Message-ID: <46F7885F.4080906@bull.net>
Date: Mon, 24 Sep 2007 11:50:23 +0200
From: Nadia Derbey <Nadia.Derbey@bull.net>
Organization: BULL/DT/OSwR&D/Linux
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To: Jarek Poplawski <jarkao2@o2.pl>
Cc: Andrew Morton <akpm@linux-foundation.org>,
       Alexey Dobriyan <adobriyan@sw.ru>, linux-kernel@vger.kernel.org
Subject: Re: 2.6.23-rc6-mm1: IPC: sleeping function called ...
References: <20070919140726.GA4603@ff.dom.local> <46F2123A.9070201@bull.net> <20070920072821.GA2065@ff.dom.local> <46F234DB.7030403@bull.net> <46F270DA.5030101@bull.net> <20070921084453.GA1758@ff.dom.local>
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Jarek Poplawski wrote:
> On Thu, Sep 20, 2007 at 03:08:42PM +0200, Nadia Derbey wrote:
> 
>>Nadia Derbey wrote:
>>
>>>Jarek Poplawski wrote:
>>>
>>>
>>>>On Thu, Sep 20, 2007 at 08:24:58AM +0200, Nadia Derbey wrote:
> 
> ...
> 
>>>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()).
> 
> 
> Yes, IMHO, it should be at least more readable when we can see where
> this RCU is really needed.
> 
> But, after 3-rd look, I have a few more doubts (btw., 3 looks are
> still not enough for me with this code, so I cerainly can miss many
> things here, and, alas, I manged to see util and msg code only):
> 

Jarek,

I'm realizing I did'nt give you an answer to issues # 1 and 3. Sorry for 
that!

> 1. ipc_lock() and ipc_lock_check() are used without ipc_ids.mutex,
> but it's probably wrong: they call idr_find() with ipc_ids pointer
> which needs this mutex, just like in similar code in: ipc_findkey(),
> ipc_get_maxid() or sysvipc_find_ipc().

I think you're completely right: the rcu_read_lock() is not enough in 
this case.
I have to solve this issue, but keeping the original way the ipc 
developers have done it: I think they didn't want to take the mutex lock 
for every single operation. E.g. sending a message to a given message 
queue shouldn't avoid creating new message queues.
I'll come up with a solution.

> 
> 2. I'm not sure this refcounting with ipc_rcu_getref/putref is SMP
> safe (memory barriers): it's not atomic, so locking is needed, but
> e.g. in do_msgsnd() kern_ipc_perm lock is used for this, while
> freeque() calls ipc_rcu_putref() with ipc_ids mutex only.
> 
> 3. Probably similar problem is possible with msr_d.r_msg which is
> read in do_msgrcv() under rcu_read_lock() only.
> 

In think here they have avoided refcoutning by using r_msg:
r_msg is initialzed to -EAGAIN before releasing the msq lock. if 
freequeue() is called it sets r_msg to EIDRM (see expunge_all(-EIDRM)).
Setting r_msg is always done under the msq lock (expunge_all() / 
pipelined_Sned()).
  Since rcu_read_lock is called right after schedule, they are sure the 
msq pointer is still valid when they re-lock it once a msg is present in 
the receive queue.

Please tell me if I'm not clear ;-)

Regards,
Nadia

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