Path: utzoo!mnetor!uunet!husc6!bloom-beacon!mit-eddie!rutgers!bellcore!tness7!petro!swrinde!ut-sally!utah-cs!utah-gr!uplherc!sp7040!obie!wes
From: wes@obie.UUCP (Barnacle Wes)
Newsgroups: comp.lang.c
Subject: Re: volatile isn't necessary, but it's there
Message-ID: <144@obie.UUCP>
Date: 10 Apr 88 19:08:28 GMT
References: <7794@alice.UUCP> <48767@sun.uucp>
Organization: the Well of Souls
Lines: 25
Summary: ++ not atomic?  Says who?

In article <48767@sun.uucp>, guy@gorodish.Sun.COM (Guy Harris) writes:
| It is worth noting, for example, that there is no guarantee that "++mutex"
| is an atomic operation.  It is, in fact, worth noting that even if "extern
| volatile char mutex;" was used, there would be no guarantee that "++mutex"
| is atomic.  In all cases, for example, "++mutex" takes an instruction to
| load "mutex" into a register, an instruction to increment it, and an
| instruction to store it, on a SPARC.  The same would be true of any other
| architecture lacking an instruction performing an "add 1 to memory"
| operation.

Another good reason NOT to use a SPARC for anything other than a
paperweight.  On a reasonable processor like the M68000, `++mutex'
becomes an atomic operation like `addq	#1,mutex'.

| This is a real problem; there were some mildly entertaining bugs on Sun-4
| caused by statements such as "++mutex;" in the kernel not being surrounded by
| "spl" calls.

Did you get these bugs squashed, or will you relase the Sun-4 SunOS
with them intact?
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