Commit-ID: ed8ebd1d514126c0e54fbdbd231427dc91c877c2
Gitweb: http://git.kernel.org/tip/ed8ebd1d514126c0e54fbdbd231427dc91c877c2
Author: Tejun Heo <tj@xxxxxxxxxx>
AuthorDate: Wed, 25 May 2016 16:11:57 -0400
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Fri, 3 Jun 2016 12:06:11 +0200
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 <tj@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Cc: Alexey Dobriyan <adobriyan@xxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Paul McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Pranith Kumar <bobby.prani@xxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: kernel-team@xxxxxx
Link: http://lkml.kernel.org/g/20160522185040.GA23664@xxxxxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
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 84f542d..1c7eec0 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,
--
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