From mboxrd@z Thu Jan  1 00:00:00 1970
Message-ID: <b6e833b996170a0c5fdeced37ce27961@swtch.com>
To: 9fans@cse.psu.edu
Subject: Re: [9fans] new system calls - semacquire, semrelease
From: "Russ Cox" <rsc@swtch.com>
Date: Sat, 25 Mar 2006 11:12:05 -0500
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> there is lock(2) in the public inferno distribution.  the implementation is amusing.

much like the old trick of storing a byte in a pipe
and using it as an interprocess lock.

the implementation of the plan 9 semaphore code,
by the way, is a bit more involved than one might
first expect.  the hard part is accessing *addr safely
without using a qlock (which would block) in semrelease.
(and you can't access *addr while holding a lock
either, since you're not allowed to touch user memory
while holding locks.)  i verified it with spin to double-check.

russ