Re: [Qemu-block] [Qemu-devel] [Libguestfs] [PATCH] tests: regressions: make test-big-heap use a temporary empty file

[Eric Blake]

On 03/21/2018 03:44 PM, Kevin Wolf wrote:
> > > You're right that file locking on a character device like /dev/null is
> > > not going to work as expected, but is it a case where fcntl() actually
> > > fails, or is it worse where the fcntl() claiming the locks "succeeds"
> > > but doesn't do what we want?  That is, what were the actual error
> > > messages you ran into?
> >
> > $ qemu-img --version
> > qemu-img version 2.10.1(qemu-2.10.1-2.fc27)
> > Copyright (c) 2003-2017 Fabrice Bellard and the QEMU Project developers
> > $ qemu-img info /dev/null
> > qemu-img: Could not open '/dev/null': Failed to get "consistent read" lock
> > Is another process using the image?
>
> Not sure where the difference is, but I can't reproduce this on
> upstream, neither git master nor the v2.10.1 tag:

Is it a case where file locking actually works, and more than one 
process is trying to lock /dev/null at once?  (qemu-img info is 
short-lived, but could there be another longer-lived process also using 
/dev/null)?

Does using -r help (if the only reason you're telling qemu-img to 
operate on /dev/null is to probe qemu-img features, can you probe those 
same features without needing to write, which in turn requests less 
locking)?

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org