From ed8ebd1d514126c0e54fbdbd231427dc91c877c2 Mon Sep 17 00:00:00 2001 From: Tejun Heo Date: Wed, 25 May 2016 16:11:57 -0400 Subject: [PATCH] percpu, locking: Revert ("percpu: Replace smp_read_barrier_depends() with lockless_dereference()") lockless_dereference() is planned to grow a sanity check to ensure that the input parameter is a pointer. __ref_is_percpu() passes in an unsinged long value which is a combination of a pointer and a flag. While it can be casted to a pointer lvalue, the casting looks messy and it's a special case anyway. Let's revert back to open-coding READ_ONCE() and explicit barrier. This doesn't cause any functional changes. Signed-off-by: Tejun Heo Signed-off-by: Peter Zijlstra (Intel) Cc: Alexey Dobriyan Cc: Andrew Morton Cc: Linus Torvalds Cc: Paul E. McKenney Cc: Paul McKenney Cc: Peter Zijlstra Cc: Pranith Kumar Cc: Thomas Gleixner Cc: kernel-team@fb.com Link: http://lkml.kernel.org/g/20160522185040.GA23664@p183.telecom.by Signed-off-by: Ingo Molnar --- include/linux/percpu-refcount.h | 12 +++++------- 1 file changed, 5 insertions(+), 7 deletions(-) diff --git a/include/linux/percpu-refcount.h b/include/linux/percpu-refcount.h index 84f542df7ff5..1c7eec09e5eb 100644 --- a/include/linux/percpu-refcount.h +++ b/include/linux/percpu-refcount.h @@ -136,14 +136,12 @@ static inline bool __ref_is_percpu(struct percpu_ref *ref, * 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. + * READ_ONCE() is required when fetching it. */ - percpu_ptr = lockless_dereference(ref->percpu_count_ptr); + percpu_ptr = READ_ONCE(ref->percpu_count_ptr); + + /* paired with smp_store_release() in __percpu_ref_switch_to_percpu() */ + smp_read_barrier_depends(); /* * Theoretically, the following could test just ATOMIC; however,