Re: [Q] Implementation of spin_lock on i386: why "rep;nop" ?

[Richard B. Johnson]

On Mon, 17 Sep 2001, Jean-Marc Saffroy wrote:

> Hi all,
> 
> One of my coworkers directed my attention to the implementation of
> spinlocks on IA-32. In spin_lock_string, we can read:
> 
> 	"cmpb $0,%0\n\t" \
> 	"rep;nop\n\t" \
> 	"jle 2b\n\t" \
> 
> The "rep;nop" line looks dubious, since the IA-32 programmer's manual from
> Intel (year 2001) mentions that the behaviour of REP is undefined when it
> is not used with string opcodes. BTW, according to the same manual, REP is
> supposed to modify ecx, but it looks like is is not the case here... which
> is fortunate, since ecx is never saved. :-)
> 
> What is the intent behind this "rep;nop" ? Does it really rely on an
> undocumented behaviour ?
> 
> 
> Regards,

Well it's now documented although you have to search a web-site to
find it. Basically, it runs the CPU at low clock-speed when it's
busy-waiting. Since most all spin-locks lock for mere microseconds
it's unlikely that it does anything useful, but it can't hurt.

Cheers,
Dick Johnson

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

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