diff options
| author | Peter Zijlstra <peterz@infradead.org> | 2019-06-13 16:43:20 +0300 |
|---|---|---|
| committer | Paul Burton <paul.burton@mips.com> | 2019-08-31 13:06:02 +0300 |
| commit | 42344113ba7a1ed7b5654cd5270af0d5698d8521 (patch) | |
| tree | 7ac40f4f47e97e27ff2909c0a58023ba6fb41b63 /arch/mips/include/asm/barrier.h | |
| parent | 1c6c1ca318585f1096d4d04bc722297c85e9fb8a (diff) | |
| download | linux-42344113ba7a1ed7b5654cd5270af0d5698d8521.tar.xz | |
mips/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 MIPS
without WEAK_REORDERING_BEYOND_LLSC) 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.
Reported-by: Andrea Parri <andrea.parri@amarulasolutions.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Diffstat (limited to 'arch/mips/include/asm/barrier.h')
| -rw-r--r-- | arch/mips/include/asm/barrier.h | 12 |
1 files changed, 10 insertions, 2 deletions
diff --git a/arch/mips/include/asm/barrier.h b/arch/mips/include/asm/barrier.h index f9a6da96aae1..9228f7386220 100644 --- a/arch/mips/include/asm/barrier.h +++ b/arch/mips/include/asm/barrier.h @@ -211,14 +211,22 @@ #define __smp_wmb() barrier() #endif +/* + * When LL/SC does imply order, it must also be a compiler barrier to avoid the + * compiler from reordering where the CPU will not. When it does not imply + * order, the compiler is also free to reorder across the LL/SC loop and + * ordering will be done by smp_llsc_mb() and friends. + */ #if defined(CONFIG_WEAK_REORDERING_BEYOND_LLSC) && defined(CONFIG_SMP) #define __WEAK_LLSC_MB " sync \n" +#define smp_llsc_mb() __asm__ __volatile__(__WEAK_LLSC_MB : : :"memory") +#define __LLSC_CLOBBER #else #define __WEAK_LLSC_MB " \n" +#define smp_llsc_mb() do { } while (0) +#define __LLSC_CLOBBER "memory" #endif -#define smp_llsc_mb() __asm__ __volatile__(__WEAK_LLSC_MB : : :"memory") - #ifdef CONFIG_CPU_CAVIUM_OCTEON #define smp_mb__before_llsc() smp_wmb() #define __smp_mb__before_llsc() __smp_wmb() |