Subject: MP Alpha kernels go multi-user
To: Ross Harvey <firstname.lastname@example.org>
From: Jason R Thorpe <email@example.com>
Date: 04/20/2001 10:38:22
[ Moved to firstname.lastname@example.org --thorpej ]
On Fri, Apr 20, 2001 at 10:14:11AM -0700, Ross Harvey wrote:
> > This makes Alpha MP kernels boot multiuser. In fact, this commit
> > is being made from my dual-CPU AlphaServer 1200 running an MP kernel.
> This really rocks! Another great job by Jason Thorpe and Bill Sommerfeld!
You should have seen how excellently the system wedged when I attemped
a parallel kernel build :-)
One more bug to fix -- I was engaged in a little too much wishful
thinking, I suppose -- I put pmap_emulate_reference() outside the
kernel_lock perimeter, since the pmap module has fine-grained locking.
However, if an interrupt comes in while you're holding the pvhead->map
lock in pmap_emulate_reference(), you will acquire the kernel_lock...
Now consider a system call of page fault on the other CPU that causes
pmap_enter() to be called, which tries to acquire the map->pvhead lock
direction (note it's already holding kernel_lock).
Now you're screwed -- one cpu did "kernel_lock -> pmap_lock" and the
other did "pmap_lock -> kernel_lock". Deadlock.
Sigh, guess I have to put pmap_emulate_reference() in the kernel_lock
perimeter for now (until kernel_lock is no longer required by interrupt
But, still, pretty cool :-)
-- Jason R. Thorpe <email@example.com>