On Wed, Mar 21, 2018 at 03:58:05PM -0500, Eric Blake wrote: > 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)?
The original test (before Pino's patch) runs ‘qemu-img info /dev/null’ purely as a way to fork qemu-img. It's a regression test for some problem we had with the code that confines qemu-img using resource limits. So really nothing much to do with qemu-img at all. The fix is to create a temporary file and run qemu-img against that. Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com virt-top is 'top' for virtual machines. Tiny program with many powerful monitoring features, net stats, disk stats, logging, etc. http://people.redhat.com/~rjones/virt-top
