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
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