Re: [9fans] sleep(), sched() and ilock()

[presotto@plan9.bell-labs.com]

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	First. Am I right when supposing that the reason ilock() doesn't call
	scheduler is that it has to be sure
	that iunlock() executes on the same cpu?

In general, if you are doing an ilock, it should be for something that
can be done quickly.  ilock is used when you're locking something that
an interrupt routine also uses and scheding in the middle of it would be
deadly.

	Second. Why doesn't sleep() simply call sched() right after it releases both
	locks? Am I unaware of some possible race condition?

Off hand, I can't see any particular reason other than tunnel vision
on our part.  Looks like all we're doing is duplicating code with the
net effect of saving 2 subroutine calls (splhi and sched).  I may try
it the other way and have Gerard Holtzman verify it again and see if
I'm missing something myself.

	Third. It seems to me that sleep() might disable interrupts on a different
	cpu than is the one that enables them again before sleep() finishes. Is it
	ok when a process brings the old processor state to the new current
	processor and leaves the old one in (say) the splhi()'d state?

Once you go frolicking through 'gotolabel(&m->sched)', the spl level
is lost, i.e., schedinit always starts hi and returns low: any
process coming out of a rescheduling comes out spllo().  That
means that calling sched() when you are splhi() is wrong.  This
refers back to your first point.  The processor that you did the sleep
on will go spllo while its looking for a new process to run and will
start that process up spllo.

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From: "Jakub Jermář" <jj@comberg.cz>
To: "9fans" <9fans@cse.psu.edu>
Subject: [9fans] sleep(), sched() and ilock()
Date: Wed, 20 Sep 2000 04:40:12 +0200
Message-ID: <000501c022ac$54713b00$9a6214d4@cz99.cz>

I am now reconsidering some aspects of sleeping, scheduling and ilocking
that I considered clear.

First. Am I right when supposing that the reason ilock() doesn't call
scheduler is that it has to be sure
that iunlock() executes on the same cpu?

Second. Why doesn't sleep() simply call sched() right after it releases both
locks? Am I unaware of some possible race condition?

Third. It seems to me that sleep() might disable interrupts on a different
cpu than is the one that enables them again before sleep() finishes. Is it
ok when a process brings the old processor state to the new current
processor and leaves the old one in (say) the splhi()'d state?

Jakub Jermar