linux-kernel - [PATCH] x86: drop a bogus cc clobber

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Date:   Mon, 19 Sep 2016 07:27:08 -0600
From:   "Jan Beulich" <JBeulich@...e.com>
To:     <mingo@...e.hu>, <tglx@...utronix.de>, <hpa@...or.com>
Cc:     <linux-kernel@...r.kernel.org>
Subject: [PATCH] x86: drop a bogus cc clobber

With the addition of uses of gcc's condition code outputs (commit
35ccfb7114 "x86, asm: Use CC_SET()/CC_OUT() in <asm/rwsem.h>") there's
now an overlap of outputs and clobbers in __down_write_trylock(). Such
overlaps are generally getting tagged with an error (occasionally even
with an ICE). I can't really tell why plain gcc 6.2 doesn't detect this
(judging by the code it is meant to), while the slightly modified one I
use does. Since condition code clobbers are never necessary on x86
(other than perhaps for documentation purposes, which doesn't really
get done consistently), remove it altogether rather than inventing
something like CC_CLOBBER (to accompany CC_SET/CC_OUT).

Signed-off-by: Jan Beulich <jbeulich@...e.com>

--- a/arch/x86/include/asm/rwsem.h
+++ b/arch/x86/include/asm/rwsem.h
@@ -154,7 +154,7 @@ static inline bool __down_write_trylock(
 		     : "+m" (sem->count), "=&a" (tmp0), "=&r" (tmp1),
 		       CC_OUT(e) (result)
 		     : "er" (RWSEM_ACTIVE_WRITE_BIAS)
-		     : "memory", "cc");
+		     : "memory");
 	return result;
 }