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Re: [Qemu-devel] [PATCH v5 3/4] monitor: more comments on lock-free flei


From: Markus Armbruster
Subject: Re: [Qemu-devel] [PATCH v5 3/4] monitor: more comments on lock-free fleids/funcs
Date: Wed, 23 May 2018 10:29:37 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/25.3 (gnu/linux)

Peter Xu <address@hidden> writes:

> On Thu, May 17, 2018 at 02:46:36PM +0200, Markus Armbruster wrote:
>> Peter Xu <address@hidden> writes:
>> 
>> > Add some explicit comment for both Readline and cpu_set/cpu_get helpers
>> > that they do not need the mon_lock protection.
>> 
>> Appreciated!
>> 
>> > Signed-off-by: Peter Xu <address@hidden>
>> > ---
>> >  monitor.c | 5 +++--
>> >  1 file changed, 3 insertions(+), 2 deletions(-)
>> >
>> > diff --git a/monitor.c b/monitor.c
>> > index d6c3c08932..ae5bca9d7c 100644
>> > --- a/monitor.c
>> > +++ b/monitor.c
>> > @@ -207,7 +207,7 @@ struct Monitor {
>> >      int suspend_cnt;            /* Needs to be accessed atomically */
>> >      bool skip_flush;
>> >      bool use_io_thr;
>> > -    ReadLineState *rs;
>> > +    ReadLineState *rs;   /* Only used in parser, so no lock needed. */
>> 
>> Pardon the ignorant question: why does "only used in parser" imply "no
>> lock needed"?
>
> Since even if the monitors can be run in multiple threads now, the
> monitor parser of a specific Monitor will still only be run in either
> the main thread or the monitor iothread.  My fault to be unclear on
> the comment.  Maybe this one is better:
>
>   It is only used in parser, and the parser of a monitor will only be
>   run either in main thread or monitor IOThread but never both, so no
>   lock is needed when accessing ReadLineState.

One further question, just to help me understand how this stuff works:
what are the conditions for the parser running in the main thread, and
what are the conditions for it running in the monitor IOThread?

[...]



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