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+================
+Shadow Variables
+================
+
+Shadow variables are a simple way for livepatch modules to associate
+additional "shadow" data with existing data structures.  Shadow data is
+allocated separately from parent data structures, which are left
+unmodified.  The shadow variable API described in this document is used
+to allocate/add and remove/free shadow variables to/from their parents.
+
+The implementation introduces a global, in-kernel hashtable that
+associates pointers to parent objects and a numeric identifier of the
+shadow data.  The numeric identifier is a simple enumeration that may be
+used to describe shadow variable version, class or type, etc.  More
+specifically, the parent pointer serves as the hashtable key while the
+numeric id subsequently filters hashtable queries.  Multiple shadow
+variables may attach to the same parent object, but their numeric
+identifier distinguishes between them.
+
+
+1. Brief API summary
+====================
+
+(See the full API usage docbook notes in livepatch/shadow.c.)
+
+A hashtable references all shadow variables.  These references are
+stored and retrieved through a <obj, id> pair.
+
+* The klp_shadow variable data structure encapsulates both tracking
+meta-data and shadow-data:
+  - meta-data
+    - obj - pointer to parent object
+    - id - data identifier
+  - data[] - storage for shadow data
+
+It is important to note that the klp_shadow_alloc() and
+klp_shadow_get_or_alloc() are zeroing the variable by default.
+They also allow to call a custom constructor function when a non-zero
+value is needed. Callers should provide whatever mutual exclusion
+is required.
+
+Note that the constructor is called under klp_shadow_lock spinlock. It allows
+to do actions that can be done only once when a new variable is allocated.
+
+* klp_shadow_get() - retrieve a shadow variable data pointer
+  - search hashtable for <obj, id> pair
+
+* klp_shadow_alloc() - allocate and add a new shadow variable
+  - search hashtable for <obj, id> pair
+  - if exists
+    - WARN and return NULL
+  - if <obj, id> doesn't already exist
+    - allocate a new shadow variable
+    - initialize the variable using a custom constructor and data when provided
+    - add <obj, id> to the global hashtable
+
+* klp_shadow_get_or_alloc() - get existing or alloc a new shadow variable
+  - search hashtable for <obj, id> pair
+  - if exists
+    - return existing shadow variable
+  - if <obj, id> doesn't already exist
+    - allocate a new shadow variable
+    - initialize the variable using a custom constructor and data when provided
+    - add <obj, id> pair to the global hashtable
+
+* klp_shadow_free() - detach and free a <obj, id> shadow variable
+  - find and remove a <obj, id> reference from global hashtable
+    - if found
+      - call destructor function if defined
+      - free shadow variable
+
+* klp_shadow_free_all() - detach and free all <*, id> shadow variables
+  - find and remove any <*, id> references from global hashtable
+    - if found
+      - call destructor function if defined
+      - free shadow variable
+
+
+2. Use cases
+============
+
+(See the example shadow variable livepatch modules in samples/livepatch/
+for full working demonstrations.)
+
+For the following use-case examples, consider commit 1d147bfa6429
+("mac80211: fix AP powersave TX vs.  wakeup race"), which added a
+spinlock to net/mac80211/sta_info.h :: struct sta_info.  Each use-case
+example can be considered a stand-alone livepatch implementation of this
+fix.
+
+
+Matching parent's lifecycle
+---------------------------
+
+If parent data structures are frequently created and destroyed, it may
+be easiest to align their shadow variables lifetimes to the same
+allocation and release functions.  In this case, the parent data
+structure is typically allocated, initialized, then registered in some
+manner.  Shadow variable allocation and setup can then be considered
+part of the parent's initialization and should be completed before the
+parent "goes live" (ie, any shadow variable get-API requests are made
+for this <obj, id> pair.)
+
+For commit 1d147bfa6429, when a parent sta_info structure is allocated,
+allocate a shadow copy of the ps_lock pointer, then initialize it:
+
+#define PS_LOCK 1
+struct sta_info *sta_info_alloc(struct ieee80211_sub_if_data *sdata,
+				const u8 *addr, gfp_t gfp)
+{
+	struct sta_info *sta;
+	spinlock_t *ps_lock;
+
+	/* Parent structure is created */
+	sta = kzalloc(sizeof(*sta) + hw->sta_data_size, gfp);
+
+	/* Attach a corresponding shadow variable, then initialize it */
+	ps_lock = klp_shadow_alloc(sta, PS_LOCK, sizeof(*ps_lock), gfp,
+				   NULL, NULL);
+	if (!ps_lock)
+		goto shadow_fail;
+	spin_lock_init(ps_lock);
+	...
+
+When requiring a ps_lock, query the shadow variable API to retrieve one
+for a specific struct sta_info:
+
+void ieee80211_sta_ps_deliver_wakeup(struct sta_info *sta)
+{
+	spinlock_t *ps_lock;
+
+	/* sync with ieee80211_tx_h_unicast_ps_buf */
+	ps_lock = klp_shadow_get(sta, PS_LOCK);
+	if (ps_lock)
+		spin_lock(ps_lock);
+	...
+
+When the parent sta_info structure is freed, first free the shadow
+variable:
+
+void sta_info_free(struct ieee80211_local *local, struct sta_info *sta)
+{
+	klp_shadow_free(sta, PS_LOCK, NULL);
+	kfree(sta);
+	...
+
+
+In-flight parent objects
+------------------------
+
+Sometimes it may not be convenient or possible to allocate shadow
+variables alongside their parent objects.  Or a livepatch fix may
+require shadow varibles to only a subset of parent object instances.  In
+these cases, the klp_shadow_get_or_alloc() call can be used to attach
+shadow variables to parents already in-flight.
+
+For commit 1d147bfa6429, a good spot to allocate a shadow spinlock is
+inside ieee80211_sta_ps_deliver_wakeup():
+
+int ps_lock_shadow_ctor(void *obj, void *shadow_data, void *ctor_data)
+{
+	spinlock_t *lock = shadow_data;
+
+	spin_lock_init(lock);
+	return 0;
+}
+
+#define PS_LOCK 1
+void ieee80211_sta_ps_deliver_wakeup(struct sta_info *sta)
+{
+	spinlock_t *ps_lock;
+
+	/* sync with ieee80211_tx_h_unicast_ps_buf */
+	ps_lock = klp_shadow_get_or_alloc(sta, PS_LOCK,
+			sizeof(*ps_lock), GFP_ATOMIC,
+			ps_lock_shadow_ctor, NULL);
+
+	if (ps_lock)
+		spin_lock(ps_lock);
+	...
+
+This usage will create a shadow variable, only if needed, otherwise it
+will use one that was already created for this <obj, id> pair.
+
+Like the previous use-case, the shadow spinlock needs to be cleaned up.
+A shadow variable can be freed just before its parent object is freed,
+or even when the shadow variable itself is no longer required.
+
+
+Other use-cases
+---------------
+
+Shadow variables can also be used as a flag indicating that a data
+structure was allocated by new, livepatched code.  In this case, it
+doesn't matter what data value the shadow variable holds, its existence
+suggests how to handle the parent object.
+
+
+3. References
+=============
+
+* https://github.com/dynup/kpatch
+The livepatch implementation is based on the kpatch version of shadow
+variables.
+
+* http://files.mkgnu.net/files/dynamos/doc/papers/dynamos_eurosys_07.pdf
+Dynamic and Adaptive Updates of Non-Quiescent Subsystems in Commodity
+Operating System Kernels (Kritis Makris, Kyung Dong Ryu 2007) presented
+a datatype update technique called "shadow data structures".