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Message-ID: <46F234DB.7030403@bull.net>
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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