Re: "xprt" reference count drops to 0

[Menyhart Zoltan <Zoltan.Menyhart@bull.net>]

J. Bruce Fields wrote:
> On Tue, Oct 05, 2010 at 10:22:30AM +0200, Menyhart Zoltan wrote:
>> Due to some race conditions, the reference count can become 0
>> while "xprt" is still on a "pool":
>
> Apologies, your email got buried in my inbox....
>
>>
>> WARNING: at lib/kref.c:43 kref_get+0x23/0x2d()
>>   [] kref_get+0x23/0x2d
>>   [] svc_xprt_get+0x12/0x14 [sunrpc]
>>   [] svc_recv+0x2db/0x78a [sunrpc]
>
> Which kernel exactly did you see this on?  Is it reproduceable?

I saw it on a 2.6.32.
It has not been corrected for the 2.6.36-rc3 yet.
The patch is for  the 2.6.36-rc3.

It is a narrow window, you need a high work load and a bit of luck  to
delay the current CPU just after"svc_xprt_enqueue()" returns.

>> I think we should increase the reference counter before adding "xprt"
>> onto any list.
>
> I don't see the xprt added to any list after the svc_xprt_get() you've
> added below.

"svc_xprt_enqueue()" has got two ways to pass an "xprt":
- via "rqstp->rq_xprt" if a worker is available,
- on the "pool->sp_sockets" list otherwise

         if (!list_empty(&pool->sp_threads)) {
                 rqstp = list_entry(pool->sp_threads.next,  struct svc_rqst,  rq_list);
                 svc_thread_dequeue(pool, rqstp);
                 rqstp->rq_xprt = xprt;
                 svc_xprt_get(xprt);
                 rqstp->rq_reserved = serv->sv_max_mesg;
                 atomic_add(rqstp->rq_reserved, &xprt->xpt_reserved);
                 pool->sp_stats.threads_woken++;
                 wake_up(&rqstp->rq_wait);
         } else {
                 list_add_tail(&xprt->xpt_ready, &pool->sp_sockets);
                 pool->sp_stats.sockets_queued++;
         }

In the 1st case, there is a "svc_xprt_get(xprt)", in the 2nd one, there is not.
Once "svc_xprt_enqueue()" returns, at some places, "svc_xprt_put(xprt)" is
invoked. If we has passed the "else" branch, the "kref" can drop down to 0.

"svc_recv()" includes:

         xprt = svc_xprt_dequeue(pool);
         if (xprt) {
                 rqstp->rq_xprt = xprt;
                 svc_xprt_get(xprt);
                 rqstp->rq_reserved = serv->sv_max_mesg;
                 atomic_add(rqstp->rq_reserved, &xprt->xpt_reserved);
         } else {

i.e. it calls "svc_xprt_get(xprt)" if it has picked up "xprt" from the "pool" list.
Yet it is too late, if the caller of "svc_xprt_enqueue()" has already had the
time to call "svc_xprt_put(xprt)".

I think "svc_xprt_get(xprt)" should be called in "svc_xprt_enqueue()" before
"list_add_tail(&xprt->xpt_ready, &pool->sp_sockets)" to keep the reference
counter valid.
It is much simpler to move the call "svc_xprt_get(xprt)" just before

	if (!list_empty(&pool->sp_threads))

Thanks,

Zoltan