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H A D | percpu.h | 509eb76e Wed Jun 05 05:20:33 CDT 2013 Will Deacon <will.deacon@arm.com> 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: <stable@vger.kernel.org> Cc: Rob Herring <rob.herring@calxeda.com> Reviewed-by: Nicolas Pitre <nico@linaro.org> Signed-off-by: Will Deacon <will.deacon@arm.com> Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk> 509eb76e Wed Jun 05 05:20:33 CDT 2013 Will Deacon <will.deacon@arm.com> 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: <stable@vger.kernel.org> Cc: Rob Herring <rob.herring@calxeda.com> Reviewed-by: Nicolas Pitre <nico@linaro.org> Signed-off-by: Will Deacon <will.deacon@arm.com> Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
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