Commit 69d927bb authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Ingo Molnar

x86/atomic: Fix smp_mb__{before,after}_atomic()

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: default avatarPeter 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: default avatarIngo Molnar <mingo@kernel.org>
parent dd471efe
......@@ -196,6 +196,9 @@ These helper barriers exist because architectures have varying implicit
ordering on their SMP atomic primitives. For example our TSO architectures
provide full ordered atomics and these barriers are no-ops.
NOTE: when the atomic RmW ops are fully ordered, they should also imply a
compiler barrier.
Thus:
atomic_fetch_add();
......
......@@ -54,7 +54,7 @@ static __always_inline void arch_atomic_add(int i, atomic_t *v)
{
asm volatile(LOCK_PREFIX "addl %1,%0"
: "+m" (v->counter)
: "ir" (i));
: "ir" (i) : "memory");
}
/**
......@@ -68,7 +68,7 @@ static __always_inline void arch_atomic_sub(int i, atomic_t *v)
{
asm volatile(LOCK_PREFIX "subl %1,%0"
: "+m" (v->counter)
: "ir" (i));
: "ir" (i) : "memory");
}
/**
......@@ -95,7 +95,7 @@ static __always_inline bool arch_atomic_sub_and_test(int i, atomic_t *v)
static __always_inline void arch_atomic_inc(atomic_t *v)
{
asm volatile(LOCK_PREFIX "incl %0"
: "+m" (v->counter));
: "+m" (v->counter) :: "memory");
}
#define arch_atomic_inc arch_atomic_inc
......@@ -108,7 +108,7 @@ static __always_inline void arch_atomic_inc(atomic_t *v)
static __always_inline void arch_atomic_dec(atomic_t *v)
{
asm volatile(LOCK_PREFIX "decl %0"
: "+m" (v->counter));
: "+m" (v->counter) :: "memory");
}
#define arch_atomic_dec arch_atomic_dec
......
......@@ -45,7 +45,7 @@ static __always_inline void arch_atomic64_add(s64 i, atomic64_t *v)
{
asm volatile(LOCK_PREFIX "addq %1,%0"
: "=m" (v->counter)
: "er" (i), "m" (v->counter));
: "er" (i), "m" (v->counter) : "memory");
}
/**
......@@ -59,7 +59,7 @@ static inline void arch_atomic64_sub(s64 i, atomic64_t *v)
{
asm volatile(LOCK_PREFIX "subq %1,%0"
: "=m" (v->counter)
: "er" (i), "m" (v->counter));
: "er" (i), "m" (v->counter) : "memory");
}
/**
......@@ -87,7 +87,7 @@ static __always_inline void arch_atomic64_inc(atomic64_t *v)
{
asm volatile(LOCK_PREFIX "incq %0"
: "=m" (v->counter)
: "m" (v->counter));
: "m" (v->counter) : "memory");
}
#define arch_atomic64_inc arch_atomic64_inc
......@@ -101,7 +101,7 @@ static __always_inline void arch_atomic64_dec(atomic64_t *v)
{
asm volatile(LOCK_PREFIX "decq %0"
: "=m" (v->counter)
: "m" (v->counter));
: "m" (v->counter) : "memory");
}
#define arch_atomic64_dec arch_atomic64_dec
......
......@@ -80,8 +80,8 @@ do { \
})
/* 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>
......
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