Re: [Qemu-devel] [RFC v2 4/8] QAPI: new QMP command option "without-bql"

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Peter Xu (peterx@redhat.com) wrote:
> On Thu, Aug 24, 2017 at 07:37:32AM +0800, Fam Zheng wrote:
> > On Wed, 08/23 18:44, Dr. David Alan Gilbert wrote:
> > > * Peter Xu (peterx@redhat.com) wrote:
> > > > Introducing this new parameter for QMP commands in general to mark out
> > > > when the command does not need BQL.  Normally QMP command executions are
> > > > done with the protection of BQL in QEMU.  However the truth is that not
> > > > all the QMP commands require the BQL.
> > > > 
> > > > This new parameter provides a way to allow QMP commands to run in
> > > > parallel when possible, without the contention on the BQL.
> > > > 
> > > > Since the default value of "without-bql" is still false, so now all QMP
> > > > commands are still protected by BQL still.
> > > > 
> > > > Signed-off-by: Peter Xu <peterx@redhat.com>
> > > 
> > > We should define what a 'without-bql' command is allowed to do:
> > >    'Commands that have without-bql set _may_ be called without the bql
> > >    being taken.  They must not take the bql or any other lock that may
> > >    become dependent on the bql.'
> 
> Sure.
> 
> > >    (Do we need to say anything about RCU?)
> 
> Could I ask how is RCU related?

My definition above said that anything declared without bql couldn't
take the bql, so couldn't block on any other thread holding the bql.
But is our command allowed to use synchronize_rcu or rcu_read_lock
that could wait for or block other threads doing rcu stuff?
Because if it did is there any guarantee that it wouldn't block?


> 
> > > 
> > > Also, 'no-bql' is shorter :-)
> > 
> > Or rather "need-bql" that defaults to true to avoid double negative (TM) with
> > "no-bql = false"?
> 
> Ok let me use "need-bql". :)

Fine by me.

Dave

> 
> -- 
> Peter Xu
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK