It was part of the patch I sent to David. In fact that was the first
implementation of it at all..
The full patch I sent around is attached as reference.
Index: include/asm-i386/rwsem.h
===================================================================
RCS file: /cvs/linux-2.4-xfs/linux/include/asm-i386/rwsem.h,v
retrieving revision 1.3
diff -u -p -r1.3 rwsem.h
--- include/asm-i386/rwsem.h 2002/05/29 22:20:17 1.3
+++ include/asm-i386/rwsem.h 2002/07/28 10:07:35
@@ -46,6 +46,7 @@ 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 *));
+extern struct rw_semaphore *FASTCALL(rwsem_downgrade_write(struct rw_semaphore *sem));
/*
* the semaphore definition
@@ -117,6 +118,29 @@ LOCK_PREFIX " incl (%%eax)\n\t" /*
}
/*
+ * 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)
@@ -144,6 +168,19 @@ LOCK_PREFIX " xadd %0,(%%eax)\n\t"
}
/*
+ * 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)
@@ -184,6 +221,7 @@ LOCK_PREFIX " xaddl %%edx,(%%eax)\n
"2:\n\t"
" decw %%dx\n\t" /* did the active count reduce to 0? */
" jnz 1b\n\t" /* jump back if not */
+ " pushl $1\n\t"
" pushl %%ecx\n\t"
" call rwsem_wake\n\t"
" popl %%ecx\n\t"
@@ -194,6 +232,35 @@ LOCK_PREFIX " xaddl %%edx,(%%eax)\n
: "a"(sem), "i"(-RWSEM_ACTIVE_WRITE_BIAS)
: "memory", "cc", "edx");
}
+
+/*
+ * downgrade write lock to read lock
+ */
+static inline void __downgrade_write(struct rw_semaphore *sem)
+{
+ __asm__ __volatile__(
+ "# beginning __downgrade_write\n\t"
+LOCK_PREFIX " addl %2,(%%eax)\n\t" /* transitions 0xZZZZ0001 -> 0xYYYY0001 */
+ " js 2f\n\t" /* jump if the lock is being waited upon */
+ "1:\n\t"
+ LOCK_SECTION_START("")
+ "2:\n\t"
+ " pushl %%ecx\n\t"
+ " pushl %%edx\n\t"
+ " call rwsem_downgrade_wake\n\t"
+ " popl %%edx\n\t"
+ " popl %%ecx\n\t"
+ " jmp 1b\n"
+ LOCK_SECTION_END
+ "# ending __downgrade_write\n"
+ : "=m"(sem->count)
+ : "a"(sem), "i"(-RWSEM_WAITING_BIAS), "m"(sem->count)
+ : "memory", "cc");
+}
+
+/*
+ * implement atomic add functionality
+ */
/*
* implement atomic add functionality
Index: include/linux/rwsem-spinlock.h
===================================================================
RCS file: /cvs/linux-2.4-xfs/linux/include/linux/rwsem-spinlock.h,v
retrieving revision 1.2
diff -u -p -r1.2 rwsem-spinlock.h
--- include/linux/rwsem-spinlock.h 2001/05/29 19:53:13 1.2
+++ include/linux/rwsem-spinlock.h 2002/07/28 10:07:37
@@ -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 */
Index: include/linux/rwsem.h
===================================================================
RCS file: /cvs/linux-2.4-xfs/linux/include/linux/rwsem.h,v
retrieving revision 1.2
diff -u -p -r1.2 rwsem.h
--- include/linux/rwsem.h 2001/05/29 19:53:13 1.2
+++ include/linux/rwsem.h 2002/07/28 10:07:37
@@ -46,6 +46,18 @@ static inline void down_read(struct rw_s
}
/*
+ * 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 @@ static inline void down_write(struct rw_
}
/*
+ * 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)
@@ -73,6 +97,16 @@ static inline void up_write(struct rw_se
rwsemtrace(sem,"Entering up_write");
__up_write(sem);
rwsemtrace(sem,"Leaving up_write");
+}
+
+/*
+ * downgrade write lock to read lock
+ */
+static inline void downgrade_write(struct rw_semaphore *sem)
+{
+ rwsemtrace(sem,"Entering downgrade_write");
+ __downgrade_write(sem);
+ rwsemtrace(sem,"Leaving downgrade_write");
}
Index: lib/rwsem-spinlock.c
===================================================================
RCS file: /cvs/linux-2.4-xfs/linux/lib/rwsem-spinlock.c,v
retrieving revision 1.1
diff -u -p -r1.1 rwsem-spinlock.c
--- lib/rwsem-spinlock.c 2001/05/02 06:22:13 1.1
+++ lib/rwsem-spinlock.c 2002/07/28 10:07:37
@@ -46,8 +46,9 @@ void init_rwsem(struct rw_semaphore *sem
* - 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 @@ static inline struct rw_semaphore *__rws
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 @@ static inline struct rw_semaphore *__rws
}
/* 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 @@ void __down_read(struct rw_semaphore *se
}
/*
+ * 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 @@ void __down_write(struct rw_semaphore *s
}
/*
+ * 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 @@ void __up_write(struct rw_semaphore *sem
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
Index: lib/rwsem.c
===================================================================
RCS file: /cvs/linux-2.4-xfs/linux/lib/rwsem.c,v
retrieving revision 1.2
diff -u -p -r1.2 rwsem.c
--- lib/rwsem.c 2001/07/11 17:53:24 1.2
+++ lib/rwsem.c 2002/07/28 10:07:37
@@ -34,8 +34,9 @@ void rwsemtrace(struct rw_semaphore *sem
* - there must be someone on the queue
* - 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;
struct list_head *next;
@@ -44,6 +45,9 @@ static inline struct rw_semaphore *__rws
rwsemtrace(sem,"Entering __rwsem_do_wake");
+ if (!wakewrite)
+ goto dont_wake_writers;
+
/* only wake someone up if we can transition the active part of the count from 0 -> 1 */
try_again:
oldcount = rwsem_atomic_update(RWSEM_ACTIVE_BIAS,sem) - RWSEM_ACTIVE_BIAS;
@@ -64,6 +68,12 @@ static inline struct rw_semaphore *__rws
wake_up_process(waiter->task);
goto out;
+ /* don't want to wake any writers */
+ dont_wake_writers:
+ waiter = list_entry(sem->wait_list.next,struct rwsem_waiter,list);
+ if (waiter->flags & RWSEM_WAITING_FOR_WRITE)
+ goto out;
+
/* grant an infinite number of read locks to the readers at the front of the queue
* - note we increment the 'active part' of the count by the number of readers (less one
* for the activity decrement we've already done) before waking any processes up
@@ -132,7 +142,7 @@ static inline struct rw_semaphore *rwsem
* - it might even be this process, since the waker takes a more active part
*/
if (!(count & RWSEM_ACTIVE_MASK))
- sem = __rwsem_do_wake(sem);
+ sem = __rwsem_do_wake(sem,1);
spin_unlock(&sem->wait_lock);
@@ -193,7 +203,7 @@ struct rw_semaphore *rwsem_wake(struct r
/* do nothing if list empty */
if (!list_empty(&sem->wait_list))
- sem = __rwsem_do_wake(sem);
+ sem = __rwsem_do_wake(sem,1);
spin_unlock(&sem->wait_lock);
@@ -202,9 +212,31 @@ struct rw_semaphore *rwsem_wake(struct r
return sem;
}
+/*
+ * downgrade a write lock into a read lock
+ * - caller incremented waiting part of count, and discovered it to be still negative
+ * - just wake up any readers at the front of the queue
+ */
+struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem)
+{
+ rwsemtrace(sem,"Entering rwsem_downgrade_wake");
+
+ spin_lock(&sem->wait_lock);
+
+ /* do nothing if list empty */
+ if (!list_empty(&sem->wait_list))
+ sem = __rwsem_do_wake(sem,0);
+
+ spin_unlock(&sem->wait_lock);
+
+ rwsemtrace(sem,"Leaving rwsem_downgrade_wake");
+ return sem;
+}
+
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
-
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