OSD(9) FreeBSD Kernel Developer's Manual OSD(9)
NAME
osd, osd_register, osd_deregister, osd_set, osd_reserve,
osd_set_reserved, osd_free_reserved, osd_get, osd_del, osd_call, osd_exit
- Object Specific Data
SYNOPSIS
#include <sys/osd.h>
typedef void
(*osd_destructor_t)(void *value);)(void *value);
typedef int
(*osd_method_t)(void *obj, void *data);)(void *obj, void *data);
int
osd_register(u_int type, osd_destructor_t destructor,
osd_method_t *methods);
void
osd_deregister(u_int type, u_int slot);
int
osd_set(u_int type, struct osd *osd, u_int slot, void *value);
void **
osd_reserve(u_int slot);
int
osd_set_reserved(u_int type, struct osd *osd, u_int slot, void **rsv,
void *value);
void
osd_free_reserved(void **rsv);
void *
osd_get(u_int type, struct osd *osd, u_int slot);
void
osd_del(u_int type, struct osd *osd, u_int slot);
int
osd_call(u_int type, u_int method, void *obj, void *data);
void
osd_exit(u_int type, struct osd *osd);
DESCRIPTION
The osd framework provides a mechanism to dynamically associate arbitrary
data at run-time with any kernel data structure which has been suitably
modified for use with osd. The one-off modification required involves
embedding a struct osd inside the kernel data structure.
An additional benefit is that after the initial change to a structure is
made, all subsequent use of osd with the structure involves no changes to
the structure's layout. By extension, if the data structure is part of
the ABI, osd provides a way of extending the structure in an ABI
preserving manner.
The details of the embedded struct osd are not relevant to consumers of
the osd framework and should not be manipulated directly.
Data associated with a structure is referenced by the osd framework using
a type/slot identifier pair. Types are statically defined in <sys/osd.h>
and provide a high-level grouping for slots to be registered under. Slot
identifiers are dynamically assigned by the framework when a data type is
registered using osd_register() and remains valid until a corresponding
call to osd_deregister().
Functions
The osd_register() function registers a type/slot identifier pair with
the osd framework for use with a new data type. The function may sleep
and therefore cannot be called from a non-sleepable context. The type
argument specifies which high-level type grouping from <sys/osd.h> the
slot identifier should be allocated under. The destructor argument
specifies an optional osd_destructor_t function pointer that will be
called for objects of the type being registered which are later destroyed
by the osd_del() function. NULL may be passed if no destructor is
required. The methods argument specifies an optional array of
osd_method_t function pointers which can be later invoked by the
osd_call() function. NULL may be passed if no methods are required. The
methods argument is currently only useful with the OSD_JAIL type
identifier.
The osd_deregister() function deregisters a previously registered
type/slot identifier pair. The function may sleep and therefore cannot
be called from a non-sleepable context. The type argument specifies
which high-level type grouping from <sys/osd.h> the slot identifier is
allocated under. The slot argument specifies the slot identifier which
is being deregistered and should be the value that was returned by
osd_register() when the data type was registered.
The osd_set() function associates a data object pointer with a kernel
data structure's struct osd member. The type argument specifies which
high-level type grouping from <sys/osd.h> the slot identifier is
allocated under. The osd argument is a pointer to the kernel data
structure's struct osd which will have the value pointer associated with
it. The slot argument specifies the slot identifier to assign the value
pointer to. The value argument points to a data object to associate with
osd.
The osd_set_reserved() function does the same as osd_set(), but with an
extra argument rsv that is internal-use memory previously allocated via
osd_reserve().
The osd_get() function returns the data pointer associated with a kernel
data structure's struct osd member from the specified type/slot
identifier pair. The type argument specifies which high-level type
grouping from <sys/osd.h> the slot identifier is allocated under. The
osd argument is a pointer to the kernel data structure's struct osd to
retrieve the data pointer from. The slot argument specifies the slot
identifier to retrieve the data pointer from.
The osd_del() function removes the data pointer associated with a kernel
data structure's struct osd member from the specified type/slot
identifier pair. The type argument specifies which high-level type
grouping from <sys/osd.h> the slot identifier is allocated under. The
osd argument is a pointer to the kernel data structure's struct osd to
remove the data pointer from. The slot argument specifies the slot
identifier to remove the data pointer from. If an osd_destructor_t
function pointer was specified at registration time, the destructor
function will be called and passed the data pointer for the type/slot
identifier pair which is being deleted.
The osd_call() function calls the specified osd_method_t function pointer
for all currently registered slots of a given type on the specified obj
and data pointers. The function may sleep and therefore cannot be called
from a non-sleepable context. The type argument specifies which high-
level type grouping from <sys/osd.h> to call the method for. The method
argument specifies the index into the osd_method_t array that was passed
to osd_register(). The obj and data arguments are passed to the method
function pointer of each slot.
The osd_exit() function removes all data object pointers from all
currently registered slots for a given type for the specified kernel data
structure's struct osd member. The type argument specifies which high-
level type grouping from <sys/osd.h> to remove data pointers from. The
osd argument is a pointer to the kernel data structure's struct osd to
remove all data object pointers for all currently registered slots from.
IMPLEMENTATION NOTES
osd uses a two dimensional matrix (array of arrays) as the data structure
to manage the external data associated with a kernel data structure's
struct osd member. The type identifier is used as the index into the
outer array, and the slot identifier is used as the index into the inner
array. To set or retrieve a data pointer for a given type/slot
identifier pair, osd_set() and osd_get() perform the equivalent of
array[type][slot], which is both constant time and fast.
If osd_set() is called on a struct osd for the first time, the array for
storing data pointers is dynamically allocated using malloc(9) with
M_NOWAIT to a size appropriate for the slot identifier being set. If a
subsequent call to osd_set() attempts to set a slot identifier which is
numerically larger than the slot used in the previous osd_set() call,
realloc(9) is used to grow the array to the appropriate size such that
the slot identifier can be used. To maximise the efficiency of any code
which calls osd_set() sequentially on a number of different slot
identifiers (e.g., during an initialisation phase) one should loop
through the slot identifiers in descending order from highest to lowest.
This will result in only a single malloc(9) call to create an array of
the largest slot size and all subsequent calls to osd_set() will proceed
without any realloc(9) calls.
It is possible for osd_set() to fail to allocate this array. To ensure
that such allocation succeeds, osd_reserve() may be called (in a non-
blocking context), and it will pre-allocate the memory via malloc(9) with
M_WAITOK. Then this pre-allocated memory is passed to
osd_set_reserved(), which will use it if necessary or otherwise discard
it. The memory may also be explicitly discarded by calling
osd_free_reserved(). As this method always allocates memory whether or
not it is ultimately needed, it should be used only rarely, such as in
the unlikely event that osd_set() fails.
The osd API is geared towards slot identifiers storing pointers to the
same underlying data structure type for a given osd type identifier.
This is not a requirement, and khelp(9) for example stores completely
different data types in slots under the OSD_KHELP type identifier.
Locking
osd internally uses a mix of mutex(9), rmlock(9) and sx(9) locks to
protect its internal data structures and state.
Responsibility for synchronising access to a kernel data structure's
struct osd member is left to the subsystem that uses the data structure
and calls the osd API.
osd_get() only acquires an rmlock in read mode, therefore making it safe
to use in the majority of contexts within the kernel including most fast
paths.
RETURN VALUES
osd_register() returns the slot identifier for the newly registered data
type.
osd_set() and osd_set_reserved() return zero on success or ENOMEM if the
specified type/slot identifier pair triggered an internal realloc(9)
which failed (osd_set_reserved() will always succeed when rsv is non-
NULL).
osd_get() returns the data pointer for the specified type/slot identifier
pair, or NULL if the slot has not been initialised yet.
osd_reserve() returns a pointer suitable for passing to
osd_set_reserved() or osd_free_reserved().
osd_call() returns zero if no method is run or the method for each slot
runs successfully. If a method for a slot returns non-zero, osd_call()
terminates prematurely and returns the method's error to the caller.
SEE ALSO
khelp(9)
HISTORY
The Object Specific Data (OSD) facility first appeared in FreeBSD 8.0.
AUTHORS
The osd facility was written by Pawel Jakub Dawidek <pjd@FreeBSD.org>.
This manual page was written by Lawrence Stewart <lstewart@FreeBSD.org>.
FreeBSD 13.1-RELEASE-p6 April 26, 2016 FreeBSD 13.1-RELEASE-p6
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