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SubjectRe: gnu asm help...
On Tue, 19 Jun 2001, Timur Tabi wrote:

> ** Reply to message from "Petr Vandrovec" <VANDROVE@vc.cvut.cz> on Tue, 19 Jun
> 2001 01:36:26 MET-1
>
>
> > No. Another CPU might increment value between LOCK INCL and
> > fetching v->counter. On ia32 architecture you are almost out of
> > luck. You can either try building atomic_inc around CMPXCHG,
> > using it as conditional store (but CMPXCHG is not available
> > on i386), or you can just guard your atomic variable with
> > spinlock - but in that case there is no reason for using atomic_t
> > at all.
>
> Oh, I see the problem. You could do something like this:
>
> cli
> mov %0, %%eax
> inc %%eax
> mov %%eax, %0
> sti
>
> and then return eax, but that won't work on SMP (whereas the "lock inc" does).
> Doing a global cli might work, though.

The Intel book(s) state that an interrupt is not acknowledged until
so many clocks (don't remember the number) after a stack operation.

Given this, is an 'attack' by another CPU allowed within this time-frame?
If not, you can do:

pushl %ebx
movl INPUT_VALUE(%esp), %eax # Get input value
movl INPUT_PTR(%esp), %ebx # Get input pointer
lock
addl %eax,(%ebx) # Add value
pushl (%ebx) # Put result on stack
popl %eax # Return value in EAX
popl %ebx

It may be worth an experiment.

In any event, you can always use a local lock to make these
operations atomic.

# Stack offsets
VALUE = 0x08
POINTER = 0x0C

.section .data
__local_lock: .long 0
.section .text

.global add_atom
.type add_atom,@function

add_atom:
pushf
cli
incb (__local_lock) # Set the lock
1: cmpb $1,(__local_lock)
jnz 1b
pushl %ebx
movl VALUE(%esp), %eax
movl POINTER(%esp), %ebx
addl %eax, (%ebx)
movl (%ebx), %eax
popl %ebx
decb (__local_lock) # Release the lock
popf
ret

The lock can also be done as:

incb (__local_lock)
1: cmpb $1,(__local_lock)
jz 2f
repz
nop
jmp 1b
2:

(maybe) the CPU being locked loops in low-power mode.



Cheers,
Dick Johnson

Penguin : Linux version 2.4.1 on an i686 machine (799.53 BogoMips).

"Memory is like gasoline. You use it up when you are running. Of
course you get it all back when you reboot..."; Actual explanation
obtained from the Micro$oft help desk.


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