Re: running out of tags in 9P (was Re: [git pull] vfs part 2)

[Al Viro <viro@ZenIV.linux.org.uk>]

On Thu, Jul 02, 2015 at 11:19:03AM +0300, Andrey Ryabinin wrote:
> Besides qemu, I've also tried kvmtool with the same result. IOW I'm seeing
> this under kvmtool as well. It just takes a bit longer to reproduce
> this in kvmtool.
> 
> > The bug I suspected to be the cause of that is in tag allocation in
> > net/9p/client.c - we could end up wrapping around 2^16 with enough pending
> > requests and that would have triggered that kind of mess.  However, Andrey
> > doesn't see that test (tag wraparound in p9_client_prepare_req()) trigger.
> > BTW, was that on the run where debugging printk in p9_client_write() *did*
> > trigger?
> 
> Yes, WARN_ON_ONCE() in p9_client_prepare_req() didn't trigger,
> but debug printk in p9_client_write() *did* trigger.

Bloody wonderful...  Could you check if v9fs_write() in qemu
hw/9pfs/virtio-9p.c ever gets to
    offset = 7;
    err = pdu_marshal(pdu, offset, "d", total);
with total > count on your testcase?