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Date: Sat, 16 Sep 2017 23:47:10 +0200 (CEST) From: Thomas Gleixner <tglx@...utronix.de> To: Linus Torvalds <torvalds@...ux-foundation.org> cc: Fengguang Wu <fengguang.wu@...el.com>, LKP <lkp@...org>, LKML <linux-kernel@...r.kernel.org>, Don Zickus <dzickus@...hat.com>, Ingo Molnar <mingo@...nel.org>, Peter Zijlstra <peterz@...radead.org> Subject: Re: d57108d4f6 ("watchdog/core: Get rid of the thread .."): BUG: unable to handle kernel NULL pointer dereference at 0000000000000208 On Sat, 16 Sep 2017, Linus Torvalds wrote: > On Sat, Sep 16, 2017 at 11:12 AM, Thomas Gleixner <tglx@...utronix.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
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