Re: [tip:smp/urgent] hotplug: Make register and unregister notifier API symmetric

[Greg KH <gregkh@linuxfoundation.org>]

On Tue, Dec 13, 2016 at 07:39:58AM +0100, Ingo Molnar wrote:
> 
> hi Greg,
> 
> Please apply this fix to v4.9-stable as well, it's now upstream.

Ok, it looks like it belongs in older stable kernels as well, based on
the fixes: tag, correct?

thanks,

greg k-h

> ----- Forwarded message from tip-bot for Michal Hocko <tipbot@zytor.com> -----
> 
> Date: Thu, 8 Dec 2016 01:13:24 -0800
> From: tip-bot for Michal Hocko <tipbot@zytor.com>
> To: linux-tip-commits@vger.kernel.org
> Cc: mingo@kernel.org, linux-kernel@vger.kernel.org, akpm@linux-foundation.org, tglx@linutronix.de, ddstreet@ieee.org, mhocko@suse.com, yuzhao@google.com, hpa@zytor.com
> Subject: [tip:smp/urgent] hotplug: Make register and unregister notifier API symmetric
> 
> Commit-ID:  777c6e0daebb3fcefbbd6f620410a946b07ef6d0
> Gitweb:     http://git.kernel.org/tip/777c6e0daebb3fcefbbd6f620410a946b07ef6d0
> Author:     Michal Hocko <mhocko@suse.com>
> AuthorDate: Wed, 7 Dec 2016 14:54:38 +0100
> Committer:  Thomas Gleixner <tglx@linutronix.de>
> CommitDate: Thu, 8 Dec 2016 10:08:41 +0100
> 
> hotplug: Make register and unregister notifier API symmetric
> 
> Yu Zhao has noticed that __unregister_cpu_notifier only unregisters its
> notifiers when HOTPLUG_CPU=y while the registration might succeed even
> when HOTPLUG_CPU=n if MODULE is enabled. This means that e.g. zswap
> might keep a stale notifier on the list on the manual clean up during
> the pool tear down and thus corrupt the list. Resulting in the following
> 
> [  144.964346] BUG: unable to handle kernel paging request at ffff880658a2be78
> [  144.971337] IP: [<ffffffffa290b00b>] raw_notifier_chain_register+0x1b/0x40
> <snipped>
> [  145.122628] Call Trace:
> [  145.125086]  [<ffffffffa28e5cf8>] __register_cpu_notifier+0x18/0x20
> [  145.131350]  [<ffffffffa2a5dd73>] zswap_pool_create+0x273/0x400
> [  145.137268]  [<ffffffffa2a5e0fc>] __zswap_param_set+0x1fc/0x300
> [  145.143188]  [<ffffffffa2944c1d>] ? trace_hardirqs_on+0xd/0x10
> [  145.149018]  [<ffffffffa2908798>] ? kernel_param_lock+0x28/0x30
> [  145.154940]  [<ffffffffa2a3e8cf>] ? __might_fault+0x4f/0xa0
> [  145.160511]  [<ffffffffa2a5e237>] zswap_compressor_param_set+0x17/0x20
> [  145.167035]  [<ffffffffa2908d3c>] param_attr_store+0x5c/0xb0
> [  145.172694]  [<ffffffffa290848d>] module_attr_store+0x1d/0x30
> [  145.178443]  [<ffffffffa2b2b41f>] sysfs_kf_write+0x4f/0x70
> [  145.183925]  [<ffffffffa2b2a5b9>] kernfs_fop_write+0x149/0x180
> [  145.189761]  [<ffffffffa2a99248>] __vfs_write+0x18/0x40
> [  145.194982]  [<ffffffffa2a9a412>] vfs_write+0xb2/0x1a0
> [  145.200122]  [<ffffffffa2a9a732>] SyS_write+0x52/0xa0
> [  145.205177]  [<ffffffffa2ff4d97>] entry_SYSCALL_64_fastpath+0x12/0x17
> 
> This can be even triggered manually by changing
> /sys/module/zswap/parameters/compressor multiple times.
> 
> Fix this issue by making unregister APIs symmetric to the register so
> there are no surprises.
> 
> Fixes: 47e627bc8c9a ("[PATCH] hotplug: Allow modules to use the cpu hotplug notifiers even if !CONFIG_HOTPLUG_CPU")
> Reported-and-tested-by: Yu Zhao <yuzhao@google.com>
> Signed-off-by: Michal Hocko <mhocko@suse.com>
> Cc: linux-mm@kvack.org
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Dan Streetman <ddstreet@ieee.org>
> Link: http://lkml.kernel.org/r/20161207135438.4310-1-mhocko@kernel.org
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> 
> ---
>  include/linux/cpu.h | 15 ++++-----------
>  kernel/cpu.c        |  2 +-
>  2 files changed, 5 insertions(+), 12 deletions(-)
> 
> diff --git a/include/linux/cpu.h b/include/linux/cpu.h
> index b886dc1..e571128 100644
> --- a/include/linux/cpu.h
> +++ b/include/linux/cpu.h
> @@ -93,22 +93,16 @@ extern bool cpuhp_tasks_frozen;
>  		{ .notifier_call = fn, .priority = pri };	\
>  	__register_cpu_notifier(&fn##_nb);			\
>  }
> -#else /* #if defined(CONFIG_HOTPLUG_CPU) || !defined(MODULE) */
> -#define cpu_notifier(fn, pri)	do { (void)(fn); } while (0)
> -#define __cpu_notifier(fn, pri)	do { (void)(fn); } while (0)
> -#endif /* #else #if defined(CONFIG_HOTPLUG_CPU) || !defined(MODULE) */
>  
> -#ifdef CONFIG_HOTPLUG_CPU
>  extern int register_cpu_notifier(struct notifier_block *nb);
>  extern int __register_cpu_notifier(struct notifier_block *nb);
>  extern void unregister_cpu_notifier(struct notifier_block *nb);
>  extern void __unregister_cpu_notifier(struct notifier_block *nb);
> -#else
>  
> -#ifndef MODULE
> -extern int register_cpu_notifier(struct notifier_block *nb);
> -extern int __register_cpu_notifier(struct notifier_block *nb);
> -#else
> +#else /* #if defined(CONFIG_HOTPLUG_CPU) || !defined(MODULE) */
> +#define cpu_notifier(fn, pri)	do { (void)(fn); } while (0)
> +#define __cpu_notifier(fn, pri)	do { (void)(fn); } while (0)
> +
>  static inline int register_cpu_notifier(struct notifier_block *nb)
>  {
>  	return 0;
> @@ -118,7 +112,6 @@ static inline int __register_cpu_notifier(struct notifier_block *nb)
>  {
>  	return 0;
>  }
> -#endif
>  
>  static inline void unregister_cpu_notifier(struct notifier_block *nb)
>  {
> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index 29de1a9..217fd2e 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -659,7 +659,6 @@ void __init cpuhp_threads_init(void)
>  	kthread_unpark(this_cpu_read(cpuhp_state.thread));
>  }
>  
> -#ifdef CONFIG_HOTPLUG_CPU
>  EXPORT_SYMBOL(register_cpu_notifier);
>  EXPORT_SYMBOL(__register_cpu_notifier);
>  void unregister_cpu_notifier(struct notifier_block *nb)
> @@ -676,6 +675,7 @@ void __unregister_cpu_notifier(struct notifier_block *nb)
>  }
>  EXPORT_SYMBOL(__unregister_cpu_notifier);
>  
> +#ifdef CONFIG_HOTPLUG_CPU
>  /**
>   * clear_tasks_mm_cpumask - Safely clear tasks' mm_cpumask for a CPU
>   * @cpu: a CPU id
> 
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