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SubjectRe: ipc,sem: sysv semaphore scalability
On 03/26/2013 02:07 PM, Sasha Levin wrote:
> On 03/26/2013 01:51 PM, Davidlohr Bueso wrote:
>> On Tue, 2013-03-26 at 13:33 -0400, Sasha Levin wrote:
>>> On 03/20/2013 03:55 PM, Rik van Riel wrote:
>>>> This series makes the sysv semaphore code more scalable,
>>>> by reducing the time the semaphore lock is held, and making
>>>> the locking more scalable for semaphore arrays with multiple
>>>> semaphores.
>>>
>>> Hi Rik,
>>>
>>> Another issue that came up is:
>>>
>>> [ 96.347341] ================================================
>>> [ 96.348085] [ BUG: lock held when returning to user space! ]
>>> [ 96.348834] 3.9.0-rc4-next-20130326-sasha-00011-gbcb2313 #318 Tainted: G W
>>> [ 96.360300] ------------------------------------------------
>>> [ 96.361084] trinity-child9/7583 is leaving the kernel with locks still held!
>>> [ 96.362019] 1 lock held by trinity-child9/7583:
>>> [ 96.362610] #0: (rcu_read_lock){.+.+..}, at: [<ffffffff8192eafb>] SYSC_semtimedop+0x1fb/0xec0
>>>
>>> It seems that we can leave semtimedop without releasing the rcu read lock.
>>>
>>> I'm a bit confused by what's going on in semtimedop with regards to rcu read lock, it
>>> seems that this behaviour is actually intentional?
>>>
>>> rcu_read_lock();
>>> sma = sem_obtain_object_check(ns, semid);
>>> if (IS_ERR(sma)) {
>>> if (un)
>>> rcu_read_unlock();
>>> error = PTR_ERR(sma);
>>> goto out_free;
>>> }
>>>
>>> When I've looked at that it seems that not releasing the read lock was (very)
>>> intentional.
>>
>> This logic was from the original code, which I also found to be quite
>> confusing.
>
> I wasn't getting this warning with the old code, so there was probably something
> else that triggers this now.
>
>>>
>>> After that, the only code path that would release the lock starts with:
>>>
>>> if (un) {
>>> ...
>>>
>>> So we won't release the lock at all if un is NULL?
>>>
>>
>> Not necessarily, we do release everything at the end of the function:
>>
>> out_unlock_free:
>> sem_unlock(sma, locknum);
>
> Ow, there's a rcu_read_unlock() in sem_unlock()? This complicates things even
> more I suspect. If un is non-NULL we'll be unlocking rcu lock twice?

It is uglier than you think...

On success, find_alloc_undo will call rcu_read_lock, so we have the
rcu_read_lock held twice :(

Some of the ipc code is quite ugly, but making too many large changes
at once is just asking for trouble. I suspect we're going to have to
untangle this one bit at a time...


--
All rights reversed.


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