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

[Nadia Derbey <Nadia.Derbey@bull.net>]

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):

Jarek,

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 ;-)

Regards,
Nadia

-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/