Re: [Xen-devel] XenStore difficulties.

[Ewan Mellor <ewan@xxxxxxxxxxxxx>]

On Mon, Nov 13, 2006 at 09:23:52PM -0800, John McCullough wrote:

> 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

I can't remember the details, but I have vague recollection of a
Xenstore-related problem where the error was ENOMEM, but that error was
misleading.

One thing you can try is to run xenstore in trace mode: export
XENSTORED_TRACE=1 before running xend, which will add "-T
/var/log/xen/xenstored-trace.log" to the xenstore command line.  This will
give you a log of every operation on the store, which will tell us whether the
error code is coming from xenstore.

Regarding the missing read -- that's probably because you are not handling
EAGAIN when doing xs_transaction_end.  You need to retry if xs_transaction_end
fails with errno = EAGAIN, being careful not to allocate memory each retry.
See xenstore_client.c for example.

Ewan.

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