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Re: [Qemu-devel] [PATCH 2/2] qemu-thread: let cur_mon be per-thread
From: |
Peter Xu |
Subject: |
Re: [Qemu-devel] [PATCH 2/2] qemu-thread: let cur_mon be per-thread |
Date: |
Wed, 11 Apr 2018 11:31:18 +0800 |
User-agent: |
Mutt/1.9.1 (2017-09-22) |
On Tue, Apr 10, 2018 at 08:54:31AM -0500, Eric Blake wrote:
> On 04/10/2018 07:49 AM, Peter Xu wrote:
> > cur_mon was only used in main loop so we don't really need that to be
> > per-thread variable. Now it's possible that we have more than one
> > thread to operate on it. Let's start to let it be per-thread variable.
> >
> > In case we'll create threads within a valid cur_mon setup, we'd better
> > let the child threads to inherit the cur_mon from parent thread too. Do
> > that for both posix and win32 threads.
> >
> > Signed-off-by: Peter Xu <address@hidden>
> > ---
> > include/monitor/monitor.h | 2 +-
> > include/qemu/thread-win32.h | 1 +
> > monitor.c | 2 +-
> > stubs/monitor.c | 2 +-
> > tests/test-util-sockets.c | 2 +-
> > util/qemu-thread-posix.c | 6 ++++++
> > util/qemu-thread-win32.c | 6 ++++++
> > 7 files changed, 17 insertions(+), 4 deletions(-)
> >
>
> > @@ -494,6 +496,9 @@ static void *qemu_thread_start(void *args)
> > void *(*start_routine)(void *) = qemu_thread_args->start_routine;
> > void *arg = qemu_thread_args->arg;
> >
> > + /* Inherit the cur_mon pointer from father thread */
>
> More typical as s/father/parent/
Fixed.
>
> > +++ b/util/qemu-thread-win32.c
>
> > @@ -339,6 +341,9 @@ static unsigned __stdcall win32_start_routine(void *arg)
> > void *(*start_routine)(void *) = data->start_routine;
> > void *thread_arg = data->arg;
> >
> > + /* Inherit the cur_mon pointer from father thread */
> > + cur_mon = data->current_monitor;
>
> Otherwise makes sense to me.
>
> I agree with your analysis that the set of existing OOB commands (just
> 'x-oob-test') has no direct use of cur_mon. I'm a little fuzzier on
> whether the OOB changes can cause cur_mon to be modified by two threads
> in parallel (monitor_qmp_dispatch_one() is futzing around with 'cur_mon'
> around the call to qmp_dispatch(), and at least
> qmp_human_monitor_command() is also futzing around with it; is there a
> case where handling qmp_human_monitor_command() in the dispatch thread
> in parallel with more input on the main thread could break?) Thus I'm
> not sure whether this is needed for 2.12 to avoid a regression.
Could I ask what's the "more input on the main thread"?
AFAIU, if we don't take x-oob-test into account, then still only the
main thread will touch (not only modify, even read) the cur_mon
variable. And as long as that's true, we are keeping the old behavior
as when we are without Out-Of-Band, then IMHO we'll be fine.
qmp_human_monitor_command() is only one QMP command handler, now it
can only be run within main thread. So even we can have the stack
like monitor_qmp_dispatch_one (which modifies cur_mon) calls
qmp_human_monitor_command (which modifies cur_mon again), still we'll
be fine too as long as they are in the same thread, just like before.
That's why I think it's not a 2.12 regression. We just need to be
prepared for what might come. Thanks,
--
Peter Xu
- [Qemu-devel] [PATCH 0/2] qemu-thread: allow cur_mon be per thread, Peter Xu, 2018/04/10
- [Qemu-devel] [PATCH 1/2] qemu-thread: always keep the posix wrapper layer, Peter Xu, 2018/04/10
- [Qemu-devel] [PATCH 2/2] qemu-thread: let cur_mon be per-thread, Peter Xu, 2018/04/10
- Re: [Qemu-devel] [PATCH 2/2] qemu-thread: let cur_mon be per-thread, Eric Blake, 2018/04/10
- Re: [Qemu-devel] [PATCH 2/2] qemu-thread: let cur_mon be per-thread,
Peter Xu <=
- Re: [Qemu-devel] [PATCH 2/2] qemu-thread: let cur_mon be per-thread, Stefan Hajnoczi, 2018/04/10
- Re: [Qemu-devel] [PATCH 2/2] qemu-thread: let cur_mon be per-thread, Peter Xu, 2018/04/10
- Re: [Qemu-devel] [PATCH 2/2] qemu-thread: let cur_mon be per-thread, Paolo Bonzini, 2018/04/11
- Re: [Qemu-devel] [PATCH 2/2] qemu-thread: let cur_mon be per-thread, Peter Xu, 2018/04/11
- Re: [Qemu-devel] [PATCH 2/2] qemu-thread: let cur_mon be per-thread, Paolo Bonzini, 2018/04/11
- Re: [Qemu-devel] [PATCH 2/2] qemu-thread: let cur_mon be per-thread, Peter Xu, 2018/04/11
- Re: [Qemu-devel] [PATCH 2/2] qemu-thread: let cur_mon be per-thread, Eric Blake, 2018/04/11
- Re: [Qemu-devel] [PATCH 2/2] qemu-thread: let cur_mon be per-thread, Peter Xu, 2018/04/12