Re: [Qemu-devel] [PATCH RFC 1/6] io: only allow return path for socket typed

[Daniel P. Berrange]

On Fri, May 19, 2017 at 03:33:12PM +0100, Dr. David Alan Gilbert wrote:
> * Daniel P. Berrange (address@hidden) 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.

Regards,
Daniel
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