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

From: Nadia Derbey
Date: Mon Sep 24 2007 - 05:44:57 EST

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

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):


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 ;-)


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