From: Peter Zijlstra <pet...@infradead.org>
[ Upstream commit 69d927bba39517d0980462efc051875b7f4db185 ]
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) <pet...@infradead.org>
Cc: Linus Torvalds <torva...@linux-foundation.org>
Cc: Peter Zijlstra <pet...@infradead.org>
Cc: Thomas Gleixner <t...@linutronix.de>
Signed-off-by: Ingo Molnar <mi...@kernel.org>
Signed-off-by: Sasha Levin <sas...@kernel.org>
---
Documentation/atomic_t.txt | 3 +++
arch/x86/include/asm/atomic.h | 8 ++++----
arch/x86/include/asm/atomic64_64.h | 8 ++++----
arch/x86/include/asm/barrier.h | 4 ++--
4 files changed, 13 insertions(+), 10 deletions(-)
diff --git a/Documentation/atomic_t.txt b/Documentation/atomic_t.txt
index 913396ac5824..ed0d814df7e0 100644
--- a/Documentation/atomic_t.txt
+++ b/Documentation/atomic_t.txt
@@ -177,6 +177,9 @@ 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();
diff --git a/arch/x86/include/asm/atomic.h b/arch/x86/include/asm/atomic.h
index 72759f131cc5..d09dd91dd0b6 100644
--- a/arch/x86/include/asm/atomic.h
+++ b/arch/x86/include/asm/atomic.h
@@ -50,7 +50,7 @@ static __always_inline void atomic_add(int i, atomic_t *v)
{
asm volatile(LOCK_PREFIX "addl %1,%0"
: "+m" (v->counter)
- : "ir" (i));
+ : "ir" (i) : "memory");
}
/**
@@ -64,7 +64,7 @@ static __always_inline void atomic_sub(int i, atomic_t *v)
{
asm volatile(LOCK_PREFIX "subl %1,%0"
: "+m" (v->counter)
- : "ir" (i));
+ : "ir" (i) : "memory");
}
/**
@@ -90,7 +90,7 @@ static __always_inline bool atomic_sub_and_test(int i,
atomic_t *v)
static __always_inline void atomic_inc(atomic_t *v)
{
asm volatile(LOCK_PREFIX "incl %0"
- : "+m" (v->counter));
+ : "+m" (v->counter) :: "memory");
}
/**
@@ -102,7 +102,7 @@ static __always_inline void atomic_inc(atomic_t *v)
static __always_inline void atomic_dec(atomic_t *v)
{
asm volatile(LOCK_PREFIX "decl %0"
- : "+m" (v->counter));
+ : "+m" (v->counter) :: "memory");
}
/**
diff --git a/arch/x86/include/asm/atomic64_64.h
b/arch/x86/include/asm/atomic64_64.h
index 738495caf05f..e6fad6bbb2ee 100644
--- a/arch/x86/include/asm/atomic64_64.h
+++ b/arch/x86/include/asm/atomic64_64.h
@@ -45,7 +45,7 @@ static __always_inline void atomic64_add(long 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 atomic64_sub(long 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");
}
/**
@@ -86,7 +86,7 @@ static __always_inline void atomic64_inc(atomic64_t *v)
{
asm volatile(LOCK_PREFIX "incq %0"
: "=m" (v->counter)
- : "m" (v->counter));
+ : "m" (v->counter) : "memory");
}
/**
@@ -99,7 +99,7 @@ static __always_inline void atomic64_dec(atomic64_t *v)
{
asm volatile(LOCK_PREFIX "decq %0"
: "=m" (v->counter)
- : "m" (v->counter));
+ : "m" (v->counter) : "memory");
}
/**
diff --git a/arch/x86/include/asm/barrier.h b/arch/x86/include/asm/barrier.h
index a04f0c242a28..bc88797cfa61 100644
--- a/arch/x86/include/asm/barrier.h
+++ b/arch/x86/include/asm/barrier.h
@@ -106,8 +106,8 @@ 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>
--
2.20.1
