Messages in this thread | | | Date | Thu, 20 Sep 2007 10:52:43 +0200 | From | Nadia Derbey <> | Subject | Re: 2.6.23-rc6-mm1: IPC: sleeping function called ... |
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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!
Regards, Nadia
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