On Mon, Apr 16, 2018 at 05:17:32PM +0800, Peter Xu wrote:
> On Mon, Apr 16, 2018 at 04:37:48PM +0800, Stefan Hajnoczi wrote:
> > On Thu, Apr 12, 2018 at 02:11:08PM +0800, Peter Xu wrote:
> > > In the future the monitor iothread may be accessing the cur_mon as
> > > well (via monitor_qmp_dispatch_one()).  Before we introduce a real
> > > Out-Of-Band command, let's convert the cur_mon variable to be a
> > > per-thread variable to make sure there won't be a race between threads.
> > >
> > > Note that thread variables are not initialized to a valid value when new
> > > thread is created.  However for our case we don't need to set it up,
> > > since the cur_mon variable is only used in such a pattern:
> > > 
> > >   old_mon = cur_mon;
> > >   cur_mon = xxx;
> > >   (do something, read cur_mon if necessary in the stack)
> > >   cur_mon = old_mon;
> > > 
> > > It plays a role as stack variable, so no need to be initialized at all.
> > > We only need to make sure the variable won't be changed unexpectedly by
> > > other threads.
> > > 
> > > Signed-off-by: Peter Xu <peterx@redhat.com>
> > > ---
> > > v3:
> > > - fix code style warning from patchew
> > > v2:
> > > - drop qemu-thread changes
> > > ---
> > >  include/monitor/monitor.h | 2 +-
> > >  monitor.c                 | 2 +-
> > >  stubs/monitor.c           | 2 +-
> > >  tests/test-util-sockets.c | 2 +-
> > >  4 files changed, 4 insertions(+), 4 deletions(-)
> > 
> > The Monitor object is not fully thread-safe, so although the correct
> > cur_mon is now accessible, code may still be unsafe.  For example,
> > monitor_get_fd(cur_mon, ...) is not thread-safe and must not be used by
> > OOB commands.
> 
> IMHO things like monitor_get_fd() should only be called in QMP
> context, so there should always be a monitor_qmp_dispatch_one() in the
> stack already (no matter whether it is in main thread or the monitor
> iothread), which means that cur_mon should have been setup.  So IMHO
> it's a programming error if monitor_get_fd() is called without correct
> cur_mon setup after this patch.

The pointer value of cur_mon is not the issue, you have made that work
correctly.  The problem is that some monitor.h APIs do not access the
Monitor object in a thread-safe fashion.

Two QMP commands executing simultaneously in the main loop thread and
the monitor IOThread can hit race conditions.  The example I gave was
the monitor_get_fd() API, which iterates and modifies the mon->fds
QLIST without a lock.

Please audit monitor.h and either make things thread-safe or document
the thread-safety rules (e.g. "This function cannot be called from
out-of-band QMP context").  This wasn't necessary before but now that
you are adding multi-threading it is.

Stefan