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Re: [Qemu-devel] [PATCH 0/3] make bh safe with hot-unplug

From: Paolo Bonzini
Subject: Re: [Qemu-devel] [PATCH 0/3] make bh safe with hot-unplug
Date: Tue, 25 Jun 2013 09:53:52 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130514 Thunderbird/17.0.6

Il 25/06/2013 08:32, liu ping fan ha scritto:
> On Tue, Jun 25, 2013 at 2:24 PM, Paolo Bonzini <address@hidden> wrote:
>> Il 25/06/2013 19:38, Liu Ping Fan ha scritto:
>>> This series relies on refcnt of object used by bh callback to run against 
>>> unplug.
>>> Open issue:
>>> Another choice may be rcu, but I think some issues are hard to resolve.
>>> Using rcu, we have two choice:
>>>   when holding object refcnt, call qemu_bh_delete(); then after grace 
>>> period, we can release.
>>>   Or making qemu_bh_delete() sync in the path of DeviceState's finalization.
>> What do you mean exactly?  qemu_bh_delete() is async, but it operates on
>> dynamically-allocated memory.
> Before DeviceState is freed, we should make sure the bh is delete, and
> not related bh is in fly.

The bh need not be deleted.  It just needs to be not-scheduled (easy,
qemu_bh_delete does that) and not running.

The latter part could be the hard one in a multi-threaded context, but I
think it's up to the device to ensure it.  It doesn't _have_ to be hard.
 For example, joining the data-plane thread would do that as well.


>> Paolo
>>> but currently, the callers of qemu_bh_delete() can not satisfy any of the 
>>> two condition.

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