Commit 0edce197 authored by Amy Griffis's avatar Amy Griffis Committed by Al Viro

[PATCH] inotify (5/5): update kernel documentation

Update kernel documentation to include a description of the inotify
kernel API.
Signed-off-by: default avatarAmy Griffis <>
Acked-by: default avatarRobert Love <>
Acked-by: default avatarJohn McCutchan <>
Signed-off-by: default avatarAl Viro <>
parent 3ca10067
......@@ -69,17 +69,135 @@ Prototypes:
int inotify_rm_watch (int fd, __u32 mask);
(iii) Internal Kernel Implementation
(iii) Kernel Interface
Each inotify instance is associated with an inotify_device structure.
Inotify's kernel API consists a set of functions for managing watches and an
event callback.
To use the kernel API, you must first initialize an inotify instance with a set
of inotify_operations. You are given an opaque inotify_handle, which you use
for any further calls to inotify.
struct inotify_handle *ih = inotify_init(my_event_handler);
You must provide a function for processing events and a function for destroying
the inotify watch.
void handle_event(struct inotify_watch *watch, u32 wd, u32 mask,
u32 cookie, const char *name, struct inode *inode)
watch - the pointer to the inotify_watch that triggered this call
wd - the watch descriptor
mask - describes the event that occurred
cookie - an identifier for synchronizing events
name - the dentry name for affected files in a directory-based event
inode - the affected inode in a directory-based event
void destroy_watch(struct inotify_watch *watch)
You may add watches by providing a pre-allocated and initialized inotify_watch
structure and specifying the inode to watch along with an inotify event mask.
You must pin the inode during the call. You will likely wish to embed the
inotify_watch structure in a structure of your own which contains other
information about the watch. Once you add an inotify watch, it is immediately
subject to removal depending on filesystem events. You must grab a reference if
you depend on the watch hanging around after the call.
inotify_get_watch(&my_watch->iwatch); // optional
s32 wd = inotify_add_watch(ih, &my_watch->iwatch, inode, mask);
inotify_put_watch(&my_watch->iwatch); // optional
You may use the watch descriptor (wd) or the address of the inotify_watch for
other inotify operations. You must not directly read or manipulate data in the
inotify_watch. Additionally, you must not call inotify_add_watch() more than
once for a given inotify_watch structure, unless you have first called either
inotify_rm_watch() or inotify_rm_wd().
To determine if you have already registered a watch for a given inode, you may
call inotify_find_watch(), which gives you both the wd and the watch pointer for
the inotify_watch, or an error if the watch does not exist.
wd = inotify_find_watch(ih, inode, &watchp);
You may use container_of() on the watch pointer to access your own data
associated with a given watch. When an existing watch is found,
inotify_find_watch() bumps the refcount before releasing its locks. You must
put that reference with:
Call inotify_find_update_watch() to update the event mask for an existing watch.
inotify_find_update_watch() returns the wd of the updated watch, or an error if
the watch does not exist.
wd = inotify_find_update_watch(ih, inode, mask);
An existing watch may be removed by calling either inotify_rm_watch() or
int ret = inotify_rm_watch(ih, &my_watch->iwatch);
int ret = inotify_rm_wd(ih, wd);
A watch may be removed while executing your event handler with the following:
inotify_remove_watch_locked(ih, iwatch);
Call inotify_destroy() to remove all watches from your inotify instance and
release it. If there are no outstanding references, inotify_destroy() will call
your destroy_watch op for each watch.
When inotify removes a watch, it sends an IN_IGNORED event to your callback.
You may use this event as an indication to free the watch memory. Note that
inotify may remove a watch due to filesystem events, as well as by your request.
If you use IN_ONESHOT, inotify will remove the watch after the first event, at
which point you may call the final inotify_put_watch.
(iv) Kernel Interface Prototypes
struct inotify_handle *inotify_init(struct inotify_operations *ops);
inotify_init_watch(struct inotify_watch *watch);
s32 inotify_add_watch(struct inotify_handle *ih,
struct inotify_watch *watch,
struct inode *inode, u32 mask);
s32 inotify_find_watch(struct inotify_handle *ih, struct inode *inode,
struct inotify_watch **watchp);
s32 inotify_find_update_watch(struct inotify_handle *ih,
struct inode *inode, u32 mask);
int inotify_rm_wd(struct inotify_handle *ih, u32 wd);
int inotify_rm_watch(struct inotify_handle *ih,
struct inotify_watch *watch);
void inotify_remove_watch_locked(struct inotify_handle *ih,
struct inotify_watch *watch);
void inotify_destroy(struct inotify_handle *ih);
void get_inotify_watch(struct inotify_watch *watch);
void put_inotify_watch(struct inotify_watch *watch);
(v) Internal Kernel Implementation
Each inotify instance is represented by an inotify_handle structure.
Inotify's userspace consumers also have an inotify_device which is
associated with the inotify_handle, and on which events are queued.
Each watch is associated with an inotify_watch structure. Watches are chained
off of each associated device and each associated inode.
off of each associated inotify_handle and each associated inode.
See fs/inotify.c for the locking and lifetime rules.
See fs/inotify.c and fs/inotify_user.c for the locking and lifetime rules.
(iv) Rationale
(vi) Rationale
Q: What is the design decision behind not tying the watch to the open fd of
the watched object?
......@@ -145,7 +263,7 @@ A: The poor user-space interface is the second biggest problem with dnotify.
file descriptor-based one that allows basic file I/O and poll/select.
Obtaining the fd and managing the watches could have been done either via a
device file or a family of new system calls. We decided to implement a
family of system calls because that is the preffered approach for new kernel
family of system calls because that is the preferred approach for new kernel
interfaces. The only real difference was whether we wanted to use open(2)
and ioctl(2) or a couple of new system calls. System calls beat ioctls.
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