linux-kernel - Re: [BUG REPORT] Soft Lockup in smp_call_function

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Date:	Sat, 30 Jan 2016 11:50:34 -0700
From:	Jeff Merkey <linux.mdb@...il.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Vlastimil Babka <vbabka@...e.cz>,
	"Peter Zijlstra (Intel)" <peterz@...radead.org>,
	Mel Gorman <mgorman@...hsingularity.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
	Andrew Lutomirski <luto@...nel.org>
Subject: Re: [BUG REPORT] Soft Lockup in smp_call_function_single+0xD8

>
> Just so you know, I have no intention of supporting this use case.  In
> fact, I'm planning to eventually stop using IST for #DB entirely, at
> which point the kernel will crash terribly if this code is
> single-stepped (except when using a hypervisor to do this single
> stepping, which is a much more sensible way to handle it).

I don't know what good using the userspace trap code is going to
help with this, a hypervisor will crash too with sysret being used
there -- I just tested it.

So it breaks all debuggers, even the faux ones that run as user space
apps.  Your
other suggestions will cause some mayhem too for debuggers.  If you
break something, I'll just unpatch it in my tree, so have fun.  LOL

Jeff