[PATCH v2 0/2] fix devm_memremap_pages() mem hotplug locking

[PATCH v2 0/2] fix devm_memremap_pages() mem hotplug locking

[Dan Williams]

Changes since v1 [1]:
* Reflowed the patches on 4.10-rc8. The v1 series no longer applies
  to -mm now that the sub-section memory hotplug support has been
  deferred to 4.12 [2].

[1]: https://lists.01.org/pipermail/linux-nvdimm/2017-February/008848.html
[2]: http://www.spinics.net/lists/linux-mm/msg121990.html

---

Ben notes that commit f931ab479dd2 "mm: fix devm_memremap_pages crash,
use mem_hotplug_{begin, done}" is incomplete and broken. Writes to
mem_hotplug.active_writer need to be coordinated under the device
hotplug lock. Otherwise, we can potentially corrupt mem_hotplug.refcount
leading to soft lockups.

---

Dan Williams (2):
      mm, devm_memremap_pages: hold device_hotplug lock over mem_hotplug_{begin,done}
      mm: validate device_hotplug is held for memory hotplug


 drivers/base/core.c    |    5 +++++
 include/linux/device.h |    1 +
 kernel/memremap.c      |    6 ++++++
 mm/memory_hotplug.c    |    2 ++
 4 files changed, 14 insertions(+)
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[PATCH v2 0/2] fix devm_memremap_pages() mem hotplug locking

[Dan Williams]

Changes since v1 [1]:
* Reflowed the patches on 4.10-rc8. The v1 series no longer applies
  to -mm now that the sub-section memory hotplug support has been
  deferred to 4.12 [2].

[1]: https://lists.01.org/pipermail/linux-nvdimm/2017-February/008848.html
[2]: http://www.spinics.net/lists/linux-mm/msg121990.html

---

Ben notes that commit f931ab479dd2 "mm: fix devm_memremap_pages crash,
use mem_hotplug_{begin, done}" is incomplete and broken. Writes to
mem_hotplug.active_writer need to be coordinated under the device
hotplug lock. Otherwise, we can potentially corrupt mem_hotplug.refcount
leading to soft lockups.

---

Dan Williams (2):
      mm, devm_memremap_pages: hold device_hotplug lock over mem_hotplug_{begin,done}
      mm: validate device_hotplug is held for memory hotplug


 drivers/base/core.c    |    5 +++++
 include/linux/device.h |    1 +
 kernel/memremap.c      |    6 ++++++
 mm/memory_hotplug.c    |    2 ++
 4 files changed, 14 insertions(+)

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[PATCH v2 1/2] mm, devm_memremap_pages: hold device_hotplug lock over mem_hotplug_{begin, done}

[Dan Williams]

The mem_hotplug_{begin,done} lock coordinates with
{get,put}_online_mems() to hold off "readers" of the current state of
memory from new hotplug actions. mem_hotplug_begin() expects exclusive
access, via the device_hotplug lock, to set mem_hotplug.active_writer.
Calling mem_hotplug_begin() without locking device_hotplug can lead to
corrupting mem_hotplug.refcount and missed wakeups / soft lockups.

Cc: <stable@vger.kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Toshi Kani <toshi.kani@hpe.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Logan Gunthorpe <logang@deltatee.com>
Fixes: f931ab479dd2 ("mm: fix devm_memremap_pages crash, use mem_hotplug_{begin, done}")
Reported-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
---
 kernel/memremap.c |    6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/kernel/memremap.c b/kernel/memremap.c
index 9ecedc28b928..06123234f118 100644
--- a/kernel/memremap.c
+++ b/kernel/memremap.c
@@ -246,9 +246,13 @@ static void devm_memremap_pages_release(struct device *dev, void *data)
 	/* pages are dead and unused, undo the arch mapping */
 	align_start = res->start & ~(SECTION_SIZE - 1);
 	align_size = ALIGN(resource_size(res), SECTION_SIZE);
+
+	lock_device_hotplug();
 	mem_hotplug_begin();
 	arch_remove_memory(align_start, align_size);
 	mem_hotplug_done();
+	unlock_device_hotplug();
+
 	untrack_pfn(NULL, PHYS_PFN(align_start), align_size);
 	pgmap_radix_release(res);
 	dev_WARN_ONCE(dev, pgmap->altmap && pgmap->altmap->alloc,
@@ -360,9 +364,11 @@ void *devm_memremap_pages(struct device *dev, struct resource *res,
 	if (error)
 		goto err_pfn_remap;
 
+	lock_device_hotplug();
 	mem_hotplug_begin();
 	error = arch_add_memory(nid, align_start, align_size, true);
 	mem_hotplug_done();
+	unlock_device_hotplug();
 	if (error)
 		goto err_add_memory;
 

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[PATCH v2 1/2] mm, devm_memremap_pages: hold device_hotplug lock over mem_hotplug_{begin, done}

[Dan Williams]

The mem_hotplug_{begin,done} lock coordinates with
{get,put}_online_mems() to hold off "readers" of the current state of
memory from new hotplug actions. mem_hotplug_begin() expects exclusive
access, via the device_hotplug lock, to set mem_hotplug.active_writer.
Calling mem_hotplug_begin() without locking device_hotplug can lead to
corrupting mem_hotplug.refcount and missed wakeups / soft lockups.

Cc: <stable@vger.kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Toshi Kani <toshi.kani@hpe.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Logan Gunthorpe <logang@deltatee.com>
Fixes: f931ab479dd2 ("mm: fix devm_memremap_pages crash, use mem_hotplug_{begin, done}")
Reported-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
---
 kernel/memremap.c |    6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/kernel/memremap.c b/kernel/memremap.c
index 9ecedc28b928..06123234f118 100644
--- a/kernel/memremap.c
+++ b/kernel/memremap.c
@@ -246,9 +246,13 @@ static void devm_memremap_pages_release(struct device *dev, void *data)
 	/* pages are dead and unused, undo the arch mapping */
 	align_start = res->start & ~(SECTION_SIZE - 1);
 	align_size = ALIGN(resource_size(res), SECTION_SIZE);
+
+	lock_device_hotplug();
 	mem_hotplug_begin();
 	arch_remove_memory(align_start, align_size);
 	mem_hotplug_done();
+	unlock_device_hotplug();
+
 	untrack_pfn(NULL, PHYS_PFN(align_start), align_size);
 	pgmap_radix_release(res);
 	dev_WARN_ONCE(dev, pgmap->altmap && pgmap->altmap->alloc,
@@ -360,9 +364,11 @@ void *devm_memremap_pages(struct device *dev, struct resource *res,
 	if (error)
 		goto err_pfn_remap;
 
+	lock_device_hotplug();
 	mem_hotplug_begin();
 	error = arch_add_memory(nid, align_start, align_size, true);
 	mem_hotplug_done();
+	unlock_device_hotplug();
 	if (error)
 		goto err_add_memory;
 

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[PATCH v2 2/2] mm: validate device_hotplug is held for memory hotplug

[Dan Williams]

mem_hotplug_begin() assumes that it can set mem_hotplug.active_writer
and run the hotplug process without racing another thread. Validate this
assumption with a lockdep assertion.

Cc: Michal Hocko <mhocko@suse.com>
Cc: Toshi Kani <toshi.kani@hpe.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Logan Gunthorpe <logang@deltatee.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reported-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
---
 drivers/base/core.c    |    5 +++++
 include/linux/device.h |    1 +
 mm/memory_hotplug.c    |    2 ++
 3 files changed, 8 insertions(+)

diff --git a/drivers/base/core.c b/drivers/base/core.c
index 8c25e68e67d7..3050e6f99403 100644
--- a/drivers/base/core.c
+++ b/drivers/base/core.c
@@ -638,6 +638,11 @@ int lock_device_hotplug_sysfs(void)
 	return restart_syscall();
 }
 
+void assert_held_device_hotplug(void)
+{
+	lockdep_assert_held(&device_hotplug_lock);
+}
+
 #ifdef CONFIG_BLOCK
 static inline int device_is_not_partition(struct device *dev)
 {
diff --git a/include/linux/device.h b/include/linux/device.h
index 491b4c0ca633..815965ee55dd 100644
--- a/include/linux/device.h
+++ b/include/linux/device.h
@@ -1135,6 +1135,7 @@ static inline bool device_supports_offline(struct device *dev)
 extern void lock_device_hotplug(void);
 extern void unlock_device_hotplug(void);
 extern int lock_device_hotplug_sysfs(void);
+void assert_held_device_hotplug(void);
 extern int device_offline(struct device *dev);
 extern int device_online(struct device *dev);
 extern void set_primary_fwnode(struct device *dev, struct fwnode_handle *fwnode);
diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index b8c11e063ff0..1635a2a085e5 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -126,6 +126,8 @@ void put_online_mems(void)
 
 void mem_hotplug_begin(void)
 {
+	assert_held_device_hotplug();
+
 	mem_hotplug.active_writer = current;
 
 	memhp_lock_acquire();

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[PATCH v2 2/2] mm: validate device_hotplug is held for memory hotplug

[Dan Williams]

mem_hotplug_begin() assumes that it can set mem_hotplug.active_writer
and run the hotplug process without racing another thread. Validate this
assumption with a lockdep assertion.

Cc: Michal Hocko <mhocko@suse.com>
Cc: Toshi Kani <toshi.kani@hpe.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Logan Gunthorpe <logang@deltatee.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reported-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
---
 drivers/base/core.c    |    5 +++++
 include/linux/device.h |    1 +
 mm/memory_hotplug.c    |    2 ++
 3 files changed, 8 insertions(+)

diff --git a/drivers/base/core.c b/drivers/base/core.c
index 8c25e68e67d7..3050e6f99403 100644
--- a/drivers/base/core.c
+++ b/drivers/base/core.c
@@ -638,6 +638,11 @@ int lock_device_hotplug_sysfs(void)
 	return restart_syscall();
 }
 
+void assert_held_device_hotplug(void)
+{
+	lockdep_assert_held(&device_hotplug_lock);
+}
+
 #ifdef CONFIG_BLOCK
 static inline int device_is_not_partition(struct device *dev)
 {
diff --git a/include/linux/device.h b/include/linux/device.h
index 491b4c0ca633..815965ee55dd 100644
--- a/include/linux/device.h
+++ b/include/linux/device.h
@@ -1135,6 +1135,7 @@ static inline bool device_supports_offline(struct device *dev)
 extern void lock_device_hotplug(void);
 extern void unlock_device_hotplug(void);
 extern int lock_device_hotplug_sysfs(void);
+void assert_held_device_hotplug(void);
 extern int device_offline(struct device *dev);
 extern int device_online(struct device *dev);
 extern void set_primary_fwnode(struct device *dev, struct fwnode_handle *fwnode);
diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index b8c11e063ff0..1635a2a085e5 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -126,6 +126,8 @@ void put_online_mems(void)
 
 void mem_hotplug_begin(void)
 {
+	assert_held_device_hotplug();
+
 	mem_hotplug.active_writer = current;
 
 	memhp_lock_acquire();

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Re: [PATCH v2 2/2] mm: validate device_hotplug is held for memory hotplug

[Ross Zwisler]

On Thu, Feb 16, 2017 at 01:53:58PM -0800, Dan Williams wrote:
> mem_hotplug_begin() assumes that it can set mem_hotplug.active_writer
> and run the hotplug process without racing another thread. Validate this
> assumption with a lockdep assertion.
> 
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Toshi Kani <toshi.kani@hpe.com>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Cc: Logan Gunthorpe <logang@deltatee.com>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Reported-by: Ben Hutchings <ben@decadent.org.uk>
> Signed-off-by: Dan Williams <dan.j.williams@intel.com>
> ---
>  drivers/base/core.c    |    5 +++++
>  include/linux/device.h |    1 +
>  mm/memory_hotplug.c    |    2 ++
>  3 files changed, 8 insertions(+)
> 
> diff --git a/drivers/base/core.c b/drivers/base/core.c
> index 8c25e68e67d7..3050e6f99403 100644
> --- a/drivers/base/core.c
> +++ b/drivers/base/core.c
> @@ -638,6 +638,11 @@ int lock_device_hotplug_sysfs(void)
>  	return restart_syscall();
>  }
>  
> +void assert_held_device_hotplug(void)
> +{
> +	lockdep_assert_held(&device_hotplug_lock);
> +}
> +
>  #ifdef CONFIG_BLOCK
>  static inline int device_is_not_partition(struct device *dev)
>  {
> diff --git a/include/linux/device.h b/include/linux/device.h
> index 491b4c0ca633..815965ee55dd 100644
> --- a/include/linux/device.h
> +++ b/include/linux/device.h
> @@ -1135,6 +1135,7 @@ static inline bool device_supports_offline(struct device *dev)
>  extern void lock_device_hotplug(void);
>  extern void unlock_device_hotplug(void);
>  extern int lock_device_hotplug_sysfs(void);
> +void assert_held_device_hotplug(void);
>  extern int device_offline(struct device *dev);
>  extern int device_online(struct device *dev);
>  extern void set_primary_fwnode(struct device *dev, struct fwnode_handle *fwnode);
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index b8c11e063ff0..1635a2a085e5 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -126,6 +126,8 @@ void put_online_mems(void)
>  
>  void mem_hotplug_begin(void)
>  {
> +	assert_held_device_hotplug();

What's the benefit to defining assert_held_device_hotplug() as a one line
wrapper, instead of just calling lockdep_assert_held(&device_hotplug_lock)
directly?

> +
>  	mem_hotplug.active_writer = current;
>  
>  	memhp_lock_acquire();
> 
> _______________________________________________
> Linux-nvdimm mailing list
> Linux-nvdimm@lists.01.org
> https://lists.01.org/mailman/listinfo/linux-nvdimm
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Re: [PATCH v2 2/2] mm: validate device_hotplug is held for memory hotplug

[Ross Zwisler]

On Thu, Feb 16, 2017 at 01:53:58PM -0800, Dan Williams wrote:
> mem_hotplug_begin() assumes that it can set mem_hotplug.active_writer
> and run the hotplug process without racing another thread. Validate this
> assumption with a lockdep assertion.
> 
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Toshi Kani <toshi.kani@hpe.com>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Cc: Logan Gunthorpe <logang@deltatee.com>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Reported-by: Ben Hutchings <ben@decadent.org.uk>
> Signed-off-by: Dan Williams <dan.j.williams@intel.com>
> ---
>  drivers/base/core.c    |    5 +++++
>  include/linux/device.h |    1 +
>  mm/memory_hotplug.c    |    2 ++
>  3 files changed, 8 insertions(+)
> 
> diff --git a/drivers/base/core.c b/drivers/base/core.c
> index 8c25e68e67d7..3050e6f99403 100644
> --- a/drivers/base/core.c
> +++ b/drivers/base/core.c
> @@ -638,6 +638,11 @@ int lock_device_hotplug_sysfs(void)
>  	return restart_syscall();
>  }
>  
> +void assert_held_device_hotplug(void)
> +{
> +	lockdep_assert_held(&device_hotplug_lock);
> +}
> +
>  #ifdef CONFIG_BLOCK
>  static inline int device_is_not_partition(struct device *dev)
>  {
> diff --git a/include/linux/device.h b/include/linux/device.h
> index 491b4c0ca633..815965ee55dd 100644
> --- a/include/linux/device.h
> +++ b/include/linux/device.h
> @@ -1135,6 +1135,7 @@ static inline bool device_supports_offline(struct device *dev)
>  extern void lock_device_hotplug(void);
>  extern void unlock_device_hotplug(void);
>  extern int lock_device_hotplug_sysfs(void);
> +void assert_held_device_hotplug(void);
>  extern int device_offline(struct device *dev);
>  extern int device_online(struct device *dev);
>  extern void set_primary_fwnode(struct device *dev, struct fwnode_handle *fwnode);
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index b8c11e063ff0..1635a2a085e5 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -126,6 +126,8 @@ void put_online_mems(void)
>  
>  void mem_hotplug_begin(void)
>  {
> +	assert_held_device_hotplug();

What's the benefit to defining assert_held_device_hotplug() as a one line
wrapper, instead of just calling lockdep_assert_held(&device_hotplug_lock)
directly?

> +
>  	mem_hotplug.active_writer = current;
>  
>  	memhp_lock_acquire();
> 
> _______________________________________________
> Linux-nvdimm mailing list
> Linux-nvdimm@lists.01.org
> https://lists.01.org/mailman/listinfo/linux-nvdimm

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Re: [PATCH v2 2/2] mm: validate device_hotplug is held for memory hotplug

[Dan Williams]

On Thu, Feb 16, 2017 at 7:08 PM, Ross Zwisler
<ross.zwisler@linux.intel.com> wrote:
> On Thu, Feb 16, 2017 at 01:53:58PM -0800, Dan Williams wrote:
>> mem_hotplug_begin() assumes that it can set mem_hotplug.active_writer
>> and run the hotplug process without racing another thread. Validate this
>> assumption with a lockdep assertion.
>>
>> Cc: Michal Hocko <mhocko@suse.com>
>> Cc: Toshi Kani <toshi.kani@hpe.com>
>> Cc: Vlastimil Babka <vbabka@suse.cz>
>> Cc: Logan Gunthorpe <logang@deltatee.com>
>> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
>> Reported-by: Ben Hutchings <ben@decadent.org.uk>
>> Signed-off-by: Dan Williams <dan.j.williams@intel.com>
>> ---
>>  drivers/base/core.c    |    5 +++++
>>  include/linux/device.h |    1 +
>>  mm/memory_hotplug.c    |    2 ++
>>  3 files changed, 8 insertions(+)
>>
>> diff --git a/drivers/base/core.c b/drivers/base/core.c
>> index 8c25e68e67d7..3050e6f99403 100644
>> --- a/drivers/base/core.c
>> +++ b/drivers/base/core.c
>> @@ -638,6 +638,11 @@ int lock_device_hotplug_sysfs(void)
>>       return restart_syscall();
>>  }
>>
>> +void assert_held_device_hotplug(void)
>> +{
>> +     lockdep_assert_held(&device_hotplug_lock);
>> +}
>> +
>>  #ifdef CONFIG_BLOCK
>>  static inline int device_is_not_partition(struct device *dev)
>>  {
>> diff --git a/include/linux/device.h b/include/linux/device.h
>> index 491b4c0ca633..815965ee55dd 100644
>> --- a/include/linux/device.h
>> +++ b/include/linux/device.h
>> @@ -1135,6 +1135,7 @@ static inline bool device_supports_offline(struct device *dev)
>>  extern void lock_device_hotplug(void);
>>  extern void unlock_device_hotplug(void);
>>  extern int lock_device_hotplug_sysfs(void);
>> +void assert_held_device_hotplug(void);
>>  extern int device_offline(struct device *dev);
>>  extern int device_online(struct device *dev);
>>  extern void set_primary_fwnode(struct device *dev, struct fwnode_handle *fwnode);
>> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
>> index b8c11e063ff0..1635a2a085e5 100644
>> --- a/mm/memory_hotplug.c
>> +++ b/mm/memory_hotplug.c
>> @@ -126,6 +126,8 @@ void put_online_mems(void)
>>
>>  void mem_hotplug_begin(void)
>>  {
>> +     assert_held_device_hotplug();
>
> What's the benefit to defining assert_held_device_hotplug() as a one line
> wrapper, instead of just calling lockdep_assert_held(&device_hotplug_lock)
> directly?
>

This allows us to keep device_hotplug_lock statically defined and an
internal implementation detail of the device core.
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Re: [PATCH v2 2/2] mm: validate device_hotplug is held for memory hotplug

[Dan Williams]

On Thu, Feb 16, 2017 at 7:08 PM, Ross Zwisler
<ross.zwisler@linux.intel.com> wrote:
> On Thu, Feb 16, 2017 at 01:53:58PM -0800, Dan Williams wrote:
>> mem_hotplug_begin() assumes that it can set mem_hotplug.active_writer
>> and run the hotplug process without racing another thread. Validate this
>> assumption with a lockdep assertion.
>>
>> Cc: Michal Hocko <mhocko@suse.com>
>> Cc: Toshi Kani <toshi.kani@hpe.com>
>> Cc: Vlastimil Babka <vbabka@suse.cz>
>> Cc: Logan Gunthorpe <logang@deltatee.com>
>> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
>> Reported-by: Ben Hutchings <ben@decadent.org.uk>
>> Signed-off-by: Dan Williams <dan.j.williams@intel.com>
>> ---
>>  drivers/base/core.c    |    5 +++++
>>  include/linux/device.h |    1 +
>>  mm/memory_hotplug.c    |    2 ++
>>  3 files changed, 8 insertions(+)
>>
>> diff --git a/drivers/base/core.c b/drivers/base/core.c
>> index 8c25e68e67d7..3050e6f99403 100644
>> --- a/drivers/base/core.c
>> +++ b/drivers/base/core.c
>> @@ -638,6 +638,11 @@ int lock_device_hotplug_sysfs(void)
>>       return restart_syscall();
>>  }
>>
>> +void assert_held_device_hotplug(void)
>> +{
>> +     lockdep_assert_held(&device_hotplug_lock);
>> +}
>> +
>>  #ifdef CONFIG_BLOCK
>>  static inline int device_is_not_partition(struct device *dev)
>>  {
>> diff --git a/include/linux/device.h b/include/linux/device.h
>> index 491b4c0ca633..815965ee55dd 100644
>> --- a/include/linux/device.h
>> +++ b/include/linux/device.h
>> @@ -1135,6 +1135,7 @@ static inline bool device_supports_offline(struct device *dev)
>>  extern void lock_device_hotplug(void);
>>  extern void unlock_device_hotplug(void);
>>  extern int lock_device_hotplug_sysfs(void);
>> +void assert_held_device_hotplug(void);
>>  extern int device_offline(struct device *dev);
>>  extern int device_online(struct device *dev);
>>  extern void set_primary_fwnode(struct device *dev, struct fwnode_handle *fwnode);
>> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
>> index b8c11e063ff0..1635a2a085e5 100644
>> --- a/mm/memory_hotplug.c
>> +++ b/mm/memory_hotplug.c
>> @@ -126,6 +126,8 @@ void put_online_mems(void)
>>
>>  void mem_hotplug_begin(void)
>>  {
>> +     assert_held_device_hotplug();
>
> What's the benefit to defining assert_held_device_hotplug() as a one line
> wrapper, instead of just calling lockdep_assert_held(&device_hotplug_lock)
> directly?
>

This allows us to keep device_hotplug_lock statically defined and an
internal implementation detail of the device core.

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