From: Peter Zijlstra <peterz@infradead.org>
To: Mateusz Guzik <mguzik@redhat.com>
Cc: linux-tip-commits@vger.kernel.org, paulmck@linux.vnet.ibm.com,
akpm@linux-foundation.org, torvalds@linux-foundation.org,
adobriyan@gmail.com, tglx@linutronix.de,
linux-kernel@vger.kernel.org, mingo@kernel.org, hpa@zytor.com
Subject: Re: [tip:locking/core] locking/barriers: Validate lockless_dereference() is used on a pointer type
Date: Tue, 7 Jun 2016 09:10:27 +0200 [thread overview]
Message-ID: <20160607071027.GJ30909@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <20160606213138.rizmdnhgdqyavnub@mguzik>
On Mon, Jun 06, 2016 at 11:31:39PM +0200, Mateusz Guzik wrote:
> The reason is that leaf_walk_rcu does get_child_rcu(pn, cindex++), which
> ends up in lockless_dereference as pn[cindex++], which is now evaluated
> twice.
Indeed, bad that; yes I think the below will work, will you send a
proper patch so I can apply?
> The simplest fix I see does the assignment later, that is:
> diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> index e9c6417..06f27fd 100644
> --- a/include/linux/compiler.h
> +++ b/include/linux/compiler.h
> @@ -551,8 +551,8 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
> */
> #define lockless_dereference(p) \
> ({ \
> - __maybe_unused const void * const _________p2 = p; \
> typeof(p) _________p1 = READ_ONCE(p); \
> + __maybe_unused const void * const _________p2 = _________p1; \
> smp_read_barrier_depends(); /* Dependency order vs. p above. */ \
> (_________p1); \
> })
>
> --
> Mateusz Guzik
next prev parent reply other threads:[~2016-06-07 7:10 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 ` [PATCH] percpu: Revert ("percpu: Replace smp_read_barrier_depends() with lockless_dereference()") Tejun Heo
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 [this message]
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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