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Re: [Qemu-devel] [PATCH v3] monitor: let cur_mon be per-thread
From: |
Stefan Hajnoczi |
Subject: |
Re: [Qemu-devel] [PATCH v3] monitor: let cur_mon be per-thread |
Date: |
Wed, 18 Apr 2018 13:12:24 +0800 |
User-agent: |
Mutt/1.9.2 (2017-12-15) |
On Tue, Apr 17, 2018 at 11:05:47AM +0200, Markus Armbruster wrote:
> Stefan Hajnoczi <address@hidden> 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 <address@hidden>
> >> > > ---
> >> > > 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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- [Qemu-devel] [PATCH v3] monitor: let cur_mon be per-thread, Peter Xu, 2018/04/12
- Re: [Qemu-devel] [PATCH v3] monitor: let cur_mon be per-thread, Eric Blake, 2018/04/12
- Re: [Qemu-devel] [PATCH v3] monitor: let cur_mon be per-thread, Marc-André Lureau, 2018/04/13
- Re: [Qemu-devel] [PATCH v3] monitor: let cur_mon be per-thread, Stefan Hajnoczi, 2018/04/16
- Re: [Qemu-devel] [PATCH v3] monitor: let cur_mon be per-thread, Peter Xu, 2018/04/16
- Re: [Qemu-devel] [PATCH v3] monitor: let cur_mon be per-thread, Stefan Hajnoczi, 2018/04/17
- Re: [Qemu-devel] [PATCH v3] monitor: let cur_mon be per-thread, Markus Armbruster, 2018/04/17
- Re: [Qemu-devel] [PATCH v3] monitor: let cur_mon be per-thread,
Stefan Hajnoczi <=
- Re: [Qemu-devel] [PATCH v3] monitor: let cur_mon be per-thread, Peter Xu, 2018/04/18
- Re: [Qemu-devel] [PATCH v3] monitor: let cur_mon be per-thread, Markus Armbruster, 2018/04/18
- Re: [Qemu-devel] [PATCH v3] monitor: let cur_mon be per-thread, Peter Xu, 2018/04/18