linux-kernel - [PATCH] x86/asm: use memory clobber in bitops that touch arbitrary memory

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Date:   Mon,  1 Apr 2019 18:24:08 +0200
From:   Alexander Potapenko <glider@...gle.com>
To:     paulmck@...ux.ibm.com, hpa@...or.com, peterz@...radead.org
Cc:     linux-kernel@...r.kernel.org, dvyukov@...gle.com,
        jyknight@...gle.com, x86@...nel.org, mingo@...hat.com
Subject: [PATCH] x86/asm: use memory clobber in bitops that touch arbitrary memory

Certain bit operations that read/write bits take a base pointer and an
arbitrarily large offset to address the bit relative to that base.
Inline assembly constraints aren't expressive enough to tell the
compiler that the assembly directive is going to touch a specific memory
location of unknown size, therefore we have to use the "memory" clobber
to indicate that the assembly is going to access memory locations other
than those listed in the inputs/outputs.

This particular patch leads to size increase of 124 kernel functions in
a defconfig build. For some of them the diff is in NOP operations, other
end up re-reading values from memory and may potentially slow down the
execution. But without these clobbers the compiler is free to cache
the contents of the bitmaps and use them as if they weren't changed by
the inline assembly.

Signed-off-by: Alexander Potapenko <glider@...gle.com>
Cc: Dmitry Vyukov <dvyukov@...gle.com>
Cc: Paul E. McKenney <paulmck@...ux.ibm.com>
Cc: H. Peter Anvin <hpa@...or.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: James Y Knight <jyknight@...gle.com>
---
 arch/x86/include/asm/bitops.h | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h
index d153d570bb04..20e4950827d9 100644
--- a/arch/x86/include/asm/bitops.h
+++ b/arch/x86/include/asm/bitops.h
@@ -111,7 +111,7 @@ clear_bit(long nr, volatile unsigned long *addr)
 	} else {
 		asm volatile(LOCK_PREFIX __ASM_SIZE(btr) " %1,%0"
 			: BITOP_ADDR(addr)
-			: "Ir" (nr));
+			: "Ir" (nr) : "memory");
 	}
 }
 
@@ -131,7 +131,7 @@ static __always_inline void clear_bit_unlock(long nr, volatile unsigned long *ad
 
 static __always_inline void __clear_bit(long nr, volatile unsigned long *addr)
 {
-	asm volatile(__ASM_SIZE(btr) " %1,%0" : ADDR : "Ir" (nr));
+	asm volatile(__ASM_SIZE(btr) " %1,%0" : ADDR : "Ir" (nr) : "memory");
 }
 
 static __always_inline bool clear_bit_unlock_is_negative_byte(long nr, volatile unsigned long *addr)
@@ -176,7 +176,7 @@ static __always_inline void __clear_bit_unlock(long nr, volatile unsigned long *
  */
 static __always_inline void __change_bit(long nr, volatile unsigned long *addr)
 {
-	asm volatile(__ASM_SIZE(btc) " %1,%0" : ADDR : "Ir" (nr));
+	asm volatile(__ASM_SIZE(btc) " %1,%0" : ADDR : "Ir" (nr) : "memory");
 }
 
 /**
@@ -197,7 +197,7 @@ static __always_inline void change_bit(long nr, volatile unsigned long *addr)
 	} else {
 		asm volatile(LOCK_PREFIX __ASM_SIZE(btc) " %1,%0"
 			: BITOP_ADDR(addr)
-			: "Ir" (nr));
+			: "Ir" (nr) : "memory");
 	}
 }
 
@@ -243,7 +243,7 @@ static __always_inline bool __test_and_set_bit(long nr, volatile unsigned long *
 	asm(__ASM_SIZE(bts) " %2,%1"
 	    CC_SET(c)
 	    : CC_OUT(c) (oldbit), ADDR
-	    : "Ir" (nr));
+	    : "Ir" (nr) : "memory");
 	return oldbit;
 }
 
@@ -283,7 +283,7 @@ static __always_inline bool __test_and_clear_bit(long nr, volatile unsigned long
 	asm volatile(__ASM_SIZE(btr) " %2,%1"
 		     CC_SET(c)
 		     : CC_OUT(c) (oldbit), ADDR
-		     : "Ir" (nr));
+		     : "Ir" (nr) : "memory");
 	return oldbit;
 }
 
@@ -326,7 +326,7 @@ static __always_inline bool variable_test_bit(long nr, volatile const unsigned l
 	asm volatile(__ASM_SIZE(bt) " %2,%1"
 		     CC_SET(c)
 		     : CC_OUT(c) (oldbit)
-		     : "m" (*(unsigned long *)addr), "Ir" (nr));
+		     : "m" (*(unsigned long *)addr), "Ir" (nr) : "memory");
 
 	return oldbit;
 }
-- 
2.21.0.392.gf8f6787159e-goog