linux-kernel - Re: [PATCH] x86/traps: Don't for in_interrupt() to return true in IST handlers

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Date:	Tue, 24 May 2016 11:51:24 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Borislav Petkov <bp@...en8.de>
Cc:	Andy Lutomirski <luto@...nel.org>, x86@...nel.org,
	Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org,
	Oleg Nesterov <oleg@...hat.com>, stable@...r.kernel.org,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH] x86/traps: Don't for in_interrupt() to return true in
 IST handlers


@Andy, its linux-kernel@...r, not lkml@...r :-)

On Tue, May 24, 2016 at 11:36:29AM +0200, Borislav Petkov wrote:
> On Tue, May 24, 2016 at 10:59:45AM +0200, Peter Zijlstra wrote:
> > Which use a sequence of: in_nmi(), in_irq(), in_softirq() to pick 1 out
> > of 4 possible contexts.
> 
> So should we make it cleaner and explicit and define a 5th context of
> priorities higher than NMI?
> 
> There's some room between those two:
> 
>  *             NMI_MASK:        0x00100000
>  * PREEMPT_NEED_RESCHED:        0x80000000
> 

A lot of pain; we'd have to go grow a whole bunch of things to 5.

Also, I don't think 5 is enough to model all the IST nesting. I'm also
not sure we really care too much; IST stuff is relatively rare. It just
means we can loose IST based trace events and the like, because its
treated as recursion.

So I think keeping it at 4 is fine, but we do want to make a semi
concious choice on how we map back to those 4.