Messages in this thread | | | Date | Tue, 6 Jan 2015 18:21:28 -0800 | From | Kent Overstreet <> | Subject | Re: [PATCH 1/2 percpu/for-3.20] percpu_ref: remove unnecessary ACCESS_ONCE() in percpu_ref_tryget_live() |
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On Tue, Jan 06, 2015 at 10:23:47AM -0500, Tejun Heo wrote: > __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.
lockless_dereference() is new - cool! I've been wanting that without even realizing it.
Acked-by: Kent Overstreet <kmo@daterainc.com>
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