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

From: Paolo Bonzini
Subject: Re: [Qemu-devel] [PATCH 2/5] hostmem: make hostmem global and RAM hotunplg safe
Date: Fri, 12 Apr 2013 12:03:40 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130311 Thunderbird/17.0.4

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
not sure it was intentional. :)  It's simpler to just move the read
inside the lock.


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