From: Tejun Heo <tj@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Alexey Dobriyan <adobriyan@gmail.com>,
akpm@linux-foundation.org, linux-kernel@vger.kernel.org,
Paul McKenney <paulmck@linux.vnet.ibm.com>,
Pranith Kumar <bobby.prani@gmail.com>,
kernel-team@fb.com
Subject: [PATCH] percpu: Revert ("percpu: Replace smp_read_barrier_depends() with lockless_dereference()")
Date: Wed, 25 May 2016 16:11:57 -0400 [thread overview]
Message-ID: <20160525201157.GI3354@mtj.duckdns.org> (raw)
In-Reply-To: <20160525195745.GH3354@mtj.duckdns.org>
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@kernel.org>
Link: http://lkml.kernel.org/g/20160522185040.GA23664@p183.telecom.by
Cc: Pranith Kumar <bobby.prani@gmail.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
---
So, something like this. Please feel free to include in the series.
Thanks.
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,
next prev parent reply other threads:[~2016-05-25 20:12 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-05-21 20:14 [PATCH] seqlock: fix raw_read_seqcount_latch() Alexey Dobriyan
2016-05-22 10:48 ` Peter Zijlstra
2016-05-22 18:50 ` Alexey Dobriyan
2016-05-23 9:36 ` Peter Zijlstra
2016-05-25 19:57 ` Tejun Heo
2016-05-25 20:11 ` Tejun Heo [this message]
2016-06-03 10:58 ` [tip:locking/core] locking/barriers: Validate lockless_dereference() is used on a pointer type tip-bot for Peter Zijlstra
2016-06-06 21:31 ` Mateusz Guzik
2016-06-07 7:10 ` Peter Zijlstra
2016-06-08 14:19 ` tip-bot for Peter Zijlstra
2016-05-27 11:11 ` [PATCH] seqlock: fix raw_read_seqcount_latch() Peter Zijlstra
2016-06-03 10:46 ` [tip:locking/core] locking/seqcount: Re-fix raw_read_seqcount_latch() tip-bot for Peter Zijlstra
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