Re: [PATCH] 2.6.0 NBD driver: remove send/recieve race for request

[Paul Clements]

Lou Langholtz wrote:

> >--- linux-2.6.0-test2-mm4-PRISTINE/drivers/block/nbd.c Sun Jul 27 12:58:51 2003
> >+++ linux-2.6.0-test2-mm4/drivers/block/nbd.c  Thu Aug  7 18:02:23 2003
> >@@ -416,11 +416,19 @@ void nbd_clear_que(struct nbd_device *lo
> >       BUG_ON(lo->magic != LO_MAGIC);
> > #endif
> >
> >+retry:
> >       do {
> >               req = NULL;
> >               spin_lock(&lo->queue_lock);
> >               if (!list_empty(&lo->queue_head)) {
> >                       req = list_entry(lo->queue_head.next, struct request, queuelist);
> >+                      if (req->ref_count > 1) { /* still in xmit */
> >+                              spin_unlock(&lo->queue_lock);
> >+                              printk(KERN_DEBUG "%s: request %p: still in use (%d), waiting...\n",
> >+                                  lo->disk->disk_name, req, req->ref_count);
> >+                              schedule_timeout(HZ); /* wait a second */
> >

> Isn't there something more deterministic than just waiting a second and
> hoping things clear up that you can use here? 

It's not exactly "hoping" something will happen...we're waiting until
do_nbd_request decrements the ref_count, which is completely
deterministic, but just not guaranteed to have already happened when
nbd_clear_que() starts. 

The use of the ref_count closes a race window (the one that I pointed
out in my response to Lou's original patch a few days ago). It protects
us in the following case(s):

1) do_nbd_request begins -- sock and file are valid
2) do_nbd_request queues the request and then calls nbd_send_req to send
the request out on the network
3) on another CPU, or after preempt, nbd-client gets a signal or an
"nbd-client -d" is called, which results in sock and file being set to
NULL, and the queue begins to be cleared, and requests that were on the
queue get freed (before do_nbd_request has finished accessing req for
the last time)


> How about not clearing the
> queue unless lo->sock is NULL and using whatever lock it is now that's
> protecting lo->sock. That way the queue clearing race can be eliminated too.

That makes sense. I hadn't done this (for compatibility reasons) since
"nbd-client -d" (disconnect) calls NBD_CLEAR_QUE before disconnecting
the socket. But I think we can just make NBD_CLEAR_QUE silently fail
(return 0) if lo->sock is set and basically turn that first
NBD_CLEAR_QUE into a noop, and let the nbd_clear_que() call at the end
of NBD_CLEAR_SOCK handle it (properly). Note that the race condition
above still applies, even with this lo->sock check in place. But this
check does eliminate the race in "nbd-client -d" where, e.g., requests
are being queued up faster than nbd_clear_que can dequeue them,
potentially making nbd_clear_que loop forever.

As a side note, NBD_CLEAR_QUE is actually now completely superfluous,
since NBD_DO_IT and NBD_CLEAR_SOCK (which always get called in
conjunction with NBD_CLEAR_QUE) contain the nbd_clear_que() call
themselves. We could just make NBD_CLEAR_QUE a noop in all cases, but I
guess we'd risk breaking some nbd client tool that's out there...

I'm testing the updated patch now, I'll send it out in a little while...

Thanks,
Paul
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