Messages in this thread | | | Date | Thu, 20 Sep 2007 15:08:42 +0200 | From | Nadia Derbey <> | Subject | Re: 2.6.23-rc6-mm1: IPC: sleeping function called ... |
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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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