[PATCH] 2.4.19-pre2, trylock for read/write semaphores

Marcelo-

Again I'm submitting a patch that adds trylock routines for 
read/write semaphores. I haven't seen it appear in either 
2.4.18 or 2.4.19-pre2.

David Howells has reviewed it and thinks its ready to be 
submitted. I've tested it and I'm confident it works correctly.
It doesn't affect the stability of the kernel because nobody's
using it, yet. All comments and concerns raised on LKML about 
this patch have been addressed. Both Christoph Hellwig and 
Kendrick Smith have expressed interest in it for the file 
system work they're doing.

Is there anything else I need to do to get it accepted into 2.4?

Brian Watson
Open SSI Clustering Project
Compaq Computer Corp
http://opensource.compaq.com/

diff -Naur linux-2.4.19-pre2/include/asm-i386/rwsem.h rwsem/include/asm-i386/rwsem.h
--- linux-2.4.19-pre2/include/asm-i386/rwsem.h	Thu Feb 28 15:16:13 2002
+++ rwsem/include/asm-i386/rwsem.h	Thu Feb 28 15:22:18 2002
@@ -4,6 +4,8 @@
  *
  * Derived from asm-i386/semaphore.h
  *
+ * Trylock by Brian Watson ([e-mail]).
+ *
  *
  * 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
@@ -117,6 +119,29 @@
 }
 
 /*
+ * 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
  */
 static inline void __down_write(struct rw_semaphore *sem)
@@ -141,6 +166,19 @@
 		: "+d"(tmp), "+m"(sem->count)
 		: "a"(sem)
 		: "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;
 }
 
 /*
diff -Naur linux-2.4.19-pre2/include/linux/rwsem-spinlock.h rwsem/include/linux/rwsem-spinlock.h
--- linux-2.4.19-pre2/include/linux/rwsem-spinlock.h	Thu Nov 22 11:46:19 2001
+++ rwsem/include/linux/rwsem-spinlock.h	Thu Feb 28 15:22:18 2002
@@ -3,6 +3,8 @@
  * Copyright (c) 2001   David Howells ([e-mail]).
  * - Derived partially from ideas by Andrea Arcangeli <[e-mail]>
  * - Derived also from comments by Linus
+ *
+ * Trylock by Brian Watson ([e-mail]).
  */
 
 #ifndef _LINUX_RWSEM_SPINLOCK_H
@@ -54,7 +56,9 @@
 
 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));
 
diff -Naur linux-2.4.19-pre2/include/linux/rwsem.h rwsem/include/linux/rwsem.h
--- linux-2.4.19-pre2/include/linux/rwsem.h	Thu Nov 22 11:46:19 2001
+++ rwsem/include/linux/rwsem.h	Thu Feb 28 15:22:18 2002
@@ -2,6 +2,8 @@
  *
  * Written by David Howells ([e-mail]).
  * Derived from asm-i386/semaphore.h
+ *
+ * Trylock by Brian Watson ([e-mail]).
  */
 
 #ifndef _LINUX_RWSEM_H
@@ -46,6 +48,18 @@
 }
 
 /*
+ * 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
  */
 static inline void down_write(struct rw_semaphore *sem)
@@ -53,6 +67,18 @@
 	rwsemtrace(sem,"Entering down_write");
 	__down_write(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;
 }
 
 /*
diff -Naur linux-2.4.19-pre2/lib/rwsem-spinlock.c rwsem/lib/rwsem-spinlock.c
--- linux-2.4.19-pre2/lib/rwsem-spinlock.c	Wed Apr 25 13:31:03 2001
+++ rwsem/lib/rwsem-spinlock.c	Thu Feb 28 15:22:18 2002
@@ -4,6 +4,8 @@
  * Copyright (c) 2001   David Howells ([e-mail]).
  * - Derived partially from idea by Andrea Arcangeli <[e-mail]>
  * - Derived also from comments by Linus
+ *
+ * Trylock by Brian Watson ([e-mail]).
  */
 #include <linux/rwsem.h>
 #include <linux/sched.h>
@@ -149,6 +151,28 @@
 }
 
 /*
+ * trylock for reading -- returns 1 if successful, 0 if contention
+ */
+int __down_read_trylock(struct rw_semaphore *sem)
+{
+	int ret = 0;
+	rwsemtrace(sem,"Entering __down_read_trylock");
+
+	spin_lock(&sem->wait_lock);
+
+	if (sem->activity>=0 && list_empty(&sem->wait_list)) {
+		/* granted */
+		sem->activity++;
+		ret = 1;
+	}
+
+	spin_unlock(&sem->wait_lock);
+
+	rwsemtrace(sem,"Leaving __down_read_trylock");
+	return ret;
+}
+
+/*
  * get a write lock on the semaphore
  * - note that we increment the waiting count anyway to indicate an exclusive lock
  */
@@ -192,6 +216,28 @@
 
  out:
 	rwsemtrace(sem,"Leaving __down_write");
+}
+
+/*
+ * trylock for writing -- returns 1 if successful, 0 if contention
+ */
+int __down_write_trylock(struct rw_semaphore *sem)
+{
+	int ret = 0;
+	rwsemtrace(sem,"Entering __down_write_trylock");
+
+	spin_lock(&sem->wait_lock);
+
+	if (sem->activity==0 && list_empty(&sem->wait_list)) {
+		/* granted */
+		sem->activity = -1;
+		ret = 1;
+	}
+
+	spin_unlock(&sem->wait_lock);
+
+	rwsemtrace(sem,"Leaving __down_write_trylock");
+	return ret;
 }
 
 /*