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#include <sys/osd.h>
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().
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.
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.
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.
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.
This manual page was written by Lawrence Stewart <Mt lstewart@FreeBSD.org>.
| OSD (9) | April 26, 2016 |
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