From: Will Deacon Date: Wed, 5 Jun 2013 10:20:33 +0000 (+0100) Subject: ARM: 7747/1: pcpu: ensure __my_cpu_offset cannot be re-ordered across barrier() X-Git-Url: https://git.karo-electronics.de/?a=commitdiff_plain;h=509eb76ebf97;p=linux-beck.git ARM: 7747/1: pcpu: ensure __my_cpu_offset cannot be re-ordered across barrier() __my_cpu_offset is non-volatile, since we want its value to be cached when we access several per-cpu variables in a row with preemption disabled. This means that we rely on preempt_{en,dis}able to hazard with the operation via the barrier() macro, so that we can't end up migrating CPUs without reloading the per-cpu offset. Unfortunately, GCC doesn't treat a "memory" clobber on a non-volatile asm block as a side-effect, and will happily re-order it before other memory clobbers (including those in prempt_disable()) and cache the value. This has been observed to break the cmpxchg logic in the slub allocator, leading to livelock in kmem_cache_alloc in mainline kernels. This patch adds a dummy memory input operand to __my_cpu_offset, forcing it to be ordered with respect to the barrier() macro. Cc: Cc: Rob Herring Reviewed-by: Nicolas Pitre Signed-off-by: Will Deacon Signed-off-by: Russell King --- diff --git a/arch/arm/include/asm/percpu.h b/arch/arm/include/asm/percpu.h index 968c0a14e0a3..209e6504922e 100644 --- a/arch/arm/include/asm/percpu.h +++ b/arch/arm/include/asm/percpu.h @@ -30,8 +30,15 @@ static inline void set_my_cpu_offset(unsigned long off) static inline unsigned long __my_cpu_offset(void) { unsigned long off; - /* Read TPIDRPRW */ - asm("mrc p15, 0, %0, c13, c0, 4" : "=r" (off) : : "memory"); + register unsigned long *sp asm ("sp"); + + /* + * Read TPIDRPRW. + * We want to allow caching the value, so avoid using volatile and + * instead use a fake stack read to hazard against barrier(). + */ + asm("mrc p15, 0, %0, c13, c0, 4" : "=r" (off) : "Q" (*sp)); + return off; } #define __my_cpu_offset __my_cpu_offset()