From: Michal Hocko <mhocko@xxxxxxxx>
x86 implementation of __down_write is using inline asm to optimize the
code flow. This however requires that it has go over an additional hop
for the slow path call_rwsem_down_write_failed which has to
save_common_regs/restore_common_regs to preserve the calling convention.
This, however doesn't add much because the fast path only saves one
register push/pop (rdx) when compared to the generic implementation:
Before:
0000000000000019 <down_write>:
19: e8 00 00 00 00 callq 1e <down_write+0x5>
1e: 55 push %rbp
1f: 48 ba 01 00 00 00 ff movabs $0xffffffff00000001,%rdx
26: ff ff ff
29: 48 89 f8 mov %rdi,%rax
2c: 48 89 e5 mov %rsp,%rbp
2f: f0 48 0f c1 10 lock xadd %rdx,(%rax)
34: 85 d2 test %edx,%edx
36: 74 05 je 3d <down_write+0x24>
38: e8 00 00 00 00 callq 3d <down_write+0x24>
3d: 65 48 8b 04 25 00 00 mov %gs:0x0,%rax
44: 00 00
46: 5d pop %rbp
47: 48 89 47 38 mov %rax,0x38(%rdi)
4b: c3 retq
After:
0000000000000019 <down_write>:
19: e8 00 00 00 00 callq 1e <down_write+0x5>
1e: 55 push %rbp
1f: 48 b8 01 00 00 00 ff movabs $0xffffffff00000001,%rax
26: ff ff ff
29: 48 89 e5 mov %rsp,%rbp
2c: 53 push %rbx
2d: 48 89 fb mov %rdi,%rbx
30: f0 48 0f c1 07 lock xadd %rax,(%rdi)
35: 48 85 c0 test %rax,%rax
38: 74 05 je 3f <down_write+0x26>
3a: e8 00 00 00 00 callq 3f <down_write+0x26>
3f: 65 48 8b 04 25 00 00 mov %gs:0x0,%rax
46: 00 00
48: 48 89 43 38 mov %rax,0x38(%rbx)
4c: 5b pop %rbx
4d: 5d pop %rbp
4e: c3 retq
This doesn't seem to justify the code obfuscation and complexity. Use
the generic implementation instead.
Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
---
arch/x86/include/asm/rwsem.h | 17 +++++------------
arch/x86/lib/rwsem.S | 9 ---------
2 files changed, 5 insertions(+), 21 deletions(-)
diff --git a/arch/x86/include/asm/rwsem.h b/arch/x86/include/asm/rwsem.h
index d79a218675bc..1b5e89b3643d 100644
--- a/arch/x86/include/asm/rwsem.h
+++ b/arch/x86/include/asm/rwsem.h
@@ -102,18 +102,11 @@ static inline int __down_read_trylock(struct rw_semaphore *sem)
static inline void __down_write(struct rw_semaphore *sem)
{
long tmp;
- asm volatile("# beginning down_write\n\t"
- LOCK_PREFIX " xadd %1,(%2)\n\t"
- /* adds 0xffff0001, returns the old value */
- " test " __ASM_SEL(%w1,%k1) "," __ASM_SEL(%w1,%k1) "\n\t"
- /* was the active mask 0 before? */
- " jz 1f\n"
- " call call_rwsem_down_write_failed\n"
- "1:\n"
- "# ending down_write"
- : "+m" (sem->count), "=d" (tmp)
- : "a" (sem), "1" (RWSEM_ACTIVE_WRITE_BIAS)
- : "memory", "cc");
+
+ tmp = atomic_long_add_return(RWSEM_ACTIVE_WRITE_BIAS,
+ (atomic_long_t *)&sem->count);
+ if (unlikely(tmp != RWSEM_ACTIVE_WRITE_BIAS))
+ rwsem_down_write_failed(sem);
}
/*
diff --git a/arch/x86/lib/rwsem.S b/arch/x86/lib/rwsem.S
index 40027db99140..ea5c7c177483 100644
--- a/arch/x86/lib/rwsem.S
+++ b/arch/x86/lib/rwsem.S
@@ -57,7 +57,6 @@
* is also the input argument to these helpers)
*
* The following can clobber %rdx because the asm clobbers it:
- * call_rwsem_down_write_failed
* call_rwsem_wake
* but %rdi, %rsi, %rcx, %r8-r11 always need saving.
*/
@@ -93,14 +92,6 @@ ENTRY(call_rwsem_down_read_failed)
ret
ENDPROC(call_rwsem_down_read_failed)
-ENTRY(call_rwsem_down_write_failed)
- save_common_regs
- movq %rax,%rdi
- call rwsem_down_write_failed
- restore_common_regs
- ret
-ENDPROC(call_rwsem_down_write_failed)
-
ENTRY(call_rwsem_wake)
/* do nothing if still outstanding active readers */
__ASM_HALF_SIZE(dec) %__ASM_HALF_REG(dx)
--
2.7.0
--
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