From ee6536f8986463f0e5b6430fc1ce392957fb3de1 Mon Sep 17 00:00:00 2001
From: Christoph Hellwig <hch@sb.bsdonline.org>
Date: Mon, 29 Jul 2002 01:18:19 +0200
Subject: Implement down_read_trylock() and down_write_trylock() and add a
 generic spinlock implementation for downgrade_write().

---
 include/asm-i386/rwsem.h       | 36 ++++++++++++++++++++++++++++++++++++
 include/linux/rwsem-spinlock.h |  3 +++
 include/linux/rwsem.h          | 25 ++++++++++++++++++++++++-
 3 files changed, 63 insertions(+), 1 deletion(-)

(limited to 'include')

diff --git a/include/asm-i386/rwsem.h b/include/asm-i386/rwsem.h
index 72f2ae078a36..7d3a3f087ed3 100644
--- a/include/asm-i386/rwsem.h
+++ b/include/asm-i386/rwsem.h
@@ -117,6 +117,29 @@ LOCK_PREFIX	"  incl      (%%eax)\n\t" /* adds 0x00000001, returns the old value
 		: "memory", "cc");
 }
 
+/*
+ * trylock for reading -- returns 1 if successful, 0 if contention
+ */
+static inline int __down_read_trylock(struct rw_semaphore *sem)
+{
+	__s32 result, tmp;
+	__asm__ __volatile__(
+		"# beginning __down_read_trylock\n\t"
+		"  movl      %0,%1\n\t"
+		"1:\n\t"
+		"  movl	     %1,%2\n\t"
+		"  addl      %3,%2\n\t"
+		"  jle	     2f\n\t"
+LOCK_PREFIX	"  cmpxchgl  %2,%0\n\t"
+		"  jnz	     1b\n\t"
+		"2:\n\t"
+		"# ending __down_read_trylock\n\t"
+		: "+m"(sem->count), "=&a"(result), "=&r"(tmp)
+		: "i"(RWSEM_ACTIVE_READ_BIAS)
+		: "memory", "cc");
+	return result>=0 ? 1 : 0;
+}
+
 /*
  * lock for writing
  */
@@ -144,6 +167,19 @@ LOCK_PREFIX	"  xadd      %%edx,(%%eax)\n\t" /* subtract 0x0000ffff, returns the
 		: "memory", "cc");
 }
 
+/*
+ * trylock for writing -- returns 1 if successful, 0 if contention
+ */
+static inline int __down_write_trylock(struct rw_semaphore *sem)
+{
+	signed long ret = cmpxchg(&sem->count,
+				  RWSEM_UNLOCKED_VALUE, 
+				  RWSEM_ACTIVE_WRITE_BIAS);
+	if (ret == RWSEM_UNLOCKED_VALUE)
+		return 1;
+	return 0;
+}
+
 /*
  * unlock after reading
  */
diff --git a/include/linux/rwsem-spinlock.h b/include/linux/rwsem-spinlock.h
index 3087c5c101f4..f4ac435bf141 100644
--- a/include/linux/rwsem-spinlock.h
+++ b/include/linux/rwsem-spinlock.h
@@ -54,9 +54,12 @@ struct rw_semaphore {
 
 extern void FASTCALL(init_rwsem(struct rw_semaphore *sem));
 extern void FASTCALL(__down_read(struct rw_semaphore *sem));
+extern int FASTCALL(__down_read_trylock(struct rw_semaphore *sem));
 extern void FASTCALL(__down_write(struct rw_semaphore *sem));
+extern int FASTCALL(__down_write_trylock(struct rw_semaphore *sem));
 extern void FASTCALL(__up_read(struct rw_semaphore *sem));
 extern void FASTCALL(__up_write(struct rw_semaphore *sem));
+extern void FASTCALL(__downgrade_write(struct rw_semaphore *sem));
 
 #endif /* __KERNEL__ */
 #endif /* _LINUX_RWSEM_SPINLOCK_H */
diff --git a/include/linux/rwsem.h b/include/linux/rwsem.h
index 320138d6643d..4a7e2bb0d7c4 100644
--- a/include/linux/rwsem.h
+++ b/include/linux/rwsem.h
@@ -45,6 +45,18 @@ static inline void down_read(struct rw_semaphore *sem)
 	rwsemtrace(sem,"Leaving down_read");
 }
 
+/*
+ * trylock for reading -- returns 1 if successful, 0 if contention
+ */
+static inline int down_read_trylock(struct rw_semaphore *sem)
+{
+	int ret;
+	rwsemtrace(sem,"Entering down_read_trylock");
+	ret = __down_read_trylock(sem);
+	rwsemtrace(sem,"Leaving down_read_trylock");
+	return ret;
+}
+
 /*
  * lock for writing
  */
@@ -55,6 +67,18 @@ static inline void down_write(struct rw_semaphore *sem)
 	rwsemtrace(sem,"Leaving down_write");
 }
 
+/*
+ * trylock for writing -- returns 1 if successful, 0 if contention
+ */
+static inline int down_write_trylock(struct rw_semaphore *sem)
+{
+	int ret;
+	rwsemtrace(sem,"Entering down_write_trylock");
+	ret = __down_write_trylock(sem);
+	rwsemtrace(sem,"Leaving down_write_trylock");
+	return ret;
+}
+
 /*
  * release a read lock
  */
@@ -85,6 +109,5 @@ static inline void downgrade_write(struct rw_semaphore *sem)
 	rwsemtrace(sem,"Leaving downgrade_write");
 }
 
-
 #endif /* __KERNEL__ */
 #endif /* _LINUX_RWSEM_H */
-- 
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