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

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Date:	Thu, 20 Sep 2007 10:52:43 +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 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!

Regards,
Nadia

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