linux-kernel - Re: 2.6.23-rc6-mm1: IPC: sleeping function called ...

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Date:	Thu, 20 Sep 2007 15:08:42 +0200
From:	Nadia Derbey <Nadia.Derbey@...l.net>
To:	Nadia Derbey <Nadia.Derbey@...l.net>
Cc:	Jarek Poplawski <jarkao2@...pl>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Alexey Dobriyan <adobriyan@...ru>, linux-kernel@...r.kernel.org
Subject: Re: 2.6.23-rc6-mm1: IPC: sleeping function called ...

Nadia Derbey wrote:
> Jarek Poplawski wrote:
> 
>> On Thu, Sep 20, 2007 at 08:24:58AM +0200, Nadia Derbey wrote:
>>
>>> Jarek Poplawski wrote:
>>>
>>>> On 18-09-2007 16:55, Nadia Derbey wrote:
>>>> ...
>>>>
>>>>
>>>>> Well, reviewing the code I found another place where the 
>>>>> rcu_read_unlock() was missing.
>>>>> I'm so sorry for the inconvenience. It's true that I should have 
>>>>> tested with CONFIG_PREEMPT=y :-(
>>>>> Now, the ltp tests pass even with this option set...
>>>>>
>>>>> In attachment you'll find a patch thhat
>>>>> 1) adds the missing rcu_read_unlock()
>>>>> 2) replaces Andrew's fix with a new one: the rcu_read_lock() is now 
>>>>> taken in ipc_lock() / ipc_lock_by_ptr() and released in 
>>>>> ipc_unlock(), exactly as it was done in the ref code.
>>>>
>>>>
>>>>
>>>> BTW, probably I miss something, but I wonder, how this RCU is working
>>>> here. E.g. in msg.c do_msgsnd() there is:
>>>>
>>>> msq = msg_lock_check(ns, msqid);
>>>> ...
>>>>
>>>> msg_unlock(msq);
>>>> schedule();
>>>>
>>>> ipc_lock_by_ptr(&msq->q_perm);
>>>>
>>>> Since msq_lock_check() gets msq with ipc_lock_check() under
>>>> rcu_read_lock(), and then goes msg_unlock(msq) (i.e. ipc_unlock())
>>>> with rcu_read_unlock(), is it valid to use this with
>>>> ipc_lock_by_ptr() yet?
>>>
>>>
>>> Before Calling msg_unlock() they call ipc_rcu_getref() that 
>>> increments a refcount in the rcu header for the msg structure. This 
>>> guarantees that the the structure won't be freed before they relock 
>>> it. Once the structure is relocked by ipc_lock_by_ptr(), they do the 
>>> symmetric operation i.e. ipc_rcu_putref().
>>
>>
>>
>> Yes, I've found this later too - sorry for bothering. I was mislead
>> by the code like this:
>>
>> struct kern_ipc_perm *ipc_lock(struct ipc_ids *ids, int id)
>> {
>>         struct kern_ipc_perm *out;
>>         int lid = ipcid_to_idx(id);
>>
>>         rcu_read_lock();
>>         out = idr_find(&ids->ipcs_idr, lid);
>>         if (out == NULL) {
>>                 rcu_read_unlock();
>>                 return ERR_PTR(-EINVAL);
>>         }
>>
>> which seems to suggest "out" is an RCU protected pointer, so, I
>> thought these refcounts were for something else. But, after looking
>> at how it's used it turns out to be ~90% wrong: probably 9 out of 10
>> places use refcouning around this,
> 
> 
> Actually, ipc_lock() is called most of the time without the 
> ipc_ids.mutex held and without refcounting (maybe you didn't look for 
> the msg_lock() sem_lock() and shm_lock() too).
> So I think disabling preemption is needed, isn't it?
> 
>> so, these rcu_read_locks() don't
>> work here at all. So, probably I miss something again, but IMHO,
>> these rcu_read_locks/unlocks could be removed here or in
>> ipc_lock_by_ptr() and it should be enough to use them directly, where
>> really needed, e.g., in msg.c do_msgrcv().
>>
> 
> I have to check for the ipc_lock_by_ptr(): may be you're right!
> 

So, here is the ipc_lock_by_ptr() status:
1) do_msgsnd(), semctl_main(GETALL), semctl_main(SETALL) and find_undo() 
call it inside a refcounting.
   ==> no rcu read section needed.

2) *_exit_ns(), ipc_findkey() and sysvipc_find_ipc() call it under the 
ipc_ids mutex lock.
   ==> no rcu read section needed.

3) do_msgrcv() is the only path where ipc_lock_by_ptr() is not called 
under refcounting
   ==> rcu read section + some more checks needed once the spnlock is
       taken.

So I completely agree with you: we might remove the rcu_read_lock() from 
the ipc_lock_by_ptr() and explicitley  call it when needed (actually, it 
is already explicitly called in do_msgrcv()).

Regards,
Nadia

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