[9fans] Kernel question: i386 test-and-set problem

[Jakub Jermar <jj@comberg.cz>]

Hi all,

I've just been studying the 9 kernel and found something very weird.
In /sys/src/9/pc/l.s, there is an assembly function called tas() that should
provide its callers with an atomic test-and-set operation. This function is
called, for instance, by lock() from taslock.c.

The weird thing is that it looks as though this function only exchanged %eax
and (%ebx) by XCHG instruction which is NOT atomic. The C source looks
satisfied with it (I have no opportunity to try it, because I am not running
Plan 9 - yet :-).
By further searching, I found (in /sys/src/lithread/386cas.s... or
something) a routine called cas(). This routine uses the proper 386 atomic
instruction CMPXCHG.

Well, what's going on? I'd expect all those C functions to call cas()
instead of tas(). Mys second question is why cas() is not the only tas()
(test-end-set) within the 386 part of the kernel. My last question is why
the cas() is outside the kernel tree.

Regards,
Jakub Jermar