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

From: Fam Zheng <famz@redhat.com>
To: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Cc: Peter Xu <peterx@redhat.com>,
	qemu-devel@nongnu.org, Paolo Bonzini <pbonzini@redhat.com>,
	"Daniel P . Berrange" <berrange@redhat.com>,
	Juan Quintela <quintela@redhat.com>,
	mdroth@linux.vnet.ibm.com, Eric Blake <eblake@redhat.com>,
	Laurent Vivier <lvivier@redhat.com>,
	Markus Armbruster <armbru@redhat.com>
Subject: Re: [Qemu-devel] [RFC v2 4/8] QAPI: new QMP command option "without-bql"
Date: Thu, 24 Aug 2017 07:37:32 +0800	[thread overview]
Message-ID: <20170823233732.GA16947@lemon> (raw)
In-Reply-To: <20170823174411.GG2648@work-vm>

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.'
>    (Do we need to say anything about RCU?)
> 
> Also, 'no-bql' is shorter :-)

Or rather "need-bql" that defaults to true to avoid double negative (TM) with
"no-bql = false"?

Fam