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From:   David Hildenbrand <david@redhat.com>
To:     linux-kernel@vger.kernel.org
Cc:     linux-mm@kvack.org, David Hildenbrand <david@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Matthew Wilcox <willy@infradead.org>,
        Michal Hocko <mhocko@suse.com>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Mike Rapoport <rppt@kernel.org>,
        Muchun Song <songmuchun@bytedance.com>,
        Oscar Salvador <osalvador@suse.de>,
        Pavel Tatashin <pasha.tatashin@soleen.com>,
        Stephen Rothwell <sfr@canb.auug.org.au>,
        linux-doc@vger.kernel.org
Subject: [PATCH v2 1/2] memory-hotplug.rst: remove locking details from admin-guide
Date:   Tue,  8 Jun 2021 15:38:54 +0200
Message-Id: <20210608133855.20397-2-david@redhat.com>
In-Reply-To: <20210608133855.20397-1-david@redhat.com>
References: <20210608133855.20397-1-david@redhat.com>
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We have the same content at Documentation/core-api/memory-hotplug.rst
and it doesn't fit into the admin-guide. The documentation was
accidentially duplicated when merging.

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Muchun Song <songmuchun@bytedance.com>
Cc: Pavel Tatashin <pasha.tatashin@soleen.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: linux-doc@vger.kernel.org
Signed-off-by: David Hildenbrand <david@redhat.com>
---
 .../admin-guide/mm/memory-hotplug.rst         | 39 -------------------
 1 file changed, 39 deletions(-)

diff --git a/Documentation/admin-guide/mm/memory-hotplug.rst b/Documentation/admin-guide/mm/memory-hotplug.rst
index c6bae2d77160..a783cf7c8e4c 100644
--- a/Documentation/admin-guide/mm/memory-hotplug.rst
+++ b/Documentation/admin-guide/mm/memory-hotplug.rst
@@ -415,45 +415,6 @@ Need more implementation yet....
  - Guard from remove if not yet.
 
 
-Locking Internals
-=================
-
-When adding/removing memory that uses memory block devices (i.e. ordinary RAM),
-the device_hotplug_lock should be held to:
-
-- synchronize against online/offline requests (e.g. via sysfs). This way, memory
-  block devices can only be accessed (.online/.state attributes) by user
-  space once memory has been fully added. And when removing memory, we
-  know nobody is in critical sections.
-- synchronize against CPU hotplug and similar (e.g. relevant for ACPI and PPC)
-
-Especially, there is a possible lock inversion that is avoided using
-device_hotplug_lock when adding memory and user space tries to online that
-memory faster than expected:
-
-- device_online() will first take the device_lock(), followed by
-  mem_hotplug_lock
-- add_memory_resource() will first take the mem_hotplug_lock, followed by
-  the device_lock() (while creating the devices, during bus_add_device()).
-
-As the device is visible to user space before taking the device_lock(), this
-can result in a lock inversion.
-
-onlining/offlining of memory should be done via device_online()/
-device_offline() - to make sure it is properly synchronized to actions
-via sysfs. Holding device_hotplug_lock is advised (to e.g. protect online_type)
-
-When adding/removing/onlining/offlining memory or adding/removing
-heterogeneous/device memory, we should always hold the mem_hotplug_lock in
-write mode to serialise memory hotplug (e.g. access to global/zone
-variables).
-
-In addition, mem_hotplug_lock (in contrast to device_hotplug_lock) in read
-mode allows for a quite efficient get_online_mems/put_online_mems
-implementation, so code accessing memory can protect from that memory
-vanishing.
-
-
 Future Work
 ===========
 
-- 
2.31.1