Re: [Qemu-devel] [PATCH v2 1/4] exec: Atomic access to bounce buffer

[Paolo Bonzini <pbonzini@redhat.com>]

On 13/03/2015 09:32, Paolo Bonzini wrote:
> 
> 
> On 13/03/2015 09:16, Fam Zheng wrote:
>>>>>> +        if (atomic_cmpxchg(&bounce.in_use, false, true)) {
>>>>
>>>> atomic_or is enough...
>> atomic_cmpxchg is here to take the ownership of bounce iff it is not in use, so
>> I think it is necessary.
> 
> It's changing false to true and true to true, so you can do
> 
>     if (atomic_or(&bounce.in_use, 1)) {
>         // was true already
>     }

... and actually, atomic_xchg is even better (on x86, atomic_or is
compiled into a cmpxchg loop, but atomic_xchg is a single instruction).

Paolo