[Xen-devel] XenStore difficulties.

[John McCullough <jmccullo@xxxxxxxxxxx>]

Hello,

I am working on forking hvm domains and I have been using xenstore to
send commands to qemu.  I have noticed that occasionally qemu's watch is
not read.  I am using the qemu fd handler methods.  I have been trying
to duplicate the missed read outside of qemu and the rest of xend, but I
haven't been able to.  However, I think I may have discovered a memory
leak in lowlevel/xs/xs.c.

I am attaching:
   - commandee.c : consumer of "commands"
   - commander.py : issuer of commands
   - xsblockingchannel.py : com channel for the commander

I am working against a changset from midsummer (11536:041be3f6b38e)
since I'm trying to iron out some of my bugs before moving forward in
the revisions.

% gcc -o commandee commandee.c /usr/lib/libxenstore.a -lpthread
% sudo ./commandee > /dev/null 
% time sudo python commander.py > /dev/null #(in separate terminal)
Traceback (most recent call last):
  File "commander.py", line 9, in ?
      xsbc = xsblockingchannel.xsblockingchannel("test")
  File "/net/xen/xsstress/xsblockingchannel.py", line 19, in __init__
        self.xs.watch(self.path, self)
        xen.lowlevel.xs.Error: (12, 'Cannot allocate memory')
        Exception xen.lowlevel.xs.Error: (2, 'No such file or
        directory') in <bound method xsblockingchannel.__del__ of
        <xsblockingchannel.xsblockingchannel instance at 0xb7ce448c>> ignored

1.02s user 5.00s system 8% cpu 1:08.61 total

Given that I am unwatching the watches in xsblockingchannel I don't
think the memory leak would be due to me.  As far as the lost watches
are concerned, I would speculate that something is wrong with qemu's
select/callback handling, or that xenstore is racing and dropping
things.  If the memory leak has already been fixed, I apologise for the
noise.  I am moving on to unix domain sockets for now, as I have been
running into this problem for a while and I don't currently have the
patience to debug it, but I thought I should mention it so that it is on
the general radar.

Once I finish with the domain sockets I should find out whether it is
the qemu select handling.

Regards,
John McCullough

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