From: Peter Hurley <peter@hurleysoftware.com>
To: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Davidlohr Bueso <davidlohr.bueso@hp.com>,
Alex Shi <alex.shi@intel.com>,
Michel Lespinasse <walken@google.com>,
Ingo Molnar <mingo@elte.hu>,
Andrew Morton <akpm@linux-foundation.org>,
Andrea Arcangeli <aarcange@redhat.com>,
Andi Kleen <andi@firstfloor.org>,
Matthew R Wilcox <matthew.r.wilcox@intel.com>,
Dave Hansen <dave.hansen@intel.com>,
Peter Zijlstra <a.p.zijlstra@chello.nl>,
Rik van Riel <riel@redhat.com>,
linux-kernel@vger.kernel.org, linux-mm <linux-mm@kvack.org>
Subject: Re: [PATCH 2/2] rwsem: do optimistic spinning for writer lock acquisition
Date: Mon, 24 Jun 2013 14:49:39 -0400 [thread overview]
Message-ID: <51C894C3.4040407@hurleysoftware.com> (raw)
In-Reply-To: <1372093876.22432.34.camel@schen9-DESK>
On 06/24/2013 01:11 PM, Tim Chen wrote:
> On Sun, 2013-06-23 at 13:03 -0700, Davidlohr Bueso wrote:
>> On Sat, 2013-06-22 at 03:57 -0400, Peter Hurley wrote:
>>> On 06/21/2013 07:51 PM, Tim Chen wrote:
>>>>
>>>> +static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
>>>> +{
>>>> + int retval = true;
>>>> +
>>>> + /* Spin only if active writer running */
>>>> + if (!sem->owner)
>>>> + return false;
>>>> +
>>>> + rcu_read_lock();
>>>> + if (sem->owner)
>>>> + retval = sem->owner->on_cpu;
>>> ^^^^^^^^^^^^^^^^^^
>>>
>>> Why is this a safe dereference? Could not another cpu have just
>>> dropped the sem (and thus set sem->owner to NULL and oops)?
>>>
>
> The rcu read lock should protect against sem->owner being NULL.
It doesn't.
Here's the comment from mutex_spin_on_owner():
/*
* Look out! "owner" is an entirely speculative pointer
* access and not reliable.
*/
Regards,
Peter Hurley
next prev parent reply other threads:[~2013-06-24 18:49 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <cover.1371855277.git.tim.c.chen@linux.intel.com>
2013-06-21 23:51 ` [PATCH 1/2] rwsem: check the lock before cpmxchg in down_write_trylock and rwsem_do_wake Tim Chen
2013-06-22 0:10 ` Alex Shi
2013-06-22 0:15 ` Davidlohr Bueso
2013-06-24 16:34 ` Tim Chen
2013-06-22 7:21 ` Peter Hurley
2013-06-23 1:16 ` Alex Shi
2013-06-23 5:10 ` Andi Kleen
2013-06-23 11:52 ` Alex Shi
2013-06-21 23:51 ` [PATCH 2/2] rwsem: do optimistic spinning for writer lock acquisition Tim Chen
2013-06-22 0:00 ` Davidlohr Bueso
2013-06-22 7:57 ` Peter Hurley
2013-06-23 20:03 ` Davidlohr Bueso
2013-06-24 17:11 ` Tim Chen
2013-06-24 18:49 ` Peter Hurley [this message]
2013-06-24 19:13 ` Tim Chen
2013-06-24 20:32 ` Peter Hurley
2013-06-24 20:17 ` Tim Chen
2013-06-24 20:48 ` Peter Hurley
2013-06-24 21:30 ` Tim Chen
2013-06-25 7:37 ` Peter Zijlstra
2013-06-25 16:00 ` Tim Chen
2013-06-24 21:58 ` Tim Chen
2013-06-24 22:08 ` Peter Hurley
2013-06-24 8:46 ` Peter Zijlstra
2013-06-24 16:36 ` Tim Chen
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