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Subject[PATCH 05/12] x86 rwsem: take advantage of new RWSEM_WAITING_BIAS semantics
Date
Now that an arbitrary number of waiting threads are represented by
a single RWSEM_WAITING_BIAS, the up_read and up_write code paths can be
slightly simplified by checking for waiting threads and no active threads
all at once.

Signed-off-by: Michel Lespinasse <walken@google.com>
---
arch/x86/include/asm/rwsem.h | 23 ++++++++++++++---------
arch/x86/lib/rwsem_64.S | 4 +---
arch/x86/lib/semaphore_32.S | 4 +---
3 files changed, 16 insertions(+), 15 deletions(-)

diff --git a/arch/x86/include/asm/rwsem.h b/arch/x86/include/asm/rwsem.h
index 606ede1..a15b84d 100644
--- a/arch/x86/include/asm/rwsem.h
+++ b/arch/x86/include/asm/rwsem.h
@@ -118,7 +118,7 @@ static inline void __down_read(struct rw_semaphore *sem)
{
asm volatile("# beginning down_read\n\t"
LOCK_PREFIX _ASM_INC "(%1)\n\t"
- /* adds 0x00000001, returns the old value */
+ /* adds 0x00000001 */
" jns 1f\n"
" call call_rwsem_down_read_failed\n"
"1:\n\t"
@@ -160,7 +160,7 @@ static inline void __down_write_nested(struct rw_semaphore *sem, int subclass)
tmp = RWSEM_ACTIVE_WRITE_BIAS;
asm volatile("# beginning down_write\n\t"
LOCK_PREFIX " xadd %1,(%2)\n\t"
- /* subtract 0x0000ffff, returns the old value */
+ /* adds 0xffff0001, returns the old value */
" test %1,%1\n\t"
/* was the count 0 before? */
" jz 1f\n"
@@ -199,12 +199,15 @@ static inline void __up_read(struct rw_semaphore *sem)
asm volatile("# beginning __up_read\n\t"
LOCK_PREFIX " xadd %1,(%2)\n\t"
/* subtracts 1, returns the old value */
- " jns 1f\n\t"
+ " cmp %4,%1\n\t"
+ /* are there waiting threads and no active threads ? */
+ " jne 1f\n"
" call call_rwsem_wake\n"
"1:\n"
"# ending __up_read\n"
: "+m" (sem->count), "=d" (tmp)
- : "a" (sem), "1" (tmp)
+ : "a" (sem), "1" (tmp),
+ "er" (RWSEM_WAITING_BIAS + RWSEM_ACTIVE_READ_BIAS)
: "memory", "cc");
}

@@ -213,17 +216,19 @@ static inline void __up_read(struct rw_semaphore *sem)
*/
static inline void __up_write(struct rw_semaphore *sem)
{
- rwsem_count_t tmp;
+ rwsem_count_t tmp = -RWSEM_ACTIVE_WRITE_BIAS;
asm volatile("# beginning __up_write\n\t"
LOCK_PREFIX " xadd %1,(%2)\n\t"
- /* tries to transition
- 0xffff0001 -> 0x00000000 */
- " jz 1f\n"
+ /* substracts 0xffff0001, returns the old value */
+ " cmp %4,%1\n\t"
+ /* are there waiting threads and no active threads ? */
+ " jne 1f\n"
" call call_rwsem_wake\n"
"1:\n\t"
"# ending __up_write\n"
: "+m" (sem->count), "=d" (tmp)
- : "a" (sem), "1" (-RWSEM_ACTIVE_WRITE_BIAS)
+ : "a" (sem), "1" (tmp),
+ "er" (RWSEM_WAITING_BIAS + RWSEM_ACTIVE_WRITE_BIAS)
: "memory", "cc");
}

diff --git a/arch/x86/lib/rwsem_64.S b/arch/x86/lib/rwsem_64.S
index 41fcf00..770a387 100644
--- a/arch/x86/lib/rwsem_64.S
+++ b/arch/x86/lib/rwsem_64.S
@@ -60,13 +60,11 @@ ENTRY(call_rwsem_down_write_failed)
ENDPROC(call_rwsem_down_write_failed)

ENTRY(call_rwsem_wake)
- decl %edx /* do nothing if still outstanding active readers */
- jnz 1f
save_common_regs
movq %rax,%rdi
call rwsem_wake
restore_common_regs
-1: ret
+ ret
ENDPROC(call_rwsem_wake)

/* Fix up special calling conventions */
diff --git a/arch/x86/lib/semaphore_32.S b/arch/x86/lib/semaphore_32.S
index 648fe47..63dbf75 100644
--- a/arch/x86/lib/semaphore_32.S
+++ b/arch/x86/lib/semaphore_32.S
@@ -103,15 +103,13 @@ ENTRY(call_rwsem_down_write_failed)

ENTRY(call_rwsem_wake)
CFI_STARTPROC
- decw %dx /* do nothing if still outstanding active readers */
- jnz 1f
push %ecx
CFI_ADJUST_CFA_OFFSET 4
CFI_REL_OFFSET ecx,0
call rwsem_wake
pop %ecx
CFI_ADJUST_CFA_OFFSET -4
-1: ret
+ ret
CFI_ENDPROC
ENDPROC(call_rwsem_wake)

--
1.7.0.1


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