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Date: Wed, 24 Jun 2015 11:04:03 +0800
From: Fam Zheng <famz@redhat.com>
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Subject: Re: [Qemu-devel] [Qemu-block] [PATCH v2 02/13] block: Introduce
 bdrv_lock and bdrv_unlock API
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To: Stefan Hajnoczi <stefanha@gmail.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>, Stefan Hajnoczi <stefanha@redhat.com>, qemu-devel@nongnu.org, qemu-block@nongnu.org

On Wed, 06/24 10:47, Fam Zheng wrote:
> On Tue, 06/16 17:07, Stefan Hajnoczi wrote:
> > On Tue, Jun 02, 2015 at 11:21:51AM +0800, Fam Zheng wrote:
> > > +/**
> > > + * bdrv_lock:
> > > + *
> > > + * Begin a temporary exclusive accessing by locking the BDS.
> > > + */
> > > +void bdrv_lock(BlockDriverState *bs);
> > > +
> > > +/**
> > > + * bdrv_unlock:
> > > + *
> > > + * End a exclusive accessing.
> > > + */
> > > +void bdrv_unlock(BlockDriverState *bs);
> > 
> > This documentation is missing important points:
> > 
> > 1. Does AioContext need to be held by the caller?  (Yes)
> 
> Yes.
> 
> > 
> > 2. Is this about thread safety?  (No, it's about exclusive access to a
> >    BDS *within* the AioContext.)
> 
> As it has to quiesce iothreads as well (for now it's even more urgent than

s/iothreads/ioeventfd/

> exclusive access within the same AioContext), I'd rather take it as yes.
> 
> Paolo, please correct me.
> 
> BTW, is there any semantics in here that we can use for multiqueue block layer?
> 
> Fam
> 
>