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From: Thomas Gleixner <tglx@linutronix.de>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Fengguang Wu <fengguang.wu@intel.com>, LKP <lkp@01.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Don Zickus <dzickus@redhat.com>, Ingo Molnar <mingo@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>
Subject: Re: d57108d4f6 ("watchdog/core: Get rid of the thread .."): BUG: unable to handle kernel NULL pointer dereference at 0000000000000208
Date: Sat, 16 Sep 2017 23:47:10 +0200 (CEST)	[thread overview]
Message-ID: <alpine.DEB.2.20.1709162342560.2105@nanos> (raw)
In-Reply-To: <CA+55aFw+j+S4_QG-6ZR6iMc0_8AKRgceG-Sh93LNCMDkiMZFqA@mail.gmail.com>

On Sat, 16 Sep 2017, Linus Torvalds wrote:
> On Sat, Sep 16, 2017 at 11:12 AM, Thomas Gleixner <tglx@linutronix.de> wrote:
> >>
> >> So I suspect your perf fix is the right one, and maybe we could/should
> >> just make people more aware of the empty cpumask issue with UP.
> >
> > Right, I just got a bit frightened as I really was not aware about that
> > 'opmtimization' which means that so far I just was lucky not to trip over
> > it.
> 
> Yeah. I can't say that I was really aware of it either in a every-day
> kind of way, it was only when I looked it up that I went "Oh, right,
> that's what we did".
> 
> So it's subtle and unexpected, and the saving grace is basically that
> empty cpumasks are really the exception to begin with. They basically
> don't happen in normal situations.

Yes and no. We get more code which uses cpumasks to store state, just like
I did, and while a lot of the cpumask functions just work as expected a
subset including for_each_cpu does not. That's confusing at best and I
rather avoid the hard to debug issues on UP, which probably gets less
testing anyway.

Thanks,

	tglx

WARNING: multiple messages have this Message-ID (diff)
From: Thomas Gleixner <tglx@linutronix.de>
To: lkp@lists.01.org
Subject: Re: d57108d4f6 ("watchdog/core: Get rid of the thread .."): BUG: unable to handle kernel NULL pointer dereference at 0000000000000208
Date: Sat, 16 Sep 2017 23:47:10 +0200	[thread overview]
Message-ID: <alpine.DEB.2.20.1709162342560.2105@nanos> (raw)
In-Reply-To: <CA+55aFw+j+S4_QG-6ZR6iMc0_8AKRgceG-Sh93LNCMDkiMZFqA@mail.gmail.com>

[-- Attachment #1: Type: text/plain, Size: 1142 bytes --]

On Sat, 16 Sep 2017, Linus Torvalds wrote:
> On Sat, Sep 16, 2017 at 11:12 AM, Thomas Gleixner <tglx@linutronix.de> wrote:
> >>
> >> So I suspect your perf fix is the right one, and maybe we could/should
> >> just make people more aware of the empty cpumask issue with UP.
> >
> > Right, I just got a bit frightened as I really was not aware about that
> > 'opmtimization' which means that so far I just was lucky not to trip over
> > it.
> 
> Yeah. I can't say that I was really aware of it either in a every-day
> kind of way, it was only when I looked it up that I went "Oh, right,
> that's what we did".
> 
> So it's subtle and unexpected, and the saving grace is basically that
> empty cpumasks are really the exception to begin with. They basically
> don't happen in normal situations.

Yes and no. We get more code which uses cpumasks to store state, just like
I did, and while a lot of the cpumask functions just work as expected a
subset including for_each_cpu does not. That's confusing at best and I
rather avoid the hard to debug issues on UP, which probably gets less
testing anyway.

Thanks,

	tglx


  reply	other threads:[~2017-09-16 21:47 UTC|newest]

Thread overview: 22+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-09-14 21:47 d57108d4f6 ("watchdog/core: Get rid of the thread .."): BUG: unable to handle kernel NULL pointer dereference at 0000000000000208 kernel test robot
2017-09-14 21:47 ` kernel test robot
2017-09-15  7:50 ` Thomas Gleixner
2017-09-15  7:50   ` Thomas Gleixner
2017-09-15 12:54   ` Thomas Gleixner
2017-09-15 12:54     ` Thomas Gleixner
2017-09-15 16:24     ` Thomas Gleixner
2017-09-15 16:24       ` Thomas Gleixner
2017-09-16 12:46       ` Fengguang Wu
2017-09-16 12:46         ` Fengguang Wu
2017-09-16 17:35         ` Thomas Gleixner
2017-09-16 17:35           ` Thomas Gleixner
2017-09-16 17:55           ` Linus Torvalds
2017-09-16 17:55             ` Linus Torvalds
2017-09-16 18:12             ` Thomas Gleixner
2017-09-16 18:12               ` Thomas Gleixner
2017-09-16 18:23               ` Linus Torvalds
2017-09-16 18:23                 ` Linus Torvalds
2017-09-16 21:47                 ` Thomas Gleixner [this message]
2017-09-16 21:47                   ` Thomas Gleixner
2017-09-16 22:02                   ` Linus Torvalds
2017-09-16 22:02                     ` Linus Torvalds

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