linux-kernel - Re: [PATCH] perf/x86/intel/bts: don't dereference ds unconditionally

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Message-ID: <87r38eu1mc.fsf@ashishki-desk.ger.corp.intel.com>
Date:   Tue, 20 Sep 2016 16:44:43 +0300
From:   Alexander Shishkin <alexander.shishkin@...ux.intel.com>
To:     Sebastian Andrzej Siewior <sebastian@...akpoint.cc>,
        Ingo Molnar <mingo@...hat.com>
Cc:     Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        linux-kernel@...r.kernel.org, vince@...ter.net, eranian@...gle.com,
        Arnaldo Carvalho de Melo <acme@...radead.org>,
        tglx@...utronix.de
Subject: Re: [PATCH] perf/x86/intel/bts: don't dereference ds unconditionally

Sebastian Andrzej Siewior <sebastian@...akpoint.cc> writes:

> From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
>
> Since commit 4d4c47412464 ("perf/x86/intel/bts: Fix BTS PMI detection")
> my box goes boom on boot:
>
> | .... node  #0, CPUs:      #1 #2 #3 #4 #5 #6 #7
> | BUG: unable to handle kernel NULL pointer dereference at 0000000000000018
> | IP: [<ffffffff8100c463>] intel_bts_interrupt+0x43/0x130
> | Call Trace:
> |  <NMI> d [<ffffffff8100b341>] intel_pmu_handle_irq+0x51/0x4b0
> |  [<ffffffff81004d47>] perf_event_nmi_handler+0x27/0x40
>
> I don't know what is going on here but ds is not always dereferenced
> unconditionally hence here the `ds' check to avoid the crash.

Good catch! I'm going to guess you don't have the NMI watchdog enabled?

Thanks,
--
Alex