On Tue, Mar 20, 2018 at 04:02:44PM +0000, Daniel P. Berrangé wrote: > No, it is unsafe - we rely on lockf() to get the mutual exclusion. > If a QEMU is running with pidfile locked, and its pid written into > it, then a 2nd QEMU comes along it will truncate the pidfile owned > by the original QEMU because the truncation happens before it has > tried to acquire the lock. The 2nd QEMU will still exit, but the > original QEMU's pid has now been lost.
That's correct, thanks for pointing it out. > We must call ftruncate() after lockf(), but before writing the new > pid into the file. That ensures there is no window in which it is > possible to see the new & old pids mixed together. I'll send a revised version doing exactly that. >From my reading of the Windows API documentation, this might not be a problem there: The file is opened with FILE_SHARE_READ, which prohibits opening the file in a writable mode and CREATE_ALWAYS will only recreate the file if it is writable. Florian
