[RFC 10/12] x86, rwsem: simplify __down_write

[Michal Hocko]

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