On Mon, Apr 01, 2019 at 06:24:08PM +0200, Alexander Potapenko wrote:
> 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 <gli...@google.com>
> Cc: Dmitry Vyukov <dvyu...@google.com>
> Cc: Paul E. McKenney <paul...@linux.ibm.com>
> Cc: H. Peter Anvin <h...@zytor.com>
> Cc: Peter Zijlstra <pet...@infradead.org>
> Cc: James Y Knight <jykni...@google.com>
Reviewed-by: Paul E. McKenney <paul...@linux.ibm.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
>
