Re: [PATCH] Fix bootloader handling when empty string is being output

[Michal Novotny <minovotn@redhat.com>]

On 08/31/2010 11:16 AM, Paolo Bonzini wrote:
> On 08/31/2010 10:47 AM, Ian Campbell wrote:
>> On Mon, 2010-08-30 at 14:10 +0100, Michal Novotny wrote:
>>> Hi,
>>> this is the patch to fix empty string as the output value
>>> of the bootloader string. If there is no output then
>>> xend is being hung indefinitely unless you press Ctrl + C keys
>>> to trigger SIGINT.
>>
>> I think a similar fix will be needed to libxl_bootloader.c, right?
>
> Yes.
>
>> Could we potentially avoid the need to use a select timeout to poll for
>> bootloader exit by including the bootloader FIFO FD and/or the PTY FDs
>> in the select's exceptfds array? Presumably if the process on the other
>> end of such an FD exits that causes some sort of exceptional condition
>> (although historically I've had trouble finding the actual specified
>> behaviour in cases like this).
>
> Using the FIFO is not possible in general because you cannot be sure 
> that the bootloader is opening it at all.  In fact, in this case both 
> libxl and xend will hang at "fifo_fd = open(fifo, O_RDONLY);" so you 
> have to apply O_NDELAY already when you open the FIFO.
>
> Also, in the case the bootloader is not opening the FIFO at all, doing 
> the waitpid in the same thread has a race if the process exits between 
> the waitpid and select.  Fixing the race requires pselect and, 
> especially in Python, introduces more complications than it removes.
>
> So, for RHEL5 xend, Michal has a better version of the patch that will 
> run the waitpid in a separate thread, and use a pipe to wake up the 
> select.  This does the same thing as /proc, but portably and without 
> the need for polling.
>
> However, for both current xend and libxl, using the PTY sounds better, 
> and it looks like an even simpler patch will work that:
>
> - uses O_RDONLY|O_NDELAY when opening the FIFO, and
>
> - exits bootloader_interact when bootloader_fd was in the returned 
> rsel but ret == 0.
>
> Paolo
So, Paolo, what do you recommend for upstream version? There's the PTY 
thing already so what should we do ? Ian, also, I don't know how it's 
working with upstream version since I found out that syntax like `xm 
create -c PVguest` with default settings (pyGrub bootloader) doesn't 
show the pyGrub at all so I don't know what's wrong with my setup. I'm 
using 2.6.32.15-xen kernel/hypervisor version with latest unstable 
user-space tools.

Any hint how this should be working Ian?

Thanks,
Michal

-- 
Michal Novotny<minovotn@redhat.com>, RHCE
Virtualization Team (xen userspace), Red Hat