Re: gnu asm help...

Re: gnu asm help...

[Petr Vandrovec]

On 19 Jun 01 at 13:21, Richard B. Johnson wrote:
> On Tue, 19 Jun 2001, Timur Tabi wrote:
> > 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.

Use spinlocks instead of global cli. Global cli can take milliseconds.
 
> The Intel book(s) state that an interrupt is not acknowledged until
> so many clocks (don't remember the number) after a stack operation.

Reread it. It says 'after operation with ss' - that is after
"mov xxxx,%ss" or "pop %ss", as it is expected that next instruction 
will be "movl yyyy,%esp". 

Before "lss ...." (it is lss in intel mnemonic...) was invented, you 
could not switch your stack safely without this feature, as NMI could 
arrive in the middle of your stack switch without blocking all interrupts 
after "mov xxxx,%ss". 

BTW, if you chain "mov %eax,%ss" back to back, they are executed
in pairs - irq can arrive after even mov, but cannot after odd (at
least on PII and PIII). But it is a bit off topic for L-K (except that 
we can try other clones, maybe someone got it wrong?)
                                        Best regards,
                                            Petr Vandrovec
                                            vandrove@vc.cvut.cz
                                            
                                                

Re: gnu asm help...

[Richard B. Johnson]

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.

Re: gnu asm help...

[Timur Tabi]

** 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.


-- 
Timur Tabi - ttabi@interactivesi.com
Interactive Silicon - http://www.interactivesi.com

Re: gnu asm help...

[Alan Cox]

> I need a simple (??) change to atomic_inc() functionality. so that i can
> increment and return the value of the variable.

Please don't blindly change atomic.h to do this. A large number of processors
don't have the x86 'xadd' functionality. Create/use seperate functions instead

Re: gnu asm help...

[Bohdan Vlasyuk]

On Tue, Jun 19, 2001 at 01:18:41AM +0200, Erik Mouw wrote:

> I also don't know the exact meaning, but here are two nice tutorials
> about inline assembly:
> 
> http://www-106.ibm.com/developerworks/linux/library/l-ia.html
> http://www.uwsg.indiana.edu/hypermail/linux/kernel/9804.2/0953.html

this one is nice too:

 http://www.delorie.com/gnu/docs/gcc/gcc_86.html

-- 
wbr

Re: gnu asm help...

[Rick Hohensee]

The C-names-in-asms stuff is explained in (g?)as.info. The explanation is
a bit strained, but after the third or fourth read it becomes fairly
sensible.


Rick Hohensee

Re: gnu asm help...

[Petr Vandrovec]

On 18 Jun 01 at 18:20, Timur Tabi wrote:
> You want to return the variable?  Try this:
> 
> static __inline__ unsigned long atomic_inc(atomic_t *v)
> {
>     __asm__ __volatile__(
>         LOCK "incl %0"
>         :"=m" (v->counter)
>         :"m" (v->counter));
> 
>     return v->counter;
> }

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. 
                                            Best regards,
                                                Petr Vandrovec
                                                vandrove@vc.cvut.cz
                                                
P.S.: Why you need to know that value? You can either rewrite
your code with atomic_dec_and_test/atomic_inc_and_test, or
you overlooked some race, or you have really strange problem.

Re: gnu asm help...

[Timur Tabi]

** Reply to message from "Raj, Ashok" <ashok.raj@intel.com> on Mon, 18 Jun 2001
15:56:50 -0700


> also if there is any reference to the gnu asm symtax, please send me a
> pointer.. 

There's lots

> i can understand what the LOCK "incl %0 means.. but not sure what the rest
> is for.

LOCK just means the x86 "lock" prefix.

incl is the 32-bit version of "inc" (incremement)

You want to return the variable?  Try this:

static __inline__ unsigned long atomic_inc(atomic_t *v)
{
    __asm__ __volatile__(
        LOCK "incl %0"
        :"=m" (v->counter)
        :"m" (v->counter));

    return v->counter;
}


-- 
Timur Tabi - ttabi@interactivesi.com
Interactive Silicon - http://www.interactivesi.com

Re: gnu asm help...

[Erik Mouw]

On Mon, Jun 18, 2001 at 03:56:50PM -0700, Raj, Ashok wrote:
> i can understand what the LOCK "incl %0 means.. but not sure what the rest
> is for.
> 
> thanks
> ashokr
> 
> static __inline__ void atomic_inc(atomic_t *v)
> {
>     __asm__ __volatile__(
>         LOCK "incl %0"
>         :"=m" (v->counter)
>         :"m" (v->counter));
> }

I also don't know the exact meaning, but here are two nice tutorials
about inline assembly:

http://www-106.ibm.com/developerworks/linux/library/l-ia.html
http://www.uwsg.indiana.edu/hypermail/linux/kernel/9804.2/0953.html



Erik

-- 
J.A.K. (Erik) Mouw, Information and Communication Theory Group, Department
of Electrical Engineering, Faculty of Information Technology and Systems,
Delft University of Technology, PO BOX 5031,  2600 GA Delft, The Netherlands
Phone: +31-15-2783635  Fax: +31-15-2781843  Email: J.A.K.Mouw@its.tudelft.nl
WWW: http://www-ict.its.tudelft.nl/~erik/

gnu asm help...

[Raj, Ashok]

Hello asm gurus..

I need a simple (??) change to atomic_inc() functionality. so that i can
increment and return the 
value of the variable.

current implementation in linux/include/asm/atomic.h does not do this job.

any help would be greatly appreciated.

ashokr


from atomic.h

also if there is any reference to the gnu asm symtax, please send me a
pointer.. 
i can understand what the LOCK "incl %0 means.. but not sure what the rest
is for.

thanks
ashokr

static __inline__ void atomic_inc(atomic_t *v)
{
    __asm__ __volatile__(
        LOCK "incl %0"
        :"=m" (v->counter)
        :"m" (v->counter));
}