linux-kernel - Re: [PATCH] x86/traps: Weaken context tracking entry assertions

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Date:	Fri, 21 Aug 2015 08:23:28 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Andy Lutomirski <luto@...nel.org>
Cc:	x86@...nel.org, Sasha Levin <sasha.levin@...cle.com>,
	Brian Gerst <brgerst@...il.com>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	linux-kernel@...r.kernel.org, Oleg Nesterov <oleg@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] x86/traps: Weaken context tracking entry assertions


* Andy Lutomirski <luto@...nel.org> wrote:

> We were asserting that we were all the way in CONTEXT_KERNEL when exception 
> handlers were called.  While having this be true is, I think, a nice goal (or 
> maybe a variant in which we assert that we're in CONTEXT_KERNEL or some new IRQ 
> context), we're not quite there.
> 
> In particular, if an IRQ interrupts the SYSCALL prologue and the IRQ handler in 
> turn causes an exception, the exception entry will be called in RCU IRQ mode but 
> with CONTEXT_USER.

Hm, so what harm would there be in making IRQ handlers enter CONTEXT_KERNEL?
Would nohz-full break?

I'd rather have a bit more tracking overhead here than lose such useful sanity 
checks.

Thanks,

	Ingo
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