From: Peter Zijlstra peterz@infradead.org
commit 69d927bba39517d0980462efc051875b7f4db185 upstream.
Recent probing at the Linux Kernel Memory Model uncovered a 'surprise'. Strongly ordered architectures where the atomic RmW primitive implies full memory ordering and smp_mb__{before,after}_atomic() are a simple barrier() (such as x86) fail for:
*x = 1; atomic_inc(u); smp_mb__after_atomic(); r0 = *y;
Because, while the atomic_inc() implies memory order, it (surprisingly) does not provide a compiler barrier. This then allows the compiler to re-order like so:
atomic_inc(u); *x = 1; smp_mb__after_atomic(); r0 = *y;
Which the CPU is then allowed to re-order (under TSO rules) like:
atomic_inc(u); r0 = *y; *x = 1;
And this very much was not intended. Therefore strengthen the atomic RmW ops to include a compiler barrier.
NOTE: atomic_{or,and,xor} and the bitops already had the compiler barrier.
Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Cc: Linus Torvalds torvalds@linux-foundation.org Cc: Peter Zijlstra peterz@infradead.org Cc: Thomas Gleixner tglx@linutronix.de Signed-off-by: Ingo Molnar mingo@kernel.org Signed-off-by: Jari Ruusu jari.ruusu@gmail.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- arch/x86/include/asm/atomic.h | 8 ++++---- arch/x86/include/asm/atomic64_64.h | 8 ++++---- arch/x86/include/asm/barrier.h | 4 ++-- 3 files changed, 10 insertions(+), 10 deletions(-)
--- a/arch/x86/include/asm/atomic.h +++ b/arch/x86/include/asm/atomic.h @@ -49,7 +49,7 @@ static __always_inline void atomic_add(i { asm volatile(LOCK_PREFIX "addl %1,%0" : "+m" (v->counter) - : "ir" (i)); + : "ir" (i) : "memory"); }
/** @@ -63,7 +63,7 @@ static __always_inline void atomic_sub(i { asm volatile(LOCK_PREFIX "subl %1,%0" : "+m" (v->counter) - : "ir" (i)); + : "ir" (i) : "memory"); }
/** @@ -89,7 +89,7 @@ static __always_inline bool atomic_sub_a static __always_inline void atomic_inc(atomic_t *v) { asm volatile(LOCK_PREFIX "incl %0" - : "+m" (v->counter)); + : "+m" (v->counter) :: "memory"); }
/** @@ -101,7 +101,7 @@ static __always_inline void atomic_inc(a static __always_inline void atomic_dec(atomic_t *v) { asm volatile(LOCK_PREFIX "decl %0" - : "+m" (v->counter)); + : "+m" (v->counter) :: "memory"); }
/** --- a/arch/x86/include/asm/atomic64_64.h +++ b/arch/x86/include/asm/atomic64_64.h @@ -44,7 +44,7 @@ static __always_inline void atomic64_add { asm volatile(LOCK_PREFIX "addq %1,%0" : "=m" (v->counter) - : "er" (i), "m" (v->counter)); + : "er" (i), "m" (v->counter) : "memory"); }
/** @@ -58,7 +58,7 @@ static inline void atomic64_sub(long i, { asm volatile(LOCK_PREFIX "subq %1,%0" : "=m" (v->counter) - : "er" (i), "m" (v->counter)); + : "er" (i), "m" (v->counter) : "memory"); }
/** @@ -85,7 +85,7 @@ static __always_inline void atomic64_inc { asm volatile(LOCK_PREFIX "incq %0" : "=m" (v->counter) - : "m" (v->counter)); + : "m" (v->counter) : "memory"); }
/** @@ -98,7 +98,7 @@ static __always_inline void atomic64_dec { asm volatile(LOCK_PREFIX "decq %0" : "=m" (v->counter) - : "m" (v->counter)); + : "m" (v->counter) : "memory"); }
/** --- a/arch/x86/include/asm/barrier.h +++ b/arch/x86/include/asm/barrier.h @@ -105,8 +105,8 @@ do { \ #endif
/* Atomic operations are already serializing on x86 */ -#define __smp_mb__before_atomic() barrier() -#define __smp_mb__after_atomic() barrier() +#define __smp_mb__before_atomic() do { } while (0) +#define __smp_mb__after_atomic() do { } while (0)
#include <asm-generic/barrier.h>