Messages in this thread Patch in this message | | | Date | Wed, 31 Oct 2012 10:41:47 +0100 | From | Krzysztof Mazur <> | Subject | Re: [PATCH v2 2/3] pppoatm: fix race condition with destroying of vcc |
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On Tue, Oct 30, 2012 at 07:20:01PM +0100, Krzysztof Mazur wrote: > On Tue, Oct 30, 2012 at 10:26:46AM -0400, Chas Williams (CONTRACTOR) wrote: > > In message <1350926091-12642-2-git-send-email-krzysiek@podlesie.net>,Krzysztof Mazur writes: > > > > as i recall from way back, this shouldnt be necessary. closing a vcc > > for an attached protocol isnt supposed to require addtional locking > > or synchronization. > > Such locking is already used by vcc_sendmsg() and I think we should do here > exacly what vcc_sendmsg() does. > > > > > vcc_release() locks the socket and vcc_destroy_socket() calls the device's > > vcc close routine and pushes a NULL skb to the attached protocol. > > this NULL push is supposed to let the attached protocol that no more > > sends and recvs can be handled. > > > > that said, the order for the device vcc close and push does seem > > reversed. since i imagine there could be a pending pppoatm_send() > > during this interval. the push of the NULL skb is allowed to wait for > > the subprotocol to finish its cleanup/shutdown. > > Yes, this problem can be probably fixed by reversing close and push > and adding some synchronization to pppoatm_unassign_vcc(), but I think > we need that locking anyway, for instance for synchronization for > checking and incrementing sk->sk_wmem_alloc, between pppoatm_send() > and vcc_sendmsg(). >
I think that the same problem exists in other drivers (net/atm/br2684.c, net/atm/clip.c, maybe other).
Reversing order of close() and push(vcc, NULL) operations seems to be a good idea, but synchronization with push(vcc, NULL) and function that calls vcc->send() must be added to all drivers. I think it's better to just use ATM socket lock - lock_sock(sk_atm(vcc)), it will fix also problems with synchronization with vcc_sendmsg() and possibly other functions (ioctl?).
I think that we should add a wrapper to vcc->send(), based on fixed pppoatm_send(), that performs required checks and takes the ATM socket lock.
But I think we should reverse those operations anyway, because some drivers may use other locks, not ATM socket lock, for proper synchronization.
Krzysiek
-- >8 -- diff --git a/net/atm/common.c b/net/atm/common.c index 0c0ad93..a0e4411 100644 --- a/net/atm/common.c +++ b/net/atm/common.c @@ -171,10 +171,10 @@ static void vcc_destroy_socket(struct sock *sk) set_bit(ATM_VF_CLOSE, &vcc->flags); clear_bit(ATM_VF_READY, &vcc->flags); if (vcc->dev) { - if (vcc->dev->ops->close) - vcc->dev->ops->close(vcc); if (vcc->push) vcc->push(vcc, NULL); /* atmarpd has no push */ + if (vcc->dev->ops->close) + vcc->dev->ops->close(vcc); while ((skb = skb_dequeue(&sk->sk_receive_queue)) != NULL) { atm_return(vcc, skb->truesize);
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