Documentation: move watch_queue to core-api

Move watch_queue documentation to the core-api index and
subdirectory.

Fixes: c73be61cede5 ("pipe: Add general notification queue support")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: David Howells <dhowells@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

1 files changed, 0 insertions, 343 deletions

diff --git a/Documentation/watch_queue.rst b/Documentation/watch_queue.rst
deleted file mode 100644
index 54f13ad5fc17..000000000000
--- a/Documentation/watch_queue.rst
+++ /dev/null
@@ -1,343 +0,0 @@
-==============================
-General notification mechanism
-==============================
-
-The general notification mechanism is built on top of the standard pipe driver
-whereby it effectively splices notification messages from the kernel into pipes
-opened by userspace.  This can be used in conjunction with::
-
-  * Key/keyring notifications
-
-
-The notifications buffers can be enabled by:
-
-	"General setup"/"General notification queue"
-	(CONFIG_WATCH_QUEUE)
-
-This document has the following sections:
-
-.. contents:: :local:
-
-
-Overview
-========
-
-This facility appears as a pipe that is opened in a special mode.  The pipe's
-internal ring buffer is used to hold messages that are generated by the kernel.
-These messages are then read out by read().  Splice and similar are disabled on
-such pipes due to them wanting to, under some circumstances, revert their
-additions to the ring - which might end up interleaved with notification
-messages.
-
-The owner of the pipe has to tell the kernel which sources it would like to
-watch through that pipe.  Only sources that have been connected to a pipe will
-insert messages into it.  Note that a source may be bound to multiple pipes and
-insert messages into all of them simultaneously.
-
-Filters may also be emplaced on a pipe so that certain source types and
-subevents can be ignored if they're not of interest.
-
-A message will be discarded if there isn't a slot available in the ring or if
-no preallocated message buffer is available.  In both of these cases, read()
-will insert a WATCH_META_LOSS_NOTIFICATION message into the output buffer after
-the last message currently in the buffer has been read.
-
-Note that when producing a notification, the kernel does not wait for the
-consumers to collect it, but rather just continues on.  This means that
-notifications can be generated whilst spinlocks are held and also protects the
-kernel from being held up indefinitely by a userspace malfunction.
-
-
-Message Structure
-=================
-
-Notification messages begin with a short header::
-
-	struct watch_notification {
-		__u32	type:24;
-		__u32	subtype:8;
-		__u32	info;
-	};
-
-"type" indicates the source of the notification record and "subtype" indicates
-the type of record from that source (see the Watch Sources section below).  The
-type may also be "WATCH_TYPE_META".  This is a special record type generated
-internally by the watch queue itself.  There are two subtypes:
-
-  * WATCH_META_REMOVAL_NOTIFICATION
-  * WATCH_META_LOSS_NOTIFICATION
-
-The first indicates that an object on which a watch was installed was removed
-or destroyed and the second indicates that some messages have been lost.
-
-"info" indicates a bunch of things, including:
-
-  * The length of the message in bytes, including the header (mask with
-    WATCH_INFO_LENGTH and shift by WATCH_INFO_LENGTH__SHIFT).  This indicates
-    the size of the record, which may be between 8 and 127 bytes.
-
-  * The watch ID (mask with WATCH_INFO_ID and shift by WATCH_INFO_ID__SHIFT).
-    This indicates that caller's ID of the watch, which may be between 0
-    and 255.  Multiple watches may share a queue, and this provides a means to
-    distinguish them.
-
-  * A type-specific field (WATCH_INFO_TYPE_INFO).  This is set by the
-    notification producer to indicate some meaning specific to the type and
-    subtype.
-
-Everything in info apart from the length can be used for filtering.
-
-The header can be followed by supplementary information.  The format of this is
-at the discretion is defined by the type and subtype.
-
-
-Watch List (Notification Source) API
-====================================
-
-A "watch list" is a list of watchers that are subscribed to a source of
-notifications.  A list may be attached to an object (say a key or a superblock)
-or may be global (say for device events).  From a userspace perspective, a
-non-global watch list is typically referred to by reference to the object it
-belongs to (such as using KEYCTL_NOTIFY and giving it a key serial number to
-watch that specific key).
-
-To manage a watch list, the following functions are provided:
-
-  * ::
-
-	void init_watch_list(struct watch_list *wlist,
-			     void (*release_watch)(struct watch *wlist));
-
-    Initialise a watch list.  If ``release_watch`` is not NULL, then this
-    indicates a function that should be called when the watch_list object is
-    destroyed to discard any references the watch list holds on the watched
-    object.
-
-  * ``void remove_watch_list(struct watch_list *wlist);``
-
-    This removes all of the watches subscribed to a watch_list and frees them
-    and then destroys the watch_list object itself.
-
-
-Watch Queue (Notification Output) API
-=====================================
-
-A "watch queue" is the buffer allocated by an application that notification
-records will be written into.  The workings of this are hidden entirely inside
-of the pipe device driver, but it is necessary to gain a reference to it to set
-a watch.  These can be managed with:
-
-  * ``struct watch_queue *get_watch_queue(int fd);``
-
-    Since watch queues are indicated to the kernel by the fd of the pipe that
-    implements the buffer, userspace must hand that fd through a system call.
-    This can be used to look up an opaque pointer to the watch queue from the
-    system call.
-
-  * ``void put_watch_queue(struct watch_queue *wqueue);``
-
-    This discards the reference obtained from ``get_watch_queue()``.
-
-
-Watch Subscription API
-======================
-
-A "watch" is a subscription on a watch list, indicating the watch queue, and
-thus the buffer, into which notification records should be written.  The watch
-queue object may also carry filtering rules for that object, as set by
-userspace.  Some parts of the watch struct can be set by the driver::
-
-	struct watch {
-		union {
-			u32		info_id;	/* ID to be OR'd in to info field */
-			...
-		};
-		void			*private;	/* Private data for the watched object */
-		u64			id;		/* Internal identifier */
-		...
-	};
-
-The ``info_id`` value should be an 8-bit number obtained from userspace and
-shifted by WATCH_INFO_ID__SHIFT.  This is OR'd into the WATCH_INFO_ID field of
-struct watch_notification::info when and if the notification is written into
-the associated watch queue buffer.
-
-The ``private`` field is the driver's data associated with the watch_list and
-is cleaned up by the ``watch_list::release_watch()`` method.
-
-The ``id`` field is the source's ID.  Notifications that are posted with a
-different ID are ignored.
-
-The following functions are provided to manage watches:
-
-  * ``void init_watch(struct watch *watch, struct watch_queue *wqueue);``
-
-    Initialise a watch object, setting its pointer to the watch queue, using
-    appropriate barriering to avoid lockdep complaints.
-
-  * ``int add_watch_to_object(struct watch *watch, struct watch_list *wlist);``
-
-    Subscribe a watch to a watch list (notification source).  The
-    driver-settable fields in the watch struct must have been set before this
-    is called.
-
-  * ::
-
-	int remove_watch_from_object(struct watch_list *wlist,
-				     struct watch_queue *wqueue,
-				     u64 id, false);
-
-    Remove a watch from a watch list, where the watch must match the specified
-    watch queue (``wqueue``) and object identifier (``id``).  A notification
-    (``WATCH_META_REMOVAL_NOTIFICATION``) is sent to the watch queue to
-    indicate that the watch got removed.
-
-  * ``int remove_watch_from_object(struct watch_list *wlist, NULL, 0, true);``
-
-    Remove all the watches from a watch list.  It is expected that this will be
-    called preparatory to destruction and that the watch list will be
-    inaccessible to new watches by this point.  A notification
-    (``WATCH_META_REMOVAL_NOTIFICATION``) is sent to the watch queue of each
-    subscribed watch to indicate that the watch got removed.
-
-
-Notification Posting API
-========================
-
-To post a notification to watch list so that the subscribed watches can see it,
-the following function should be used::
-
-	void post_watch_notification(struct watch_list *wlist,
-				     struct watch_notification *n,
-				     const struct cred *cred,
-				     u64 id);
-
-The notification should be preformatted and a pointer to the header (``n``)
-should be passed in.  The notification may be larger than this and the size in
-units of buffer slots is noted in ``n->info & WATCH_INFO_LENGTH``.
-
-The ``cred`` struct indicates the credentials of the source (subject) and is
-passed to the LSMs, such as SELinux, to allow or suppress the recording of the
-note in each individual queue according to the credentials of that queue
-(object).
-
-The ``id`` is the ID of the source object (such as the serial number on a key).
-Only watches that have the same ID set in them will see this notification.
-
-
-Watch Sources
-=============
-
-Any particular buffer can be fed from multiple sources.  Sources include:
-
-  * WATCH_TYPE_KEY_NOTIFY
-
-    Notifications of this type indicate changes to keys and keyrings, including
-    the changes of keyring contents or the attributes of keys.
-
-    See Documentation/security/keys/core.rst for more information.
-
-
-Event Filtering
-===============
-
-Once a watch queue has been created, a set of filters can be applied to limit
-the events that are received using::
-
-	struct watch_notification_filter filter = {
-		...
-	};
-	ioctl(fd, IOC_WATCH_QUEUE_SET_FILTER, &filter)
-
-The filter description is a variable of type::
-
-	struct watch_notification_filter {
-		__u32	nr_filters;
-		__u32	__reserved;
-		struct watch_notification_type_filter filters[];
-	};
-
-Where "nr_filters" is the number of filters in filters[] and "__reserved"
-should be 0.  The "filters" array has elements of the following type::
-
-	struct watch_notification_type_filter {
-		__u32	type;
-		__u32	info_filter;
-		__u32	info_mask;
-		__u32	subtype_filter[8];
-	};
-
-Where:
-
-  * ``type`` is the event type to filter for and should be something like
-    "WATCH_TYPE_KEY_NOTIFY"
-
-  * ``info_filter`` and ``info_mask`` act as a filter on the info field of the
-    notification record.  The notification is only written into the buffer if::
-
-	(watch.info & info_mask) == info_filter
-
-    This could be used, for example, to ignore events that are not exactly on
-    the watched point in a mount tree.
-
-  * ``subtype_filter`` is a bitmask indicating the subtypes that are of
-    interest.  Bit 0 of subtype_filter[0] corresponds to subtype 0, bit 1 to
-    subtype 1, and so on.
-
-If the argument to the ioctl() is NULL, then the filters will be removed and
-all events from the watched sources will come through.
-
-
-Userspace Code Example
-======================
-
-A buffer is created with something like the following::
-
-	pipe2(fds, O_TMPFILE);
-	ioctl(fds[1], IOC_WATCH_QUEUE_SET_SIZE, 256);
-
-It can then be set to receive keyring change notifications::
-
-	keyctl(KEYCTL_WATCH_KEY, KEY_SPEC_SESSION_KEYRING, fds[1], 0x01);
-
-The notifications can then be consumed by something like the following::
-
-	static void consumer(int rfd, struct watch_queue_buffer *buf)
-	{
-		unsigned char buffer[128];
-		ssize_t buf_len;
-
-		while (buf_len = read(rfd, buffer, sizeof(buffer)),
-		       buf_len > 0
-		       ) {
-			void *p = buffer;
-			void *end = buffer + buf_len;
-			while (p < end) {
-				union {
-					struct watch_notification n;
-					unsigned char buf1[128];
-				} n;
-				size_t largest, len;
-
-				largest = end - p;
-				if (largest > 128)
-					largest = 128;
-				memcpy(&n, p, largest);
-
-				len = (n->info & WATCH_INFO_LENGTH) >>
-					WATCH_INFO_LENGTH__SHIFT;
-				if (len == 0 || len > largest)
-					return;
-
-				switch (n.n.type) {
-				case WATCH_TYPE_META:
-					got_meta(&n.n);
-				case WATCH_TYPE_KEY_NOTIFY:
-					saw_key_change(&n.n);
-					break;
-				}
-
-				p += len;
-			}
-		}
-	}