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Re: [Qemu-devel] [PATCH 10/10] qdev: fix create in place obj's life cycl
From: |
Avi Kivity |
Subject: |
Re: [Qemu-devel] [PATCH 10/10] qdev: fix create in place obj's life cycle problem |
Date: |
Wed, 29 Aug 2012 10:13:32 -0700 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:14.0) Gecko/20120717 Thunderbird/14.0 |
On 08/27/2012 06:01 PM, Jan Kiszka wrote:
> On 2012-08-27 22:53, Avi Kivity wrote:
> > On 08/27/2012 12:38 PM, Jan Kiszka wrote:
> >>>> Even worse, apply
> >>>> restrictions on how the dispatched objects, the regions, have to be
> >>>> treated because of this.
> >>>
> >>> Please elaborate.
> >>
> >> The fact that you can't manipulate a memory region object arbitrarily
> >> after removing it from the mapping because you track the references to
> >> the object that the region points to, not the region itself. The region
> >> remains in use by the dispatching layer and potentially the called
> >> device, even after deregistration.
> >
> > That object will be a container_of() the region, usually literally but
> > sometimes only in spirit. Reference counting the regions means they
> > cannot be embedded into other objects any more.
>
> I cannot follow the logic of the last sentence. Reference counting just
> means that we track if there are users left, not necessarily that we
> perform asynchronous releases. We can simply wait for those users to
> complete.
I don't see how. Suppose you add a reference count to MemoryRegion.
How do you delay its containing object's destructor from running? Do
you iterate over all member MemoryRegion and examine their reference counts?
Usually a reference count controls the lifetime of the reference counted
object, what are you suggesting here?
> >
> > We can probably figure out a way to flush out accesses. After switching
> > to rcu, for example, all we need is synchronize_rcu() in a
> > non-deadlocking place. But my bet is that it will not be needed.
>
> If you properly flush out accesses, you don't need to track at device
> level anymore - and mess with abstraction layers. That's my whole point.
To flush out an access you need either rwlock_write_lock() or
synchronize_rcu() (depending on the implementation). But neither of
these can be run from an rcu read-side critical section or
rwlock_read_lock().
You could defer the change to a bottom half, but if the hardware demands
that the change be complete before returning, that doesn't work.
--
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.
- Re: [Qemu-devel] [PATCH 10/10] qdev: fix create in place obj's life cycle problem, (continued)
- Re: [Qemu-devel] [PATCH 10/10] qdev: fix create in place obj's life cycle problem, Avi Kivity, 2012/08/27
- Re: [Qemu-devel] [PATCH 10/10] qdev: fix create in place obj's life cycle problem, Jan Kiszka, 2012/08/27
- Re: [Qemu-devel] [PATCH 10/10] qdev: fix create in place obj's life cycle problem, Avi Kivity, 2012/08/27
- Re: [Qemu-devel] [PATCH 10/10] qdev: fix create in place obj's life cycle problem, Jan Kiszka, 2012/08/27
- Re: [Qemu-devel] [PATCH 10/10] qdev: fix create in place obj's life cycle problem, Avi Kivity, 2012/08/27
- Re: [Qemu-devel] [PATCH 10/10] qdev: fix create in place obj's life cycle problem, Jan Kiszka, 2012/08/27
- Re: [Qemu-devel] [PATCH 10/10] qdev: fix create in place obj's life cycle problem, Avi Kivity, 2012/08/27
- Re: [Qemu-devel] [PATCH 10/10] qdev: fix create in place obj's life cycle problem, Jan Kiszka, 2012/08/27
- Re: [Qemu-devel] [PATCH 10/10] qdev: fix create in place obj's life cycle problem, Avi Kivity, 2012/08/27
- Re: [Qemu-devel] [PATCH 10/10] qdev: fix create in place obj's life cycle problem, Jan Kiszka, 2012/08/27
- Re: [Qemu-devel] [PATCH 10/10] qdev: fix create in place obj's life cycle problem,
Avi Kivity <=
- Re: [Qemu-devel] [PATCH 10/10] qdev: fix create in place obj's life cycle problem, Jan Kiszka, 2012/08/29
- Re: [Qemu-devel] [PATCH 10/10] qdev: fix create in place obj's life cycle problem, Avi Kivity, 2012/08/29
- Re: [Qemu-devel] [PATCH 10/10] qdev: fix create in place obj's life cycle problem, Jan Kiszka, 2012/08/29
- Re: [Qemu-devel] [PATCH 10/10] qdev: fix create in place obj's life cycle problem, liu ping fan, 2012/08/27
- Re: [Qemu-devel] [PATCH 10/10] qdev: fix create in place obj's life cycle problem, liu ping fan, 2012/08/27
- Re: [Qemu-devel] [PATCH 10/10] qdev: fix create in place obj's life cycle problem, Jan Kiszka, 2012/08/28
- Re: [Qemu-devel] [PATCH 10/10] qdev: fix create in place obj's life cycle problem, Paolo Bonzini, 2012/08/28
- Re: [Qemu-devel] [PATCH 10/10] qdev: fix create in place obj's life cycle problem, Avi Kivity, 2012/08/29
- Re: [Qemu-devel] [PATCH 10/10] qdev: fix create in place obj's life cycle problem, Jan Kiszka, 2012/08/29
- Re: [Qemu-devel] [PATCH 10/10] qdev: fix create in place obj's life cycle problem, Avi Kivity, 2012/08/29