osd_get (9)

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Copyright (c) 2010 Lawrence Stewart <lstewart@FreeBSD.org>
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(The comments found at the beginning of the groff file "man9/osd_get.9freebsd".)

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

In sys/osd.h Ft typedef void Fn *osd_destructor_t void *value Ft typedef int Fn *osd_method_t void *obj void *data Ft int Fo osd_register Fa u_int type Fa osd_destructor_t destructor Fa osd_method_t *methods Fc Ft void Fo osd_deregister Fa u_int type Fa u_int slot Fc Ft int Fo osd_set Fa u_int type Fa struct osd *osd Fa u_int slot Fa void *value Fc Ft void ** Fo osd_reserve Fa u_int slot Fc Ft int Fo osd_set_reserved Fa u_int type Fa struct osd *osd Fa u_int slot Fa void **rsv Fa void *value Fc Ft void Fo osd_free_reserved Fa void **rsv Fc Ft void * Fo osd_get Fa u_int type Fa struct osd *osd Fa u_int slot Fc Ft void Fo osd_del Fa u_int type Fa struct osd *osd Fa u_int slot Fc Ft int Fo osd_call Fa u_int type Fa u_int method Fa void *obj Fa void *data Fc Ft void Fo osd_exit Fa u_int type Fa struct osd *osd Fc

DESCRIPTION

The 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 . The one-off modification required involves embedding a Vt 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 with the structure involves no changes to the structure's layout. By extension, if the data structure is part of the ABI, provides a way of extending the structure in an ABI preserving manner.

The details of the embedded Vt struct osd are not relevant to consumers of the framework and should not be manipulated directly.

Data associated with a structure is referenced by the framework using a type/slot identifier pair. Types are statically defined in 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 Fn osd_register and remains valid until a corresponding call to Fn osd_deregister .

Functions

The Fn osd_register function registers a type/slot identifier pair with the framework for use with a new data type. The function may sleep and therefore cannot be called from a non-sleepable context. The Fa type argument specifies which high-level type grouping from In sys/osd.h the slot identifier should be allocated under. The Fa 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 Fn osd_del function. NULL may be passed if no destructor is required. The Fa methods argument specifies an optional array of osd_method_t function pointers which can be later invoked by the Fn osd_call function. NULL may be passed if no methods are required. The Fa methods argument is currently only useful with the OSD_JAIL type identifier.

The Fn 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 Fa type argument specifies which high-level type grouping from In sys/osd.h the slot identifier is allocated under. The Fa slot argument specifies the slot identifier which is being deregistered and should be the value that was returned by Fn osd_register when the data type was registered.

The Fn osd_set function associates a data object pointer with a kernel data structure's Vt struct osd member. The Fa type argument specifies which high-level type grouping from In sys/osd.h the slot identifier is allocated under. The Fa osd argument is a pointer to the kernel data structure's Vt struct osd which will have the Fa value pointer associated with it. The Fa slot argument specifies the slot identifier to assign the Fa value pointer to. The Fa value argument points to a data object to associate with Fa osd .

The Fn osd_set_reserved function does the same as Fn osd_set , but with an extra argument Fa rsv that is internal-use memory previously allocated via Fn osd_reserve .

The Fn osd_get function returns the data pointer associated with a kernel data structure's Vt struct osd member from the specified type/slot identifier pair. The Fa type argument specifies which high-level type grouping from In sys/osd.h the slot identifier is allocated under. The Fa osd argument is a pointer to the kernel data structure's Vt struct osd to retrieve the data pointer from. The Fa slot argument specifies the slot identifier to retrieve the data pointer from.

The Fn osd_del function removes the data pointer associated with a kernel data structure's Vt struct osd member from the specified type/slot identifier pair. The Fa type argument specifies which high-level type grouping from In sys/osd.h the slot identifier is allocated under. The Fa osd argument is a pointer to the kernel data structure's Vt struct osd to remove the data pointer from. The Fa 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 Fn osd_call function calls the specified osd_method_t function pointer for all currently registered slots of a given type on the specified Fa obj and Fa data pointers. The function may sleep and therefore cannot be called from a non-sleepable context. The Fa type argument specifies which high-level type grouping from In sys/osd.h to call the method for. The Fa method argument specifies the index into the osd_method_t array that was passed to Fn osd_register . The Fa obj and Fa data arguments are passed to the method function pointer of each slot.

The Fn osd_exit function removes all data object pointers from all currently registered slots for a given type for the specified kernel data structure's Vt struct osd member. The Fa type argument specifies which high-level type grouping from In sys/osd.h to remove data pointers from. The Fa osd argument is a pointer to the kernel data structure's Vt struct osd to remove all data object pointers for all currently registered slots from.

IMPLEMENTATION NOTES

uses a two dimensional matrix (array of arrays) as the data structure to manage the external data associated with a kernel data structure's Vt 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, Fn osd_set and Fn osd_get perform the equivalent of array[type][slot], which is both constant time and fast.

If Fn osd_set is called on a Vt 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 Fn osd_set attempts to set a slot identifier which is numerically larger than the slot used in the previous Fn 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 Fn 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 Fn osd_set will proceed without any realloc(9) calls.

It is possible for Fn osd_set to fail to allocate this array. To ensure that such allocation succeeds, Fn 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 Fn osd_set_reserved , which will use it if necessary or otherwise discard it. The memory may also be explicitly discarded by calling Fn 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 Fn osd_set fails.

The API is geared towards slot identifiers storing pointers to the same underlying data structure type for a given 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

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 Vt struct osd member is left to the subsystem that uses the data structure and calls the API.

Fn 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

Fn osd_register returns the slot identifier for the newly registered data type.

Fn osd_set and Fn osd_set_reserved return zero on success or ENOMEM if the specified type/slot identifier pair triggered an internal realloc(9) which failed Fn ( osd_set_reserved will always succeed when Fa rsv is non-NULL).

Fn osd_get returns the data pointer for the specified type/slot identifier pair, or NULL if the slot has not been initialised yet.

Fn osd_reserve returns a pointer suitable for passing to Fn osd_set_reserved or Fn osd_free_reserved .

Fn 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, Fn 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 Fx 8.0 .

AUTHORS

An -nosplit The facility was written by An Pawel Jakub Dawidek Aq Mt pjd@FreeBSD.org .

This manual page was written by An Lawrence Stewart Aq Mt lstewart@FreeBSD.org .