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Re: [Qemu-devel] [PATCH RFC 1/6] io: only allow return path for socket t
From: |
Dr. David Alan Gilbert |
Subject: |
Re: [Qemu-devel] [PATCH RFC 1/6] io: only allow return path for socket typed |
Date: |
Fri, 19 May 2017 19:41:34 +0100 |
User-agent: |
Mutt/1.8.2 (2017-04-18) |
* Daniel P. Berrange (address@hidden) wrote:
> 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.
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.
Dave
> Regards,
> Daniel
> --
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--
Dr. David Alan Gilbert / address@hidden / Manchester, UK
- Re: [Qemu-devel] [PATCH RFC 1/6] io: only allow return path for socket typed, (continued)
- Re: [Qemu-devel] [PATCH RFC 1/6] io: only allow return path for socket typed, Dr. David Alan Gilbert, 2017/05/19
- Re: [Qemu-devel] [PATCH RFC 1/6] io: only allow return path for socket typed, Daniel P. Berrange, 2017/05/19
- Re: [Qemu-devel] [PATCH RFC 1/6] io: only allow return path for socket typed, Dr. David Alan Gilbert, 2017/05/19
- Re: [Qemu-devel] [PATCH RFC 1/6] io: only allow return path for socket typed, Daniel P. Berrange, 2017/05/19
- Re: [Qemu-devel] [PATCH RFC 1/6] io: only allow return path for socket typed, Dr. David Alan Gilbert, 2017/05/19
- Re: [Qemu-devel] [PATCH RFC 1/6] io: only allow return path for socket typed, Daniel P. Berrange, 2017/05/19
- Re: [Qemu-devel] [PATCH RFC 1/6] io: only allow return path for socket typed,
Dr. David Alan Gilbert <=
- Re: [Qemu-devel] [PATCH RFC 1/6] io: only allow return path for socket typed, Daniel P. Berrange, 2017/05/22
[Qemu-devel] [PATCH RFC 2/6] migration: isolate return path on src, Peter Xu, 2017/05/19
[Qemu-devel] [PATCH RFC 3/6] migration: fix leak of src file on dst, Peter Xu, 2017/05/19
[Qemu-devel] [PATCH RFC 4/6] migration: shut src return path unconditionally, Peter Xu, 2017/05/19