RE: write_unlock: replace clear_bit with byte store

Christoph Lameter
2005-04-29T00:50:53Z

On Thu, 28 Apr 2005, Chen, Kenneth W wrote:

> Christoph Lameter wrote on Thursday, April 28, 2005 5:22 PM
> > How do I do a store with release and nontemporal semantics without asm?
>
> Use ia64_st1_rel(), and add a wrapper in gcc_intrin.h. Though that only
> takes care of store with release semantics.

Hmm... How about this one. Its still better than the cmpxchg for C and you
can do all the tricks you want with the C code later. I hope the bitfields
allocate the lowest bits first?

---

write_lock uses a cmpxchg like the regular spin_lock but write_unlock uses
clear_bit which requires a load and then a loop over a cmpxchg. The
following patch makes write_unlock simply use a nontemporal store to clear
the highest 8 bits. We will then still have the lower 3 bytes (24 bits)
left to count the readers.

Signed-off-by: Christoph Lameter <clameter@sgi.com>

Index: linux-2.6.11/include/asm-ia64/spinlock.h
=================================--- linux-2.6.11.orig/include/asm-ia64/spinlock.h 2005-03-01 23:37:48.000000000 -0800
+++ linux-2.6.11/include/asm-ia64/spinlock.h 2005-04-28 17:48:11.000000000 -0700
@@ -117,8 +117,8 @@ do { \
#define spin_unlock_wait(x) do { barrier(); } while ((x)->lock)

typedef struct {
- volatile unsigned int read_counter : 31;
- volatile unsigned int write_lock : 1;
+ volatile unsigned int read_counter : 24;
+ volatile unsigned int write_lock : 8;
#ifdef CONFIG_PREEMPT
unsigned int break_lock;
#endif
@@ -174,6 +174,13 @@ do { \
(result = 0); \
})

+static inline void _raw_write_unlock(rwlock_t *x)
+{
+ u8 *y = (u8 *)x;
+ barrier();
+ asm volatile ("st1.rel.nta [%0] = r0\n\t" :: "r"(y+3) : "memory" );
+}
+
#else /* !ASM_SUPPORTED */

#define _raw_write_lock(l) \
@@ -195,14 +202,14 @@ do { \
(ia64_val = 0); \
})

+static inline void _raw_write_unlock(rwlock_t *x)
+{
+ barrier();
+ x->write_lock = 0;
+}
+
#endif /* !ASM_SUPPORTED */

#define _raw_read_trylock(lock) generic_raw_read_trylock(lock)

-#define _raw_write_unlock(x) \
-({ \
- smp_mb__before_clear_bit(); /* need barrier before releasing lock... */ \
- clear_bit(31, (x)); \
-})
-
#endif /* _ASM_IA64_SPINLOCK_H */