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

[Nadia Derbey]

Nadia Derbey wrote:
> 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!

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()).

Regards,
Nadia

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