Re: HPET regression in 2.6.26 versus 2.6.25 -- why Yinghai's revert may have failed

quoted

Considering these 3 consecutive commits (according to 'git log')from late
Feb. 2008, between kernel versions 2.6.25 and 2.6.26-rc1:
---------------------------------------------------------

700efc1b...:  the last kernel I can build and run just fine.

3def3d6d...:  this one builds, but locks up in inet_init() once the sequence
of function calls reaches synchronize_rcu().  Reverting here works, but is
trivial and silly, just reproducing 700efc1b...

1e934dda...:  attempting to revert the changes from 3def3d6d... (just one
commit before!) already fails.
---------------------------------------------------------

This last commit has an effect on my machine that prevents attempts to
revert 3def3d6d... from working as intended.  This may explain why
Yinghai's patch providing the revert for 2.6.27-rc3 did not work.
(Hopefully none of the other changes between Feb. and Aug. would also keep
the revert from working, but I wouldn't bet my life on it....)

The 3d... and 1e... commits are quite small, touching only 4 files total,
and both commits involve calls to insert_resource().  Something on my 2
problem machines is behaving badly in this area.

I wonder if it would help to revert both the 3def3d6d... and 1e934dda...
commits.  If there are 2 (or more) problematic commits, then of course
it wouldn't help to revert just one of the two commits.  This is one of
the nastiest type of debugging scenario, when there is more than one
cause of the observed problem, although in such case the multiple
causes are often related in some way.

Thanks for this Bill.  I got home pretty late last night, so I only tried a
few things before hitting the sack.

Your suggestion is something I was planning, but didn't get to yet.  It
seems like any change after 3def3d6d that touches insert_resource() causes
kernels to lock up on 2 of my 3 machines.

Mike Galbraith sent an offlist reply with a very good idea for finding out
whether a commit _before_ 3def3d6d is the actual cause of my troubles.  I
am more intrigued by this possibility than the idea you and I had about
reverting both 3def3d6d and 1e934dda and moving forward from there.

If Mike's idea doesn't seem to go anywhere -- if I cannot find a kernel
that works by applying the 3def3d6d changes to _previous_ kernel revisions,
then I plan to create a branch at 700efc1b and try moving forward toward
2.6.26 (skipping the 3def3d6d and 1e934dda commits, of course) until the
kernel freezes again.

Now I have plenty of things to try!


Thx,
Dave W.