linux-kernel - Re: Regression bisected to f2f84b05e02b (bug: consolidate warn_slowpath

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Message-ID: <20200612044757.GA10703@tower>
Date:   Fri, 12 Jun 2020 16:47:57 +1200
From:   Michael Cree <mcree@...on.net.nz>
To:     Matt Turner <mattst88@...il.com>
Cc:     Kees Cook <keescook@...omium.org>,
        Linux-Arch <linux-arch@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-alpha <linux-alpha@...r.kernel.org>,
        Richard Henderson <rth@...ddle.net>,
        Ivan Kokshaysky <ink@...assic.park.msu.ru>
Subject: Re: Regression bisected to f2f84b05e02b (bug: consolidate
 warn_slowpath_fmt() usage)

On Thu, Jun 11, 2020 at 09:23:52PM -0700, Matt Turner wrote:
> Since I noticed earlier that using maxcpus=1 on a 2-CPU system
> prevented the system from hanging, I tried disabling CONFIG_SMP on my
> 1-CPU system as well. In doing so, I discovered that the RCU torture
> module (RCU_TORTURE_TEST) triggers some null pointer dereferences on
> Alpha when CONFIG_SMP is set, but works successfully when CONFIG_SMP
> is unset.
> 
> That seems likely to be a symptom of the same underlying problem that
> started this thread, don't you think? If so, I'll focus my attention
> on that.

I wonder if that is related to user space segfaults we are now seeing
on SMP systems but not UP systems while building Alpha debian-ports.
It's happening in the test-suites of builds of certain software
(such as autogen and guile) but they always build successfully with
the test suite passing on a UP system.

When investigating I seem to recall it was a NULL (or near NULL)
pointer dereference but couldn't make any sense of how it might
have got into such an obviously wrong state.

Cheers,
Michael.