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Date: Tue, 30 Jun 2009 11:23:28 +0530
Message-ID: <abe8a1fd0906292253ye4595c9wb0c085b7d89aea7@mail.gmail.com>
Subject: Re: Inline Assembly queries
From: kernel mailz <kernelmailz@googlemail.com>
To: Ian Lance Taylor <iant@google.com>
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Consider atomic_add and atomic_add_return in kernel code.

On Tue, Jun 30, 2009 at 2:59 AM, Ian Lance Taylor<iant@google.com> wrote:
> kernel mailz <kernelmailz@googlemail.com> writes:
>
>> I tried a small example
>>
>> int *p = 0x1000;
>> int a = *p;
>> asm("sync":::"memory");
>> a = *p;
>>
>> and
>>
>> volatile int *p = 0x1000;
>> int a = *p;
>> asm("sync");
>> a = *p
>>
>> Got the same assembly.
>> Which is right.
>>
>> So does it mean, if proper use of volatile is done, there is no need
>> of "memory" ?
>
> You have to consider the effects of inlining, which may bring in other
> memory loads and stores through non-volatile pointers.
>
> Ian
>
Consider

static __inline__ void atomic_add(int a, atomic_t *v)
{
        int t;

        __asm__ __volatile__(
"1:     lwarx   %0,0,%3         # atomic_add\n\
        add     %0,%2,%0\n"
        PPC405_ERR77(0,%3)
"       stwcx.  %0,0,%3 \n\
        bne-    1b"
        : "=&r" (t), "+m" (v->counter)
        : "r" (a), "r" (&v->counter)
        : "cc");
}

static __inline__ int atomic_add_return(int a, atomic_t *v)
{
        int t;

        __asm__ __volatile__(
        LWSYNC_ON_SMP
"1:     lwarx   %0,0,%2         # atomic_add_return\n\
        add     %0,%1,%0\n"
        PPC405_ERR77(0,%2)
"       stwcx.  %0,0,%2 \n\
        bne-    1b"
        ISYNC_ON_SMP
        : "=&r" (t)
        : "r" (a), "r" (&v->counter)
        : "cc", "memory");

        return t;
}

I am not able to figure out why "memory" is added in latter

-TZ