Message-ID: <46F398C3.3000804@bull.net>
Date:	Fri, 21 Sep 2007 12:11:15 +0200
From:	Nadia Derbey <Nadia.Derbey@...l.net>
To:	Jarek Poplawski <jarkao2@...pl>
Cc:	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 ...

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):
> 
> 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().
> 
> 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.

OK, but freeque() freeary() and shm_destroy() are special cases:
we have the following path:

mutex_lock(ipc_ids.mutex)
...
ipc_lock(ipcp)
... do whatever cleaning is needed ...
ipc_rmid(ipcp)
ipc_unlock(ipcp)
....
ipc_rcu_putref(ipcp)

Once the rmid has been done the ipc structure is considered as not 
visible anymore from the user side ==> any syscall called with the 
corresponding id will return invalid.
The only thing that could happen is that this structure be reused for a 
newly allocated ipc structure. But this too cannot happen since we are 
under the ipc_ids mutex lock.

Am I wrong?

Answers to the other questions in separate e-mails

Regards,
Nadia

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