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Date: Wed, 14 Nov 2012 10:47:57 +0100
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH v6 1/8] atomic: introduce atomic operations
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To: liu ping fan <qemulist@gmail.com>
Cc: Peter Maydell <peter.maydell@linaro.org>, Stefan Hajnoczi <stefanha@gmail.com>, Marcelo Tosatti <mtosatti@redhat.com>, qemu-devel@nongnu.org, Avi Kivity <avi@redhat.com>, Anthony Liguori <anthony@codemonkey.ws>, Jan Kiszka <jan.kiszka@siemens.com>

Il 14/11/2012 10:38, liu ping fan ha scritto:
> On Tue, Nov 13, 2012 at 6:11 PM, Paolo Bonzini <pbonzini@redhat.com> wr=
ote:
>>>> Il 05/11/2012 06:38, Liu Ping Fan ha scritto:
>>>>> From: Liu Ping Fan <pingfank@linux.vnet.ibm.com>
>>>>>
>>>>> If out of global lock, we will be challenged by SMP in low level,
>>>>> so need atomic ops.
>>>>>
>>>>> This file is a wrapper of GCC atomic builtin.
>>>>
>>>> I still object to this.
>>>>
>>>> I know it enforces type-safety, but it is incomplete.  It doesn't
>>>
>>> Although it is incomplete, but the rest cases are rarely used.  Linux
>>> faces such issue, and the "int" version is enough, so I think we can
>>> borrow experience from there.
>>
>> One of the two places that use __sync_* require 64-bit accesses.  My
> Yes, these two places are not easy to fix.

Which shows that Linux's atomic_t is not suited for QEMU, in my opinion.

>> RCU prototype required pointer-sized access, which you cannot make typ=
e-
> But I think that your RCU prototype should rely on atomic of CPU, not
> gcc=91s atomic.

What's the difference?  gcc's atomic produces the same instructions as
hand-written assembly (or should).

> Otherwise, it could be slow (I guess something like spinlock there).

Not sure what you mean.  I'm using two things: 1) write barriers; 2)
atomic_xchg of a pointer for fast, wait-free enqueuing of call_rcu
callbacks.

Paolo