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

[presotto@plan9.bell-labs.com]

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As I said in the last message, there has to be synchronization
external to the sleep code to make such a situation work.  For
example

	lock list
	add to list
	unlock list
	sleep
	lock list
	free
	unlock list

and another process will do

	lock list
	if(something to wakeup)
		wakeup
	unlock list

Things calling sleep/wakeup can and do perform such
synchronization.  It's part of writing a multithreaded
kernel, i.e., snchronizing accesses to structures.
You don't haphazardly free things when there are
possible accesses to them outstanding in other threads,
or at least I try not to.

Unfortunately, postnote is an asyncronous event often
initiated by someone that's trying to stop something.
It doesn't know about this external synchronization.

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Doesn't
	/*
	 * Expects that only one process can call wakeup for any given Rendez
	 */
	int
	wakeup(Rendez *r)
	{

mean that process p cannot continue after the sleep?

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From: miller@hamnavoe.demon.co.uk
To: 9fans@cse.psu.edu
Subject: Re: [9fans] Kernel question: i386 test-and-set problem
Date: Wed, 2 Aug 2000 15:51:33 0100
Message-ID: <E13Jzoi-000AkU-0A@finch-post-10.mail.demon.net>

> If it were at all possible for wakeup to be called with
> r already freed, the code would be wrong to begin with
> since r is an argument to wakeup.

For things to go wrong it's not necessary for wakeup to be
called after r is freed; what I said was "the free(r) and
malloc() could happen *while* or even before wakeup(r) runs".
It's sufficient to have sleep(r) and wakeup(r) racing on
two processors.  We could have this interleaving of events:

sleep(r) is called
  wakeup(r) is called
sleep tests wakeup condition, returns
  wakeup is delayed by an interrupt on its processor
caller of sleep deallocates structure containing r
some other process reallocates r and clobbers r->p
  wakeup resumes, dereferences r->p, ka-boom!

If you think sleep and wakeup are sufficiently interlocked
by higher level considerations that this can't happen,
then let's use your scenario with postnote, mutatis mutandis
to apply to the existing kernel algorithm:

process x calls postnote:
	postnote(p):
		p->notepending = 1
		lock(p->rlock)
		r = p->r
		if r != 0
			if(r->p == p && p->r == r)
				r->p = 0
				p->r = 0
				ready(p)
		unlock(p->rlock)

Immediately after the unlock(p->rlock) is executed,
process q calls wakeup

	process q:
		wakeup(r):    {wakeup condition is satisfied}
			coherence()
			p = r->p
			if p != 0
				lock(p->rlock)
				if (r->p == p && p->r == r)
				    r->p = 0
				    p->r = 0	
				    ready(p)
			    unlock(p->rlock)

During the call to coherence() process q is interrupted.
Process p now continues after the sleep:

	process p:
		sleep(r);
		free(r)

Process y now does

		xxx = malloc(234);
		xxx->a = 12;

And finally process q resumes, and loads and dereferences
r->p which is no longer vald.  ka-boom again.

-- Richard