[PATCH] hotplug: make register and unregister notifier API symmetric

[PATCH] hotplug: make register and unregister notifier API symmetric

[Michal Hocko]

[Let's CC more people - the thread started
http://lkml.kernel.org/r/1480540516-6458-1-git-send-email-yuzhao@google.com]

On Fri 02-12-16 09:56:26, Dan Streetman wrote:
> On Fri, Dec 2, 2016 at 9:44 AM, Michal Hocko <mhocko@kernel.org> wrote:
> > On Fri 02-12-16 15:38:48, Michal Hocko wrote:
> >> On Fri 02-12-16 09:24:35, Dan Streetman wrote:
> >> > On Fri, Dec 2, 2016 at 8:46 AM, Michal Hocko <mhocko@kernel.org> wrote:
> >> > > On Wed 30-11-16 13:15:16, Yu Zhao wrote:
> >> > >> __unregister_cpu_notifier() only removes registered notifier from its
> >> > >> linked list when CPU hotplug is configured. If we free registered CPU
> >> > >> notifier when HOTPLUG_CPU=n, we corrupt the linked list.
> >> > >>
> >> > >> To fix the problem, we can either use a static CPU notifier that walks
> >> > >> through each pool or just simply disable CPU notifier when CPU hotplug
> >> > >> is not configured (which is perfectly safe because the code in question
> >> > >> is called after all possible CPUs are online and will remain online
> >> > >> until power off).
> >> > >>
> >> > >> v2: #ifdef for cpu_notifier_register_done during cleanup.
> >> > >
> >> > > this ifedfery is just ugly as hell. I am also wondering whether it is
> >> > > really needed. __register_cpu_notifier and __unregister_cpu_notifier are
> >> > > noops for CONFIG_HOTPLUG_CPU=n. So what's exactly that is broken here?
> >> >
> >> > hmm, that's interesting, __unregister_cpu_notifier is always a noop if
> >> > HOTPLUG_CPU=n, but __register_cpu_notifier is only a noop if
> >> > HOTPLUG_CPU=n *and* MODULE.  If !MODULE, __register_cpu_notifier does
> >>
> >> OK, I've missed the MODULE part
> >>
> >> > actually register!  This was added by commit
> >> > 47e627bc8c9a70392d2049e6af5bd55fae61fe53 ('hotplug: Allow modules to
> >> > use the cpu hotplug notifiers even if !CONFIG_HOTPLUG_CPU') and looks
> >> > like it's to allow built-ins to register so they can notice during
> >> > boot when cpus are initialized.
> >>
> >> I cannot say I wound understand the motivation but that is not really
> >> all that important.
> >>
> >> > IMHO, that is the real problem - sure, without HOTPLUG_CPU, nobody
> >> > should ever get a notification that a cpu is dying, but that doesn't
> >> > mean builtins that register notifiers will never unregister their
> >> > notifiers and then free them.
> >>
> >> Yes that is true. That suggests that __unregister_cpu_notifier should
> >> the the symmetric thing to the __register_cpu_notifier for
> >> CONFIG_MODULE, right?
> >
> > I meant the following. Completely untested
> 
> agreed, but also needs the non-__ version, and kernel/cpu.c needs
> tweaking to move those functions out of the #ifdef CONFIG_HOTPLUG_CPU
> section.

OK, this is still only compile tested. Yu Zhao, assuming you were able
to trigger the original problem could you test with the below patch
please?
---

Re: [PATCH] hotplug: make register and unregister notifier API symmetric

[kbuild test robot]

[-- Attachment #1: Type: text/plain, Size: 1030 bytes --]

Hi Michal,

[auto build test ERROR on linus/master]
[also build test ERROR on v4.9-rc7 next-20161202]
[if your patch is applied to the wrong git tree, please drop us a note to help improve the system]

url:    https://github.com/0day-ci/linux/commits/Michal-Hocko/hotplug-make-register-and-unregister-notifier-API-symmetric/20161203-114815
config: i386-randconfig-r0-201648 (attached as .config)
compiler: gcc-5 (Debian 5.4.1-2) 5.4.1 20160904
reproduce:
        # save the attached .config to linux build tree
        make ARCH=i386 

All errors (new ones prefixed by >>):

   arch/x86/oprofile/built-in.o: In function `nmi_timer_shutdown':
>> nmi_timer_int.c:(.text+0x238b): undefined reference to `__unregister_cpu_notifier'
   arch/x86/oprofile/built-in.o: In function `nmi_shutdown':
   nmi_int.c:(.text+0x2793): undefined reference to `__unregister_cpu_notifier'

---
0-DAY kernel test infrastructure                Open Source Technology Center
https://lists.01.org/pipermail/kbuild-all                   Intel Corporation

[-- Attachment #2: .config.gz --]
[-- Type: application/gzip, Size: 21659 bytes --]

Re: [PATCH] hotplug: make register and unregister notifier API symmetric

[kbuild test robot]

[-- Attachment #1: Type: text/plain, Size: 1179 bytes --]

Hi Michal,

[auto build test ERROR on linus/master]
[also build test ERROR on v4.9-rc7 next-20161202]
[if your patch is applied to the wrong git tree, please drop us a note to help improve the system]

url:    https://github.com/0day-ci/linux/commits/Michal-Hocko/hotplug-make-register-and-unregister-notifier-API-symmetric/20161203-114815
config: i386-randconfig-s1-201648 (attached as .config)
compiler: gcc-6 (Debian 6.2.0-3) 6.2.0 20160901
reproduce:
        # save the attached .config to linux build tree
        make ARCH=i386 

All errors (new ones prefixed by >>):

   mm/built-in.o: In function `zs_unregister_cpu_notifier':
>> zsmalloc.c:(.text.unlikely+0xc7e): undefined reference to `__unregister_cpu_notifier'
   arch/x86/oprofile/built-in.o: In function `nmi_timer_shutdown':
   nmi_timer_int.c:(.text+0x1c7d): undefined reference to `__unregister_cpu_notifier'
   arch/x86/oprofile/built-in.o: In function `nmi_shutdown':
   nmi_int.c:(.text+0x2319): undefined reference to `__unregister_cpu_notifier'

---
0-DAY kernel test infrastructure                Open Source Technology Center
https://lists.01.org/pipermail/kbuild-all                   Intel Corporation

[-- Attachment #2: .config.gz --]
[-- Type: application/gzip, Size: 21074 bytes --]

Re: [PATCH] hotplug: make register and unregister notifier API symmetric

[Michal Hocko]

On Sat 03-12-16 13:15:42, kbuild test robot wrote:
> Hi Michal,
> 
> [auto build test ERROR on linus/master]
> [also build test ERROR on v4.9-rc7 next-20161202]
> [if your patch is applied to the wrong git tree, please drop us a note to help improve the system]
> 
> url:    https://github.com/0day-ci/linux/commits/Michal-Hocko/hotplug-make-register-and-unregister-notifier-API-symmetric/20161203-114815
> config: i386-randconfig-r0-201648 (attached as .config)
> compiler: gcc-5 (Debian 5.4.1-2) 5.4.1 20160904
> reproduce:
>         # save the attached .config to linux build tree
>         make ARCH=i386 
> 
> All errors (new ones prefixed by >>):
> 
>    arch/x86/oprofile/built-in.o: In function `nmi_timer_shutdown':
> >> nmi_timer_int.c:(.text+0x238b): undefined reference to `__unregister_cpu_notifier'
>    arch/x86/oprofile/built-in.o: In function `nmi_shutdown':
>    nmi_int.c:(.text+0x2793): undefined reference to `__unregister_cpu_notifier'

Ohh, right. I have missed that unregister functions definitions are
guarded as well. This patch should hopefully be correct finally.
Please note it also exports register/unregister callbacks when
CONFIG_HOTPLUG_CPU is not defined which is not really needed strictly
speaking because those are only used when !MODULE but I would rather
not make the code more complicated. If maintainers prefer I can guard
exports separately of course.
---

Re: [PATCH] hotplug: make register and unregister notifier API symmetric

[Yu Zhao]

On Fri, Dec 02, 2016 at 04:19:36PM +0100, Michal Hocko wrote:
> [Let's CC more people - the thread started
> http://lkml.kernel.org/r/1480540516-6458-1-git-send-email-yuzhao@google.com]
> 
> On Fri 02-12-16 09:56:26, Dan Streetman wrote:
> > On Fri, Dec 2, 2016 at 9:44 AM, Michal Hocko <mhocko@kernel.org> wrote:
> > > On Fri 02-12-16 15:38:48, Michal Hocko wrote:
> > >> On Fri 02-12-16 09:24:35, Dan Streetman wrote:
> > >> > On Fri, Dec 2, 2016 at 8:46 AM, Michal Hocko <mhocko@kernel.org> wrote:
> > >> > > On Wed 30-11-16 13:15:16, Yu Zhao wrote:
> > >> > >> __unregister_cpu_notifier() only removes registered notifier from its
> > >> > >> linked list when CPU hotplug is configured. If we free registered CPU
> > >> > >> notifier when HOTPLUG_CPU=n, we corrupt the linked list.
> > >> > >>
> > >> > >> To fix the problem, we can either use a static CPU notifier that walks
> > >> > >> through each pool or just simply disable CPU notifier when CPU hotplug
> > >> > >> is not configured (which is perfectly safe because the code in question
> > >> > >> is called after all possible CPUs are online and will remain online
> > >> > >> until power off).
> > >> > >>
> > >> > >> v2: #ifdef for cpu_notifier_register_done during cleanup.
> > >> > >
> > >> > > this ifedfery is just ugly as hell. I am also wondering whether it is
> > >> > > really needed. __register_cpu_notifier and __unregister_cpu_notifier are
> > >> > > noops for CONFIG_HOTPLUG_CPU=n. So what's exactly that is broken here?
> > >> >
> > >> > hmm, that's interesting, __unregister_cpu_notifier is always a noop if
> > >> > HOTPLUG_CPU=n, but __register_cpu_notifier is only a noop if
> > >> > HOTPLUG_CPU=n *and* MODULE.  If !MODULE, __register_cpu_notifier does
> > >>
> > >> OK, I've missed the MODULE part
> > >>
> > >> > actually register!  This was added by commit
> > >> > 47e627bc8c9a70392d2049e6af5bd55fae61fe53 ('hotplug: Allow modules to
> > >> > use the cpu hotplug notifiers even if !CONFIG_HOTPLUG_CPU') and looks
> > >> > like it's to allow built-ins to register so they can notice during
> > >> > boot when cpus are initialized.
> > >>
> > >> I cannot say I wound understand the motivation but that is not really
> > >> all that important.
> > >>
> > >> > IMHO, that is the real problem - sure, without HOTPLUG_CPU, nobody
> > >> > should ever get a notification that a cpu is dying, but that doesn't
> > >> > mean builtins that register notifiers will never unregister their
> > >> > notifiers and then free them.
> > >>
> > >> Yes that is true. That suggests that __unregister_cpu_notifier should
> > >> the the symmetric thing to the __register_cpu_notifier for
> > >> CONFIG_MODULE, right?
> > >
> > > I meant the following. Completely untested
> > 
> > agreed, but also needs the non-__ version, and kernel/cpu.c needs
> > tweaking to move those functions out of the #ifdef CONFIG_HOTPLUG_CPU
> > section.
> 
> OK, this is still only compile tested. Yu Zhao, assuming you were able
> to trigger the original problem could you test with the below patch
> please?

This patch (plus the latest fix in this thread) solves the problem.

Just for the record, the problem is when CONFIG_HOTPLUG_CPU=n, changing
/sys/module/zswap/parameters/compressor multiple times will cause:

[  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

> ---
> From c812fe4e519914aa37f092d3a0321038fadcdde7 Mon Sep 17 00:00:00 2001
> From: Michal Hocko <mhocko@suse.com>
> Date: Fri, 2 Dec 2016 16:06:56 +0100
> Subject: [PATCH] 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. 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")
> Cc: stable # zswap needs it 4.3+
> Reported-by: Yu Zhao <yuzhao@google.com>
> Signed-off-by: Michal Hocko <mhocko@suse.com>
> ---
>  include/linux/cpu.h | 15 ++++-----------
>  1 file changed, 4 insertions(+), 11 deletions(-)
> 
> diff --git a/include/linux/cpu.h b/include/linux/cpu.h
> index 797d9c8e9a1b..c8938eb21e34 100644
> --- a/include/linux/cpu.h
> +++ b/include/linux/cpu.h
> @@ -105,22 +105,16 @@ 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;
> @@ -130,7 +124,6 @@ static inline int __register_cpu_notifier(struct notifier_block *nb)
>  {
>  	return 0;
>  }
> -#endif
>  
>  static inline void unregister_cpu_notifier(struct notifier_block *nb)
>  {
> -- 
> 2.10.2
> 
> -- 
> Michal Hocko
> SUSE Labs

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Re: [PATCH] hotplug: make register and unregister notifier API symmetric

[Michal Hocko]

On Mon 05-12-16 12:59:02, Yu Zhao wrote:
> On Fri, Dec 02, 2016 at 04:19:36PM +0100, Michal Hocko wrote:
> > [Let's CC more people - the thread started
> > http://lkml.kernel.org/r/1480540516-6458-1-git-send-email-yuzhao@google.com]
> > 
> > On Fri 02-12-16 09:56:26, Dan Streetman wrote:
> > > On Fri, Dec 2, 2016 at 9:44 AM, Michal Hocko <mhocko@kernel.org> wrote:
> > > > On Fri 02-12-16 15:38:48, Michal Hocko wrote:
> > > >> On Fri 02-12-16 09:24:35, Dan Streetman wrote:
> > > >> > On Fri, Dec 2, 2016 at 8:46 AM, Michal Hocko <mhocko@kernel.org> wrote:
> > > >> > > On Wed 30-11-16 13:15:16, Yu Zhao wrote:
> > > >> > >> __unregister_cpu_notifier() only removes registered notifier from its
> > > >> > >> linked list when CPU hotplug is configured. If we free registered CPU
> > > >> > >> notifier when HOTPLUG_CPU=n, we corrupt the linked list.
> > > >> > >>
> > > >> > >> To fix the problem, we can either use a static CPU notifier that walks
> > > >> > >> through each pool or just simply disable CPU notifier when CPU hotplug
> > > >> > >> is not configured (which is perfectly safe because the code in question
> > > >> > >> is called after all possible CPUs are online and will remain online
> > > >> > >> until power off).
> > > >> > >>
> > > >> > >> v2: #ifdef for cpu_notifier_register_done during cleanup.
> > > >> > >
> > > >> > > this ifedfery is just ugly as hell. I am also wondering whether it is
> > > >> > > really needed. __register_cpu_notifier and __unregister_cpu_notifier are
> > > >> > > noops for CONFIG_HOTPLUG_CPU=n. So what's exactly that is broken here?
> > > >> >
> > > >> > hmm, that's interesting, __unregister_cpu_notifier is always a noop if
> > > >> > HOTPLUG_CPU=n, but __register_cpu_notifier is only a noop if
> > > >> > HOTPLUG_CPU=n *and* MODULE.  If !MODULE, __register_cpu_notifier does
> > > >>
> > > >> OK, I've missed the MODULE part
> > > >>
> > > >> > actually register!  This was added by commit
> > > >> > 47e627bc8c9a70392d2049e6af5bd55fae61fe53 ('hotplug: Allow modules to
> > > >> > use the cpu hotplug notifiers even if !CONFIG_HOTPLUG_CPU') and looks
> > > >> > like it's to allow built-ins to register so they can notice during
> > > >> > boot when cpus are initialized.
> > > >>
> > > >> I cannot say I wound understand the motivation but that is not really
> > > >> all that important.
> > > >>
> > > >> > IMHO, that is the real problem - sure, without HOTPLUG_CPU, nobody
> > > >> > should ever get a notification that a cpu is dying, but that doesn't
> > > >> > mean builtins that register notifiers will never unregister their
> > > >> > notifiers and then free them.
> > > >>
> > > >> Yes that is true. That suggests that __unregister_cpu_notifier should
> > > >> the the symmetric thing to the __register_cpu_notifier for
> > > >> CONFIG_MODULE, right?
> > > >
> > > > I meant the following. Completely untested
> > > 
> > > agreed, but also needs the non-__ version, and kernel/cpu.c needs
> > > tweaking to move those functions out of the #ifdef CONFIG_HOTPLUG_CPU
> > > section.
> > 
> > OK, this is still only compile tested. Yu Zhao, assuming you were able
> > to trigger the original problem could you test with the below patch
> > please?
> 
> This patch (plus the latest fix in this thread) solves the problem.
> 
> Just for the record, the problem is when CONFIG_HOTPLUG_CPU=n, changing
> /sys/module/zswap/parameters/compressor multiple times will cause:
> 
> [  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

Thanks for this additional information which I have added to the
changelog. I have also added your Tested-by unless you have any
objections and will repost soon.
-- 
Michal Hocko
SUSE Labs

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[PATCH] hotplug: make register and unregister notifier API symmetric

[Michal Hocko]

From: Michal Hocko <mhocko@suse.com>

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")
Cc: stable # zswap requires for 4.3+
Reported-and-tested-by: Yu Zhao <yuzhao@google.com>
Signed-off-by: Michal Hocko <mhocko@suse.com>
---
Hi,
the previous version of the patch has been sent [1] and the reporter has
confirmed it works as expected. I have also added more information to
the changelog. Does this looks sensible?

[1] http://lkml.kernel.org/r/20161205060840.GC30758@dhcp22.suse.cz

 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 797d9c8e9a1b..c8938eb21e34 100644
--- a/include/linux/cpu.h
+++ b/include/linux/cpu.h
@@ -105,22 +105,16 @@ 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;
@@ -130,7 +124,6 @@ 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 341bf80f80bd..73fb59fda809 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -578,7 +578,6 @@ 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)
@@ -595,6 +594,7 @@ 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
-- 
2.10.2

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