Re: [Qemu-devel] [PATCH v3] monitor: let cur_mon be per-thread

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Tue, Apr 17, 2018 at 11:05:47AM +0200, Markus Armbruster wrote:
> Stefan Hajnoczi <stefanha@redhat.com> writes:
> 
> > 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.
> 
> Code working with the current thread's monitor via thread-local cur_mon
> is easier to analyze in some ways than code working with a Monitor *
> parameter: the latter can interfere with some other thread's monitor,
> and you may have to argue what values the parameter can take.
> 
> You might want to replace parameters by cur_mon in certain cases.
> 
> Funnily, the plan used to be the opposite.  Commit 376253ece48: "On the
> mid or long term, those use case will be obsoleted so that [cur_mon] can
> be removed again."

Either way, the issue I described can still happen since two QMP
commands for a single Monitor object can execute simultaneously in the
main loop thread and the monitor IOThread.

I'm basically warning that QMP multi-threading isn't a solved problem
yet.  It needs to be solved by a combination of making things
thread-safe, documentation, and assertions so code fails loudly and
early when called from an unsupported context.

Stefan

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