Re: Race condition? /tmp/net.ifaces and pre-pivot

[Warren Togami <wtogami-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

On 07/23/2009 05:37 PM, Warren Togami wrote:
> I am running into an odd issue that is supposedto be impossible.
>
> pre-pivot/20write-ifcfg.sh is often being run but fails to write
> net.*.ifcfg files because /tmp/net.ifaces does not exist at that moment.
> The mount was otherwise successful, and rdbreak before switch_root sees
> that /tmp/net.ifaces exists.
>
> With plain "root=dhcp" with a NFS rootfs, it seems to never happen.
> However with "root=dhcp bridge", perhaps 25-75% of the time it is
> failing to write ifcfg files.
>
> Some kind of race going on? An inspection of the code seems to me that
> /tmp/net.ifaces should have already been created prior to pre-pivot?
>

<dillow> warren: if you don't have any ip= lines on the command line, 
/tmp/net.ifaces gets created by netroot after the handler successfully 
completes
<dillow> warren: I think that races with the check loop in init
<dillow> warren: single processor box?
<dillow> warren: it looks like you could get scheduled away from netroot 
and the init command loop could notice that root is mounted and make it 
to the pre-pivot hook during a single quantum
<dillow> warren: as a test, you could move the line '[ ! -f 
/tmp/net.ifaces ] && echo $netif > /tmp/net.ifaces' in netroot in front 
of the handler call
<dillow> warren: that should work around your issue. I don't think it is 
a long term fix, though -- the multiple NIC case would need some work to 
be clean
<dillow> warren: this would only affect NFS, as the other devices would 
create /dev/root and exit that way to the mount loop
<dillow> so they wouldn't sail through the mount loop due to the 
existence of $NEWROOT/proc
<warren> dillow: single processor qemu VM
<warren> dillow: so this race was happening before, only we didn't 
notice it since we weren't relying on anything
<warren> dillow: any suggestions of a substitution that wont break dash?
<dillow> warren: I think it got introduced with the initqueue stuff or a 
combination of that and changing the loops

Harald, any ideas?

Warren Togami
wtogami-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org
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