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

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Message-ID: <46F2123A.9070201@bull.net>
Date:	Thu, 20 Sep 2007 08:24:58 +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 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().

Regards,
Nadia
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