linux-kernel - Re: [ANNOUNCE] v5.14-rc5-rt8

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Message-ID: <7ad059482e56e88bfbe13830cfa06dffee9d071f.camel@gmx.de>
Date:   Sun, 15 Aug 2021 15:03:35 +0200
From:   Mike Galbraith <efault@....de>
To:     Vlastimil Babka <vbabka@...e.cz>,
        Clark Williams <williams@...hat.com>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        RT <linux-rt-users@...r.kernel.org>
Subject: Re: [ANNOUNCE] v5.14-rc5-rt8

On Sun, 2021-08-15 at 11:35 +0200, Vlastimil Babka wrote:
> On 8/15/21 6:17 AM, Mike Galbraith wrote:
> > On Sat, 2021-08-14 at 21:08 +0200, Vlastimil Babka wrote:
> > >
> > > Aha! That's helpful. Hopefully it's just a small issue where we
> > > opportunistically test flags on a page that's protected by the local
> > > lock we didn't take yet, and I didn't realize there's the VM_BUG_ON
> > > which can trigger if our page went away (which we would have realized
> > > after taking the lock).
> >
> > Speaking of optimistic peeking perhaps going badly, why is the below
> > not true?  There's protection against ->partial going disappearing
> > during a preemption... but can't it just as easily appear, so where is
> > that protection?
>
> If it appears, it appears, we don't care, we just leave it there and
> won't use it.
>
> > If the other side of that window is safe, it could use a comment so
> > dummies reading this code don't end up asking mm folks why the heck
> > they don't just take the darn lock and be done with it instead of tap
> > dancing all around thething :)
>
> Well, with your patch, ->partial might appear just after the unlock, so
> does that really change anything?

Viewed from pov consumption is optional, it makes sense.

	-Mike