Re: [V9fs-developer] 9pfs hangs since 4.7

[Greg Kurz <groug@kaod.org>]

On Sat, 7 Jan 2017 17:19:10 +0000
Al Viro <viro@ZenIV.linux.org.uk> wrote:

> On Sat, Jan 07, 2017 at 04:10:45PM +0100, Greg Kurz wrote:
> > > virtqueue_push(), but pdu freeing is delayed until v9fs_flush() gets woken
> > > up.  In the meanwhile, another request arrives into the slot of freed by
> > > that virtqueue_push() and we are out of pdus.
> > >   
> > 
> > Indeed. Even if this doesn't seem to be the problem here, I guess this should
> > be fixed.  
> 
> 	FWIW, there's something that looks like an off-by-one in
> v9fs_device_realize_common():
>     /* initialize pdu allocator */
>     QLIST_INIT(&s->free_list);
>     QLIST_INIT(&s->active_list);
>     for (i = 0; i < (MAX_REQ - 1); i++) {
>         QLIST_INSERT_HEAD(&s->free_list, &s->pdus[i], next);
>         s->pdus[i].s = s;
>         s->pdus[i].idx = i;
>     }
> 
> Had been there since the original merge of 9p support into qemu - that code
> had moved around a bit, but it had never inserted s->pdus[MAX_REQ - 1] into
> free list.  So your scenario with failing pdu_alloc() is still possible.

Indeed, this (MAX_REQ - 1) thing looks wrong. Thanks for poiting that out.

> In that log the total amount of pending requests has reached 128 for the
> first time right when the requests had stopped being handled and even
> though it had dropped below that shortly after, extra requests being put
> into queue had not been processed at all...
> 
> I'm not familiar with qemu guts enough to tell if that's a plausible scenario,
> though... shouldn't subsequent queue insertions (after enough slots had been
> released) simply trigger virtio_queue_notify_vq() again?  It *is* a bug
> (if we get a burst filling a previously empty queue all at once, there won't
> be any slots becoming freed), but that's obviously not the case here -
> slots were getting freed, after all.