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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