Re: [Qemu-devel] [PATCH 2/5] hostmem: make hostmem global and RAM hotunplg safe

[liu ping fan]

On Fri, Apr 12, 2013 at 6:03 PM, Paolo Bonzini <address@hidden> wrote:
> Il 12/04/2013 10:38, Stefan Hajnoczi ha scritto:
>>> > Yes, protect. Supposed we have HostMem A, and it will become B. Then
>>> > hostmem_lookup will either see A or B.  If it see A, it should use A
>>> > refcnt agaist hostmem_listener_commit to drop A.  This refcnt has no
>>> > relation with mr's object's refcnt.
>> My question is why you are accessing cur_hostmem outside hostmem_lock
>> but then assigning it inside the lock on the next line...
>>
>>>>> > >> +    cur_hostmem = next_hostmem;
>> ...here.
>>
>> If you want an atomic exchange then tmp = cur_hostmem should be inside
>> the lock.
>
> It will work because readers will grab either the hostmem_lock or the
> BQL, while writers will grab both.  A kind of local/global lock, but I'm
No only hostmem_lock is used to protect readers from writers.  While
the writers are protected agaist each other by biglock.  So leaving
the "cur_hostmem = next_hostmem" outside to reflect the lock's rules.

Regards,
Pingfan
> not sure it was intentional. :)  It's simpler to just move the read
> inside the lock.
>
> Paolo