Subject: Re: gmcgarry_ctxsw problems
To: Jason R Thorpe <email@example.com>
From: Gregory McGarry <firstname.lastname@example.org>
Date: 02/06/2003 12:04:52
Jason R Thorpe wrote:
> I've been looking at Greg's new context switch code on the above-mentioned
> branch, and have identified some problems with it:
> 1. sched_lock is released too early. Specifically, it is released
> before having completely switched away from the outgoing proc.
> This is a race condition that will break MP systems (and has
> in the past, until cpu_switch() was fixed on i386 and alpha).
> 2. It does not properly handle switching to an idle context. On
> UP systems, this means proc0's PCB. On MP systems, this means
> the CPU's idle PCB. On the VAX, this means the CPU's interrupt
> 3. It doesn't not have any way to communicate the info that can
> currently be passed between cpu_switch() and idle(). For the
> most part, that means "remembering if we're switching to the
> same process again" -- this is more complicated in MP systems.
> For this reason, I think the approach taken by Greg's new code needs
> to be reexamined. I think probably a better way to handle it is for
> cpu_switch() to actually call an MI C function to get the new process
> to switch to. That is, leave it mostly the same as it is now, just
> replace the code that looks at the run queues with a function call.
Certainly I didn't completely consider that MP ports were switching to
an idle context. The solution was bogus for that case.
I was trying to avoid a callback, only to simplify that cpu_switch()
does only the switch, nothing more. I still like the idea of
cpu_idle(), only because it makes the code more readable.
A callback is what FreeBSD does. I'm happy enough with that solution.
At least we can progress from there.
-- Gregory McGarry <email@example.com>