Re: [Qemu-devel] race condition when exec'ing "qemu -incoming" followed by monitor "cont"

["Daniel P. Berrange" <berrange@redhat.com>]

On Fri, Apr 09, 2010 at 12:03:54PM -0400, Laine Stump wrote:
> (Please forgive (and correct!) any inaccuracies in my description of 
> qemu's workings - I've only recently started looking at it directly, 
> rather than through the lens of libvirt)
> 
> libvirt implements a "domain restore" operation by:
> 
> 0) start with a previously saved domain image in a file
> 
> 1) open the domain image, and connect it to a pipe
> 
> 2) fork, connect the pipe to stdin, and exec qemu with "-incoming exec:cat"
> 
> 3) execute "cont" in that qemu's monitor.
> 
> (for those familiar with the code, you can look at the 
> src/qemu/eqmu_driver.c:qemudDomainRestore() in the libvirt source).
> 
> Although this works successfully for most people, I'm consistently 
> seeing a problem on my particular hardware (Intel Core 2 Duo 2.2Ghz) 
> that causes this domain restore to fail. It seems that the "cont" 
> command takes effect before the restore is completed (possibly/probably 
> before it even starts?) resulting in a failed restore - the domain is 
> left in some random state, sometimes rebooting spontaneously, sometimes 
> just hung.
> 
> If I insert a usleep(250 * 1000) between starting up qemu with 
> "-incoming exec:cat" and issuing "cont" to start the CPUs, the restore 
> is successful 100% of the time.
> 
> I've been told that once the incoming migration starts, the monitor will 
> be non-responsive until it is complete. This should mean that as long as 
> the "cont" isn't issued until after the migration starts, it will be 
> blocked until the migration is complete, thus protecting us from the 
> race; for this reason (along with the fact that a 250msec sleep is 
> enough to cure the problem) I'm thinking it's likely the "cont" happens 
> before the migration starts.
> 
> There is, of course, an "info migrate" command in the monitor that could 
> be used to assure the migration had completed before issuing "cont", but 
> that command only works for outgoing migrations, not incoming 
> (presumably if it was available, checking the info prior to the 
> migration starting would return "not started" (or something similar), 
> and once it had started, the entire monitor interface would block until 
> the migrate was completed).

Yep, I'd really like to see a 'info migrate' or equivalent that works
for incoming migration, even if it can't give us progress info, just
the status report is important to detect failure & completion.

Daniel
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