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Date: Fri, 12 Apr 2013 12:03:40 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH 2/5] hostmem: make hostmem global and RAM
	hotunplg safe
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To: Stefan Hajnoczi <stefanha@gmail.com>
Cc: Peter Maydell <peter.maydell@linaro.org>, Anthony Liguori <aliguori@us.ibm.com>, Jan Kiszka <jan.kiszka@siemens.com>, Marcelo Tosatti <mtosatti@redhat.com>, liu ping fan <qemulist@gmail.com>, qemu-devel@nongnu.org, Vasilis Liaskovitis <vasilis.liaskovitis@profitbricks.com>

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.

Paolo