
+
+/*
+ * implement atomic add functionality
+ */
+static inline void rwsem_atomic_add(int delta, struct rw_semaphore *sem)
+{
+	__asm__ __volatile__(
+LOCK_PREFIX	"addl %1,%0"
+		:"=m"(sem->count)
+		:"ir"(delta), "m"(sem->count));
+}
+
+/*
+ * implement exchange and add functionality
+ */
+static inline int rwsem_atomic_update(int delta, struct rw_semaphore *sem)
+{
+	int tmp = delta;
+
+	__asm__ __volatile__(
+LOCK_PREFIX	"xadd %0,(%2)"
+		: "+r"(tmp), "=m"(sem->count)
+		: "r"(sem), "m"(sem->count)
+		: "memory");
+
+	return tmp+delta;
+}
+
+#endif /* __KERNEL__ */
+#endif /* _I386_RWSEM_H */