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SubjectRe: [PATCH] read/write semaphore trylock routines - 2.4.6
David Howells wrote:
> Given that the implementation in that asm-i386/rwsem.h is based around usage
> of the XADD instruction which exists everywhere the CMPXCHG instruction does
> (I believe), you don't really need to put the #if clause around it.
>
> You should also add spinlock versions too, which will cover most other archs.


Sorry, David. If I had read the copyright line for rwsem.h I would
have sent the original e-mail to you.

I've added the generic spinlock versions. I've also renamed the i386
versions to __down_*_trylock() and added stubs to
include/linux/rwsem.h, as you did for the up_*() and down_*()
routines.

The generic versions were compiled but not tested. Probably not a big
deal since I snagged most of the code from your __down_*() routines.

--
Brian Watson | "The common people of England... so
Linux Kernel Developer | jealous of their liberty, but like the
SSI Clustering Laboratory | common people of most other countries
Compaq Computer Corp | never rightly considering wherein it
Los Angeles, CA | consists..."
| -Adam Smith, Wealth of Nations, 1776

mailto:Brian.J.Watson@compaq.com
http://opensource.compaq.com/diff -ur linux/include/asm-i386/rwsem.h rwsem_trylock/include/asm-i386/rwsem.h
--- linux/include/asm-i386/rwsem.h Fri Apr 27 15:48:24 2001
+++ rwsem_trylock/include/asm-i386/rwsem.h Tue Jul 10 19:32:05 2001
@@ -117,6 +117,24 @@
}

/*
+ * trylock for reading -- returns 1 if successful, 0 if contention
+ */
+static inline int __down_read_trylock(struct rw_semaphore *sem)
+{
+ signed long old, new;
+
+repeat:
+ old = (volatile signed long)sem->count;
+ if (old < RWSEM_UNLOCKED_VALUE)
+ return 0;
+ new = old + RWSEM_ACTIVE_READ_BIAS;
+ if (cmpxchg(&sem->count, old, new) == old)
+ return 1;
+ else
+ goto repeat;
+}
+
+/*
* lock for writing
*/
static inline void __down_write(struct rw_semaphore *sem)
@@ -141,6 +159,19 @@
: "+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 -ur linux/include/linux/rwsem-spinlock.h rwsem_trylock/include/linux/rwsem-spinlock.h
--- linux/include/linux/rwsem-spinlock.h Wed Jul 4 14:20:44 2001
+++ rwsem_trylock/include/linux/rwsem-spinlock.h Tue Jul 10 19:32:05 2001
@@ -54,7 +54,9 @@

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 -ur linux/include/linux/rwsem.h rwsem_trylock/include/linux/rwsem.h
--- linux/include/linux/rwsem.h Wed Jul 4 14:20:44 2001
+++ rwsem_trylock/include/linux/rwsem.h Tue Jul 10 19:32:05 2001
@@ -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)
@@ -53,6 +65,18 @@
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 -ur linux/lib/rwsem-spinlock.c rwsem_trylock/lib/rwsem-spinlock.c
--- linux/lib/rwsem-spinlock.c Wed Apr 25 13:31:03 2001
+++ rwsem_trylock/lib/rwsem-spinlock.c Tue Jul 10 18:59:24 2001
@@ -149,6 +149,28 @@
}

/*
+ * 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++;
+ spin_unlock(&sem->wait_lock);
+ ret = 1;
+ }
+
+ 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 +214,28 @@

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;
+ spin_unlock(&sem->wait_lock);
+ ret = 1;
+ }
+
+ rwsemtrace(sem,"Leaving __down_write_trylock");
+ return ret;
}

/*
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