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 16:32:26 -0400 [thread overview]
Message-ID: <51C8ACDA.4080400@hurleysoftware.com> (raw)
In-Reply-To: <1372101180.22432.58.camel@schen9-DESK>
On 06/24/2013 03:13 PM, Tim Chen wrote:
> On Mon, 2013-06-24 at 14:49 -0400, Peter Hurley wrote:
>> 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.
>> */
>>
>
> In mutex_spin_on_owner, after rcu_read_lock, the owner_running()
> function de-references the owner pointer.
Only after establishing the following preconditions:
1. snapshot of owner is non-NULL
2. mutex->owner == snapshot owner
3. memory holding mutex has not been freed (that's what the
rcu_read_lock() is for)
Only then is the owner dereferenced and only through the snapshot
(not the now-possibly-rewritten sem->owner).
> I'm using similar logic in rw-sem.
With crucial details absent.
Regards,
Peter Hurley
next prev parent reply other threads:[~2013-06-24 20:32 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
2013-06-24 19:13 ` Tim Chen
2013-06-24 20:32 ` Peter Hurley [this message]
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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