__ref_is_percpu() needs the implied ACCESS_ONCE() in
lockless_dereference() 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 <t...@kernel.org>
Cc: Kent Overstreet <k...@daterainc.com>
---
include/linux/percpu-refcount.h | 20 +++++++++++++++++---
1 file changed, 17 insertions(+), 3 deletions(-)
--- a/include/linux/percpu-refcount.h
+++ b/include/linux/percpu-refcount.h
@@ -128,8 +128,22 @@ static inline void percpu_ref_kill(struc
static inline bool __ref_is_percpu(struct percpu_ref *ref,
unsigned long __percpu
**percpu_countp)
{
- /* paired with smp_store_release() in percpu_ref_reinit() */
- unsigned long percpu_ptr = lockless_dereference(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, meaning that
+ * ACCESS_ONCE() is required when fetching it.
+ *
+ * Also, we need a data dependency barrier to be paired with
+ * smp_store_release() in __percpu_ref_switch_to_percpu().
+ *
+ * Use lockless deref which contains both.
+ */
+ percpu_ptr = lockless_dereference(ref->percpu_count_ptr);
/*
* Theoretically, the following could test just ATOMIC; however,
@@ -233,7 +247,7 @@ 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);
}
--
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