[libvirt] [PATCH] virtlockd: acquire locks on re-exec

[Daniel P. Berrangé]

On Fri, Mar 02, 2018 at 03:24:53PM -0700, Jim Fehlig wrote:
> On 03/02/2018 11:12 AM, Daniel P. Berrangé wrote:
> > On Fri, Mar 02, 2018 at 04:52:23PM +0000, Daniel P. Berrangé wrote:
> > > On Thu, Mar 01, 2018 at 04:42:36PM -0700, Jim Fehlig wrote:
> > > > Locks held by virtlockd are dropped on re-exec.
> > > > 
> > > > virtlockd       94306 POSIX 5.4G WRITE 0     0   0 /tmp/test.qcow2
> > > > virtlockd       94306 POSIX   5B WRITE 0     0   0 /run/virtlockd.pid
> > > > virtlockd       94306 POSIX   5B WRITE 0     0   0 /run/virtlockd.pid
> > > > 
> > > > Acquire locks in PostExecRestart code path.
> > > 
> > > This is really strange and should *not* be happening.  POSIX locks
> > > are supposed to be preserved across execve() if the FD has CLOEXEC
> > > unset, and you don't fork() before the exec.
> > 
> > [snip]
> > 
> > > So I wonder what we've screwed up to cause the locks to get
> > > released - reaquiring them definitely isn't desirable as we
> > > should not loose them in the first place !
> > 
> > This is really very strange. The problem seems to be the existance of
> > threads at time of execve().
> > 
> > If you spawn a thread and the thread exits, and you execve the locks
> > are preserved.
> > 
> > If you spawn a thread and the thread is still running, and you execve
> > the locks are lost.
> 
> Indeed you are correct. I'm seeing the same behavior with the below
> modifications to your demo. The lock is preserved after execve when
> BREAK_FLOCK is 0, but removed when BREAK_FLOCK is 1.

I tried RHEL7 vintage kernel and saw the same behaviour, so this seems
to be a fairly long standing issue.

In theory we can make virNetServer single-threaded by passing max_workers==0
when creating it, but I just tried that and hit deadlock, so it seems that
code has bit-rotted. If I we can fix that, then I think leaving it single
threaded is the easiest way to deal with this. Failing that, we would have
to arrange to cleanly terminate all threads before re-exec'ing.


Regards,
Daniel
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