[PATCH] x86: __down_read_trylock fix 32b build failure

[Michal Hocko]

Ingo has noticed the following compilation error with CONFIG_X86_32=y
./arch/x86/include/asm/rwsem.h:106:2: error: âasmâ operand has impossible constraints

The reason seems to be that 32b doesn't like ret being input and output
argument sharing the same register with sem which is only the input. Fix
this by making ret output only and use %3 (aka sem) for xadd.

ret initialization is not needed now because this is done implicitly
by the asm even for the fast path as both sem and ret share the same
register.

Reported-by: Ingo Molnar <mingo@xxxxxxxxxx>
Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
---
 arch/x86/include/asm/rwsem.h | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/x86/include/asm/rwsem.h b/arch/x86/include/asm/rwsem.h
index d759c5f70f49..453744c1d347 100644
--- a/arch/x86/include/asm/rwsem.h
+++ b/arch/x86/include/asm/rwsem.h
@@ -102,9 +102,9 @@ static inline int __down_read_trylock(struct rw_semaphore *sem)
 #define ____down_write(sem, slow_path)			\
 ({							\
 	long tmp;					\
-	struct rw_semaphore* ret = sem;			\
+	struct rw_semaphore* ret;			\
 	asm volatile("# beginning down_write\n\t"	\
-		     LOCK_PREFIX "  xadd      %1,(%2)\n\t"	\
+		     LOCK_PREFIX "  xadd      %1,(%3)\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? */\
@@ -112,7 +112,7 @@ static inline int __down_read_trylock(struct rw_semaphore *sem)
 		     "  call " slow_path "\n"		\
 		     "1:\n"				\
 		     "# ending down_write"		\
-		     : "+m" (sem->count), "=d" (tmp), "+a" (ret)	\
+		     : "+m" (sem->count), "=d" (tmp), "=a" (ret)	\
 		     : "a" (sem), "1" (RWSEM_ACTIVE_WRITE_BIAS) \
 		     : "memory", "cc");			\
 	ret;						\
-- 
2.8.0.rc3

-- 
Michal Hocko
SUSE Labs
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