On Fri, May 19, 2017 at 07:41:34PM +0100, Dr. David Alan Gilbert wrote: > * Daniel P. Berrange (berra...@redhat.com) wrote: > > On Fri, May 19, 2017 at 03:33:12PM +0100, Dr. David Alan Gilbert wrote: > > > * Daniel P. Berrange (berra...@redhat.com) wrote: > > > > > shutdown() is safe, in that it stops any other threads accessing the > > > > > fd > > > > > but doesn't allow it's reallocation until the close; We perform the > > > > > close only when we've joined all other threads that were using the fd. > > > > > Any of the threads that do new calls on the fd get an error and > > > > > quickly > > > > > fall down their error paths. > > > > > > > > Ahh that's certainly an interesting scenario. That would certainly be > > > > a problem with the migration code when this was originally written. > > > > It had two QEMUFile structs each with an 'int fd' field, so when you > > > > close 'fd' on one QEMUFile struct, it wouldn't update the other QEMUFile > > > > used by another thread. > > > > > > > > Since we switched over to use QIOChannel though, I think the thread > > > > scenario you describe should be avoided entirely. When you have multiple > > > > QEMUFile objects, they each have a reference counted pointer to the same > > > > underlying QIOChannel object instance. So when QEMUFile triggers a call > > > > to qio_channel_close() in one thread, that'll set fd=-1 in the > > > > QIOChannel. > > > > Since the other threads have a reference to the same QIOChannel object, > > > > they'll now see this fd == -1 straightaway. > > > > > > > > So, IIUC, this should make the need for shutdown() redundant (at least > > > > for the thread race conditions you describe). > > > > > > That's not thread safe unless you're doing some very careful locking. > > > Consider: > > > T1 T2 > > > oldfd=fd tmp=fd > > > fd=-1 > > > close(oldfd) > > > unrelated open() > > > read(tmp,... > > > > > > In practice every use of fd will be a copy into a tmp and then the > > > syscall; the unrelated open() could happen in another thread. > > > (OK, the gap between the tmp and the read is tiny, although if we're > > > doing multiple operations chances are the compiler will optimise > > > it to the top of a loop). > > > > > > There's no way to make that code safe. > > > > Urgh, yes, I see what you mean. > > > > Currently the QIOChannelCommand implementation, uses a pair of anonymous > > pipes for stdin/out to the child process. I wonder if we could switch > > that to use socketpair() instead, thus letting us shutdown() on it too. > > > > Though I guess it would be sufficient for qio_channel_shutdown() to > > merely kill the child PID, while leaving the FDs open, as then you'd > > get EOF and/or EPIPE on the read/writes. > > Yes, I guess it's a question of which one is more likely to actually > kill the exec child off; the socketpair is more likely to cause the > source side migration code to cancel cleanly, although a kill -9 > should sort out a wayward exec child.
I'm also curious if there's any real world performance difference between using a pipe vs socketpair to communicate with a child process Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
