Re: [RFC PATCH] mm: silence soft lockups from unlock_page

From: Linus Torvalds
Date: Thu Aug 06 2020 - 13:08:05 EST

On Wed, Aug 5, 2020 at 10:21 PM Hugh Dickins <hughd@xxxxxxxxxx> wrote:
> Something I was interested to realize in looking at this: trylock_page()
> on a contended lock is now much less likely to jump the queue and
> succeed than before, since your lock holder hands off the page lock to
> the next holder: much smaller window than waiting for the next to wake
> to take it. Nothing wrong with that, but effect might be seen somewhere.

Yeah, the window is smaller, but it's not gone.

It *might* be interesting to actually do the handover directly from
"unlock_page()", and avoid clearing (and then setting) the bit

Something like the attached TOTALLY UNTESTED patch.

NOTE! Sometimes when I say something is untested, I think the patch is
fine because it's simple and straightforward, I just didn't test it.

This time it's both untested and very very subtle indeed. Did I get
the hand-over case SMP memory barriers right? Did I screw something
else up?

So this might be complete garbage. I really don't know. But it might
close the window for an unfair trylock (or lucky page_lock())

Or maybe it just makes page locking break entirely. It's a very real risk.

The upside may not be worth it.


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