From owner-freebsd-hackers  Tue Oct  3  0: 1:39 2000
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Date: Tue, 03 Oct 2000 00:01:39 -0700 (PDT)
From: John Baldwin <jhb@FreeBSD.org>
To: Jan Mikkelsen <janm@transactionsite.com>
Subject: Re: atomic operations
Cc: FreeBSD Hackers <freebsd-hackers@FreeBSD.ORG>, 
Cc: FreeBSD Hackers <freebsd-hackers@FreeBSD.ORG>,
	Kevin Mills <kmills@aventail.com>
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On 25-Sep-00 Jan Mikkelsen wrote:
> Kevin Mills <kmills@aventail.com> wrote:
>>I found the atomic_* functions in <machine/atomic.h>, but noticed that they
>>have no return value.  What I need is a function that increments/decrements
>>the given value *and* returns the new value in an atomic operation.  I
>>suppose this is possible, yes?  How would one modify the assembly to make
>>this work?
> 
> 
> Atomic decrement, in the Intel style:
> 
> long atomic_decrement(volatile long* address)
> {
>   asm {
>     mov ecx, [address]
>     mov eax, -1
>     lock xadd [ecx], eax
>     dec eax
>   }
>  /* Return value in EAX */
> }
> 
> An untested conversion into the GNU/AT&T style:
> 
> long atomic_decrement(volatile long* address)
> {
>  asm("movl 8(%ebp),%ecx");
>  asm("movl $-1, %eax");
>  asm("lock xaddl %eax,(%ecx)");
>  asm("decl %eax");
>  /* Return value in %eax */
> }

Uh, there is no xaddl instruction in the x86 instruction set.  There is
a fetchadd instruction in ia64, but that doesn't help much here.  You
can use a loop with the atomic_cmpset_* primitives though to achieve this.
e.g.:

volatile int value;
int save, increment;

   value = 3; increment = 4;
   do {
      save = value;
   } while (atomic_cmpset_int(&value, save, save + increment) == 0);
   foo = some_array[save + increment];

You can use this to control access to a circular buffer w/o needing a
lock to obtain new entries for example.  This will only work with -current
though.

-- 

John Baldwin <jhb@FreeBSD.org> -- http://www.FreeBSD.org/~jhb/
PGP Key: http://www.baldwin.cx/~john/pgpkey.asc
"Power Users Use the Power to Serve!"  -  http://www.FreeBSD.org/


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