__ref_is_percpu() needs ACCESS_ONCE() on @ref->percpu_count_ptr because the value is tested for !__PERCPU_REF_ATOMIC, which may be set asynchronously, and then used as a pointer. If the compiler generates a separate fetch when using it as a pointer, __PERCPU_REF_ATOMIC may be set in between contaminating the pointer value.
percpu_ref_tryget_live() also uses ACCESS_ONCE() to test __PERCPU_REF_DEAD; however, there's no reason for this. I just copied ACCESS_ONCE() usage blindly from __ref_is_percpu(). All it does is confusing people trying to understand what's going on. This patch removes the unnecessary ACCESS_ONCE() usage from percpu_ref_tryget_live() and adds a comment explaining why __ref_is_percpu() needs it. Signed-off-by: Tejun Heo <[email protected]> Cc: Kent Overstreet <[email protected]> --- include/linux/percpu-refcount.h | 14 ++++++++++++-- 1 file changed, 12 insertions(+), 2 deletions(-) --- a/include/linux/percpu-refcount.h +++ b/include/linux/percpu-refcount.h @@ -128,7 +128,17 @@ static inline void percpu_ref_kill(struc static inline bool __ref_is_percpu(struct percpu_ref *ref, unsigned long __percpu **percpu_countp) { - unsigned long percpu_ptr = ACCESS_ONCE(ref->percpu_count_ptr); + unsigned long percpu_ptr; + + /* + * The value of @ref->percpu_count_ptr is tested for + * !__PERCPU_REF_ATOMIC, which may be set asynchronously, and then + * used as a pointer. If the compiler generates a separate fetch + * when using it as a pointer, __PERCPU_REF_ATOMIC may be set in + * between contaminating the pointer value. Make sure that the + * value is fetched once. + */ + percpu_ptr = ACCESS_ONCE(ref->percpu_count_ptr); /* paired with smp_store_release() in percpu_ref_reinit() */ smp_read_barrier_depends(); @@ -215,7 +225,7 @@ static inline bool percpu_ref_tryget_liv if (__ref_is_percpu(ref, &percpu_count)) { this_cpu_inc(*percpu_count); ret = true; - } else if (!(ACCESS_ONCE(ref->percpu_count_ptr) & __PERCPU_REF_DEAD)) { + } else if (!(ref->percpu_count_ptr & __PERCPU_REF_DEAD)) { ret = atomic_long_inc_not_zero(&ref->count); } -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
