Re: [Fix PATCH] cpu/hotplug: Fix bug report when add "nosmt" parameter with CONFIG_HOTPLUG_CPU=N

[Greg KH <gregkh@linuxfoundation.org>]

On Mon, Mar 25, 2019 at 09:51:23PM +0800, lantianyu1986@gmail.com wrote:
> From: Lan Tianyu <Tianyu.Lan@microsoft.com>
> 
> When add "nosmt" parameter, kernel still boots up all logical cpus once
> and set CR4.MCE on each CPU. This is to avoid shutting down machine
> when a broadacasted MCE is observed CR4.MCE=0b. (Detail please see comment
> in the cpu_smt_allowed()). Smt cpus will bring up and bring down during
> kernel boot with "nosmt" parameter.
> 
> When CONFIG_HOTPLUG_CPU=Y, CPU_DYING callbacks will be called inside
> stop-machine and irq is disabled. This happens in the take_cpu_down()
> callback. When CONFIG_HOTPLUG_CPU=N,CPU_DYING callbacks will be called
> with irq enabled.
> 
> smpcfd_dying_cpu() is one of CPU_DYING callbacks and it assumes to be
> called when irq is disabled. smpcfd_dying_cpu() calls flush_smp_call_
> function_queue() which requires to be called with irq disabled.
> 
> When CONFIG_HOTPLUG_CPU=N and add "nosmt" parameter, smpcfd_dying_cpu()
> is called with irq enalbed and this triggers BUG_ON(!irqs_disabled())
> in the irq_work_run_list(). This patch is to fix the issue.
> 
> Fixes: 0cc3cd21657b ("cpu/hotplug: Boot HT siblings at least once")
> Signed-off-by: Lan Tianyu <Tianyu.Lan@microsoft.com>
> ---
>  kernel/smp.c | 5 +++++
>  1 file changed, 5 insertions(+)

<formletter>

This is not the correct way to submit patches for inclusion in the
stable kernel tree.  Please read:
    https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html
for how to do this properly.

</formletter>