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

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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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