linux-kernel - [patch 0/2] cpu/hotplug: Prevent damage with SMP=y and HOTPLUG

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Date:   Tue, 26 Mar 2019 17:36:04 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     Tianyu Lan <Tianyu.Lan@...rosoft.com>,
        Konrad Wilk <konrad.wilk@...cle.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Mukesh Ojha <mojha@...eaurora.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Jiri Kosina <jkosina@...e.cz>, Rik van Riel <riel@...riel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Micheal Kelley <michael.h.kelley@...rosoft.com>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Borislav Petkov <bp@...en8.de>, x86@...nel.org
Subject: [patch 0/2] cpu/hotplug: Prevent damage with SMP=y and HOTPLUG_CPU=n

Tianyu reported a crash with SMP=y and HOTPLUG_CPU=n plus 'nosmt' on the
kernel command line.

  https://lkml.kernel.org/r/1553521883-20868-1-git-send-email-Tianyu.Lan@microsoft.com

The reason is a bug in the hotplug code which does not handle the fact,
that HOTPLUG_CPU=n cannot tear down a CPU completely.

Unfortunately HOTPLUG_CPU cannot be enforced as some architectures do not
support it at all.

The fix is only a workaround because a full solution is not possible due to
the limitations of HOTPLUG_CPU=n. So the CPU stays around in an undead state.

As 'nosmt' has become popular recently, the proper solution for X86 is to
enforce HOTPLUG_CPU when SMP is enabled.

Thanks,

	tglx
----
 arch/x86/Kconfig |    8 +-------
 kernel/cpu.c     |   20 ++++++++++++++++++--
 2 files changed, 19 insertions(+), 9 deletions(-)