Re: [Qemu-devel] [PATCH V3 01/11] atomic: introduce atomic operations

[liu ping fan <qemulist@gmail.com>]

On Tue, Sep 11, 2012 at 4:04 PM, Avi Kivity <avi@redhat.com> wrote:
> On 09/11/2012 10:51 AM, Liu Ping Fan wrote:
>> 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.
>>
>> Signed-off-by: Liu Ping Fan <pingfank@linux.vnet.ibm.com>
>> ---
>>  include/qemu/atomic.h |   63 +++++++++++++++++++++++++++++++++++++++++++++++++
>>  1 files changed, 63 insertions(+), 0 deletions(-)
>>  create mode 100644 include/qemu/atomic.h
>>
>> diff --git a/include/qemu/atomic.h b/include/qemu/atomic.h
>> new file mode 100644
>> index 0000000..f17145d
>> --- /dev/null
>> +++ b/include/qemu/atomic.h
>> @@ -0,0 +1,63 @@
>> +/*
>> + * Simple wrapper of gcc Atomic-Builtins
>> + *
>> + * This work is licensed under the terms of the GNU GPL, version 2 or later.
>> + * See the COPYING file in the top-level directory.
>> + *
>> + */
>> +#ifndef __QEMU_ATOMIC_H
>> +#define __QEMU_ATOMIC_H
>> +
>> +typedef struct Atomic {
>> +    int counter;
>> +} Atomic;
>
> Best to mark counter 'volatile'.
>
>> +
>> +static inline void atomic_set(Atomic *v, int i)
>> +{
>> +    v->counter = i;
>> +}
>> +
>> +static inline int atomic_read(Atomic *v)
>> +{
>> +    return v->counter;
>> +}
>>
>
> So these two operations don't get mangled by the optimizer.
>
Browsing linux code and reading lkml, find some similar material. But
they have moved volatile from ->counter to function - atomic_read().
As to atomic_read(), I think it need to prevent optimizer from
refetching issue, but as to atomic_set(), do we need ?

Regards,
pingfan
>
>
>
> --
> error compiling committee.c: too many arguments to function