[PATCH] rwsem trylock and downgrade part II

[David Howells <dhowells@redhat.com>]

Hi Linus, Christoph,

I'm not sure what happened, but I think I applied an old incomplete patch when
patching 2.5.28, one that was missing Christoph's rwsem-spinlock stuff and the
trylock stuff. The attached patch adds these back (as per Christoph's patch),
it also adds a missing export to lib/rwsem.c.

When compiled for PentiumIII SMP, I see:

	dhowells>nm vmlinux | grep rwsem
	c01c5dbd t .text.lock.rwsem
	c02995a0 ? __kstrtab_rwsem_down_read_failed
	c02995b7 ? __kstrtab_rwsem_down_write_failed
	c02995da ? __kstrtab_rwsem_downgrade_wake
	c02995cf ? __kstrtab_rwsem_wake
	c02a1cf8 ? __ksymtab_rwsem_down_read_failed
	c02a1d00 ? __ksymtab_rwsem_down_write_failed
	c02a1d10 ? __ksymtab_rwsem_downgrade_wake
	c02a1d08 ? __ksymtab_rwsem_wake
	c01c5870 T rwsem_down_read_failed
	c01c5a20 T rwsem_down_write_failed
	c01c5cf0 T rwsem_downgrade_wake
	c01c5bd0 T rwsem_wake

When compiled for i386 SMP, I see:

	dhowells>nm vmlinux | grep 'rwsem\|__\(up\|down.*\)_\(read\|write\)'
	c01be49a t .text.lock.rwsem_spinlock
	c01bdf8c T __down_read
	c01be080 T __down_read_trylock
	c01be0f4 T __down_write
	c01be1ec T __down_write_trylock
	c01be3e4 T __downgrade_write
	c028c0f9 ? __kstrtab___down_read
	c028c120 ? __kstrtab___down_read_trylock
	c028c142 ? __kstrtab___down_write
	c028c160 ? __kstrtab___down_write_trylock
	c028c1c0 ? __kstrtab___downgrade_write
	c028c183 ? __kstrtab___up_read
	c028c19b ? __kstrtab___up_write
	c028c0e0 ? __kstrtab_init_rwsem
	c0295b80 ? __ksymtab___down_read
	c0295b88 ? __ksymtab___down_read_trylock
	c0295b90 ? __ksymtab___down_write
	c0295b98 ? __ksymtab___down_write_trylock
	c0295bb0 ? __ksymtab___downgrade_write
	c0295ba0 ? __ksymtab___up_read
	c0295ba8 ? __ksymtab___up_write
	c0295b78 ? __ksymtab_init_rwsem
	c01be264 T __up_read
	c01be2fc T __up_write
	c01bdf6c T init_rwsem

So I think I've got all the functions there and that they're all exported now.

David

diff -uNr linux-2.5.29/include/asm-i386/rwsem.h linux-rwsem-2529/include/asm-i386/rwsem.h
--- linux-2.5.29/include/asm-i386/rwsem.h	Mon Jul 29 08:26:56 2002
+++ linux-rwsem-2529/include/asm-i386/rwsem.h	Mon Jul 29 08:32:49 2002
@@ -118,6 +118,29 @@
 }
 
 /*
+ * 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)
@@ -145,6 +168,19 @@
 }
 
 /*
+ * 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
  */
 static inline void __up_read(struct rw_semaphore *sem)
diff -uNr linux-2.5.29/include/linux/rwsem-spinlock.h linux-rwsem-2529/include/linux/rwsem-spinlock.h
--- linux-2.5.29/include/linux/rwsem-spinlock.h	Thu Jul 11 19:35:21 2002
+++ linux-rwsem-2529/include/linux/rwsem-spinlock.h	Mon Jul 29 08:32:49 2002
@@ -54,9 +54,12 @@
 
 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 -uNr linux-2.5.29/include/linux/rwsem.h linux-rwsem-2529/include/linux/rwsem.h
--- linux-2.5.29/include/linux/rwsem.h	Mon Jul 29 08:26:57 2002
+++ linux-rwsem-2529/include/linux/rwsem.h	Mon Jul 29 08:32:49 2002
@@ -46,6 +46,18 @@
 }
 
 /*
+ * 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)
@@ -56,6 +68,18 @@
 }
 
 /*
+ * 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
  */
 static inline void up_read(struct rw_semaphore *sem)
@@ -85,6 +109,5 @@
 	rwsemtrace(sem,"Leaving downgrade_write");
 }
 
-
 #endif /* __KERNEL__ */
 #endif /* _LINUX_RWSEM_H */
diff -uNr linux-2.5.29/lib/rwsem-spinlock.c linux-rwsem-2529/lib/rwsem-spinlock.c
--- linux-2.5.29/lib/rwsem-spinlock.c	Thu Jul 11 19:35:17 2002
+++ linux-rwsem-2529/lib/rwsem-spinlock.c	Mon Jul 29 08:32:49 2002
@@ -46,8 +46,9 @@
  *   - the 'waiting count' is non-zero
  * - the spinlock must be held by the caller
  * - woken process blocks are discarded from the list after having flags zeroised
+ * - writers are only woken if wakewrite is non-zero
  */
-static inline struct rw_semaphore *__rwsem_do_wake(struct rw_semaphore *sem)
+static inline struct rw_semaphore *__rwsem_do_wake(struct rw_semaphore *sem, int wakewrite)
 {
 	struct rwsem_waiter *waiter;
 	int woken;
@@ -56,7 +57,14 @@
 
 	waiter = list_entry(sem->wait_list.next,struct rwsem_waiter,list);
 
-	/* try to grant a single write lock if there's a writer at the front of the queue
+	if (!wakewrite) {
+		if (waiter->flags & RWSEM_WAITING_FOR_WRITE)
+			goto out;
+		goto dont_wake_writers;
+	}
+
+	/* if we are allowed to wake writers try to grant a single write lock if there's a
+	 * writer at the front of the queue
 	 * - we leave the 'waiting count' incremented to signify potential contention
 	 */
 	if (waiter->flags & RWSEM_WAITING_FOR_WRITE) {
@@ -68,16 +76,19 @@
 	}
 
 	/* grant an infinite number of read locks to the readers at the front of the queue */
+ dont_wake_writers:
 	woken = 0;
-	do {
+	while (waiter->flags&RWSEM_WAITING_FOR_READ) {
+		struct list_head *next = waiter->list.next;
+
 		list_del(&waiter->list);
 		waiter->flags = 0;
 		wake_up_process(waiter->task);
 		woken++;
 		if (list_empty(&sem->wait_list))
 			break;
-		waiter = list_entry(sem->wait_list.next,struct rwsem_waiter,list);
-	} while (waiter->flags&RWSEM_WAITING_FOR_READ);
+		waiter = list_entry(next,struct rwsem_waiter,list);
+	}
 
 	sem->activity += woken;
 
@@ -149,6 +160,28 @@
 }
 
 /*
+ * 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
  */
@@ -195,6 +228,28 @@
 }
 
 /*
+ * 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;
+}
+
+/*
  * release a read lock on the semaphore
  */
 void __up_read(struct rw_semaphore *sem)
@@ -222,18 +277,40 @@
 
 	sem->activity = 0;
 	if (!list_empty(&sem->wait_list))
-		sem = __rwsem_do_wake(sem);
+		sem = __rwsem_do_wake(sem, 1);
 
 	spin_unlock(&sem->wait_lock);
 
 	rwsemtrace(sem,"Leaving __up_write");
 }
 
+/*
+ * downgrade a write lock into a read lock
+ * - just wake up any readers at the front of the queue
+ */
+void __downgrade_write(struct rw_semaphore *sem)
+{
+	rwsemtrace(sem,"Entering __rwsem_downgrade");
+
+	spin_lock(&sem->wait_lock);
+
+	sem->activity = 1;
+	if (!list_empty(&sem->wait_list))
+		sem = __rwsem_do_wake(sem,0);
+
+	spin_unlock(&sem->wait_lock);
+
+	rwsemtrace(sem,"Leaving __rwsem_downgrade");
+}
+
 EXPORT_SYMBOL(init_rwsem);
 EXPORT_SYMBOL(__down_read);
+EXPORT_SYMBOL(__down_read_trylock);
 EXPORT_SYMBOL(__down_write);
+EXPORT_SYMBOL(__down_write_trylock);
 EXPORT_SYMBOL(__up_read);
 EXPORT_SYMBOL(__up_write);
+EXPORT_SYMBOL(__downgrade_write);
 #if RWSEM_DEBUG
 EXPORT_SYMBOL(rwsemtrace);
 #endif
diff -uNr linux-2.5.29/lib/rwsem.c linux-rwsem-2529/lib/rwsem.c
--- linux-2.5.29/lib/rwsem.c	Mon Jul 29 08:26:57 2002
+++ linux-rwsem-2529/lib/rwsem.c	Mon Jul 29 08:35:47 2002
@@ -236,6 +236,7 @@
 EXPORT_SYMBOL_NOVERS(rwsem_down_read_failed);
 EXPORT_SYMBOL_NOVERS(rwsem_down_write_failed);
 EXPORT_SYMBOL_NOVERS(rwsem_wake);
+EXPORT_SYMBOL_NOVERS(rwsem_downgrade_wake);
 #if RWSEM_DEBUG
 EXPORT_SYMBOL(rwsemtrace);
 #endif