Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!tut.cis.ohio-state.edu!ucbvax!BARILVM.BITNET!P85025
From: P85025@BARILVM.BITNET (Doron Shikmoni)
Newsgroups: comp.lang.asm370
Subject: Re: STOSM/STNSM
Message-ID: <8911121448.AA18138@brazos.rice.edu>
Date: 12 Nov 89 12:19:45 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Reply-To: IBM 370 Assembly Programming Discussion List <ASM370%UCF1VM.BITNET@ricevm1.rice.edu>
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From Valdis:

>CS is pretty simple really - there's a good description on page A-40
>of the S/370 PrincOp (GA22-7000-10).  All it does is fetch a value,
>compare it, and if it's the same update.  It's used for serializing
>code.

This is a dangerously incomplete description. See below.

>       Example (cribbing from A-40.. :) - you have a lock word, that
>if it's a 0 you can proceed, but if it's a 1 you have to wait.  The
>person who gets the lock sets it to 1 when he enters, and to 0 when he leaves.
>
>       SR     R1,R1     get a zero.
>       LA     R2,1      get a one.
>LOOP   CS     R1,R2,LOCKWORD  do it
>       BC     4,LOOP    if somebody else got it, spin.
>... locked code here
>       ST     R1,LOCKWORD clear the flag

This code demonstrates one of the common bugs introduced via CS. It does
not maintain any serialization!!!! See below.

>What the CS will do is:
>       Fetch LOCKWORD
>       Compare to R1
>       If EQUAL, STORE R2 at LONGWORD
>       else set condition code.

There's your bug. The last line should be replaced with the following two:

else LOAD LOCKWORD into R1.
set condition code.

NOW, please read your code section again. Assuming that you meant "0"
to read "unlocked" and "1" to read "locked", you can see that your code
allows everyone to be in the critical section at the same time. No spin
will ever occur; if get bad CC on the first try, R1 is loaded with "1";
then, the next (single) spin will get a CC of zero, and your lock has
been picked...

This is a very common error done with CS and CDS; and since actual lock
clashes are rare, statistically, such bugs may be sitting in many programs,
waiting for the best time to incarnate, a-la Murphy....

The corrected code will look like this (making minimal change to your
example):

       LA     R2,1      get a one.
LOOP   SR     R1,R1     get a zero.
       CS     R1,R2,LOCKWORD  do it
       BC     4,LOOP    if somebody else got it, spin.
... locked code here
       ST     R1,LOCKWORD clear the flag

As a matter of taste, I'd also change the "BC  4" to "BNE". But this
is only a matter of taste, which is not arguable.

Regards
Doron