Re: [libvirt] [PATCH] command: avoid potential deadlock on handshake

On 06/07/2012 12:51 AM, Michal Privoznik wrote: I decided the commit message was confusing, so I updated it before pushing: There is a theoretical problem of an extreme bug where we can get into deadlock due to command handshaking. Thanks to a pair of pipes, we have a situation where the parent thinks the child reported an error and is waiting for a message from the child to explain the error; but at the same time the child thinks it reported success and is waiting for the parent to acknowledge the success; so both processes are now blocked. Thankfully, I don't think this deadlock is possible without at least one other bug in the code, but I did see exactly that sort of situation prior to commit da831af - I saw a backtrace where a double close bug in the parent caused the parent to read from the wrong fd and assume the child failed, even though the child really sent success. This potential deadlock is not quite like commit 858c247 (a deadlock due to multiple readers on one pipe preventing a write from completing), although the solution is similar - always close unused pipe fds before blocking, rather than after. -- Eric Blake eblake(a)redhat.com +1-919-301-3266 Libvirt virtualization library http://libvirt.org