Re: [PATCH][PPPOL2TP]: Fix SMP oops in pppol2tp driver

James Chapman
2008-02-20T16:03:04Z

Jarek Poplawski wrote:
> On Mon, Feb 18, 2008 at 10:09:24PM +0000, James Chapman wrote:
> ...
>> Unfortunately the ISP's syslog stops. But I've been able to borrow
>> two Quad Xeon boxes and have reproduced the problem.
>>
>> Here's a new version of the patch. The patch avoids disabling irqs
>> and fixes the sk_dst_get() usage that DaveM mentioned. But even with
>> this patch, lockdep still complains if hundreds of ppp sessions are
>> inserted into a tunnel as rapidly as possible (lockdep trace is below).
>> I can stop these errors by wrapping the call to ppp_input() in
>> pppol2tp_recv_dequeue_skb() with local_irq_save/restore. What is a
>> better fix?
>
> I send here my proposal: it's intended for testing and to check one of
> possible solutions here. IMHO your lockdep reports show there is no
> use to change anything around sk_dst_lock: it would need the global
> change of this lock to fix this problem. So the fix should be done
> around pch->upl lock and this means changing ppp_generic.

Hmm, I need to study the lockdep report again. It seems I'm misreading
the lockdep output. :(

> In the patch below I've used trylock in places which seem to allow
> for skipping some things (while config is changed only) or simply
> don't need this lock because there is no ppp struct. This could be
> modified to add some waiting loop if necessary. Another option is to
> change the write side of this lock: it looks like more vulnerable if
> something missed because there are more locks involved, but probably
> should be enough to solve this problem too.
>
> I think pppol2tp need to be first checked only with hlist_lock bh
> patch, unless there were some lockdep reports on these other locks
> too. (BTW, I added ppp maintainer to CC - I hope we get Paul's opinion
> on this.)

I tried your ppp_generic patch with only the hlist_lock bh patch in
pppol2tp and it seems to fix the ppp create/delete issue. However, when
I added much more traffic into the test (flood pings over ppp interfaces
while repeatedly creating/deleting the L2TP (PPP) sessions) I get a soft
lockup detected in pppol2tp_xmit() after anything between 1 minute and
an hour. :( I'm investigating that now.

Thanks for your help!

> (testing patch #1)
> ---
>
> drivers/net/ppp_generic.c | 33 +++++++++++++++++++++++----------
> 1 files changed, 23 insertions(+), 10 deletions(-)
>
> diff --git a/drivers/net/ppp_generic.c b/drivers/net/ppp_generic.c
> index 4dc5b4b..5cbc534 100644
> --- a/drivers/net/ppp_generic.c
> +++ b/drivers/net/ppp_generic.c
> @@ -1473,7 +1473,7 @@ void
> ppp_input(struct ppp_channel *chan, struct sk_buff *skb)
> {
> struct channel *pch = chan->ppp;
> - int proto;
> + int proto, locked;
>
> if (!pch || skb->len == 0) {
> kfree_skb(skb);
> @@ -1481,8 +1481,13 @@ ppp_input(struct ppp_channel *chan, struct sk_buff *skb)
> }
>
> proto = PPP_PROTO(skb);
> - read_lock_bh(&pch->upl);
> - if (!pch->ppp || proto >= 0xc000 || proto == PPP_CCPFRAG) {
> + /*
> + * We use trylock to avoid dependency between soft-irq-safe upl lock
> + * and soft-irq-unsafe sk_dst_lock.
> + */
> + local_bh_disable();
> + locked = read_trylock(&pch->upl);
> + if (!locked || !pch->ppp || proto >= 0xc000 || proto == PPP_CCPFRAG) {
> /* put it on the channel queue */
> skb_queue_tail(&pch->file.rq, skb);
> /* drop old frames if queue too long */
> @@ -1493,7 +1498,10 @@ ppp_input(struct ppp_channel *chan, struct sk_buff *skb)
> } else {
> ppp_do_recv(pch->ppp, skb, pch);
> }
> - read_unlock_bh(&pch->upl);
> +
> + if (locked)
> + read_unlock(&pch->upl);
> + local_bh_enable();
> }
>
> /* Put a 0-length skb in the receive queue as an error indication */
> @@ -1506,16 +1514,18 @@ ppp_input_error(struct ppp_channel *chan, int code)
> if (!pch)
> return;
>
> - read_lock_bh(&pch->upl);
> - if (pch->ppp) {
> + /* a trylock comment in ppp_input() */
> + local_bh_disable();
> + if (read_trylock(&pch->upl) && pch->ppp) {
> skb = alloc_skb(0, GFP_ATOMIC);
> if (skb) {
> skb->len = 0; /* probably unnecessary */
> skb->cb[0] = code;
> ppp_do_recv(pch->ppp, skb, pch);
> }
> + read_unlock(&pch->upl);
> }
> - read_unlock_bh(&pch->upl);
> + local_bh_enable();
> }
>
> /*
> @@ -2044,10 +2054,13 @@ int ppp_unit_number(struct ppp_channel *chan)
> int unit = -1;
>
> if (pch) {
> - read_lock_bh(&pch->upl);
> - if (pch->ppp)
> + /* a trylock comment in ppp_input() */
> + local_bh_disable();
> + if (read_trylock(&pch->upl) && pch->ppp) {
> unit = pch->ppp->file.index;
> - read_unlock_bh(&pch->upl);
> + read_unlock(&pch->upl);
> + }
> + local_bh_enable();
> }
> return unit;
> }
> --

--
James Chapman
Katalix Systems Ltd
http://www.katalix.com
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