Re: [Qemu-devel] [PATCH 2/2] qemu-thread: let cur_mon be per-thread

[Eric Blake <eblake@redhat.com>]

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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 <peterx@redhat.com>
> ---
>  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/

> +++ 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.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org


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