Copyright (c) 2010 Lawrence Stewart <lstewart@FreeBSD.org>
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$FreeBSD: releng/10.2/share/man/man9/osd.9 233648 2012-03-29 05:02:12Z eadler $
.Dd January 5, 2011 .Dt OSD 9 .Os .Sh NAME .Nm osd , .Nm osd_register , .Nm osd_deregister , .Nm osd_set , .Nm osd_get , .Nm osd_del , .Nm osd_call , .Nm osd_exit .Nd Object Specific Data .Sh SYNOPSIS n sys/osd.h .Ft typedef void .Fn "\*(lp*osd_destructor_t\*(rp" "void *value" .Ft typedef int .Fn "\*(lp*osd_method_t\*(rp" "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_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 .Sh DESCRIPTION The .Nm 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 .Nm . The one-off modification required involves embedding a .Vt "struct osd" inside the kernel data structure.
p An additional benefit is that after the initial change to a structure is made, all subsequent use of .Nm with the structure involves no changes to the structure's layout. By extension, if the data structure is part of the ABI, .Nm provides a way of extending the structure in an ABI preserving manner.
p The details of the embedded .Vt "struct osd" are not relevant to consumers of the .Nm framework and should not be manipulated directly.
p Data associated with a structure is referenced by the .Nm framework using a type/slot identifier pair. Types are statically defined in n 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 . .Ss Functions The .Fn osd_register function registers a type/slot identifier pair with the .Nm 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 n 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.
p 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 n 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.
p 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 n 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 .
p 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 n 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.
p 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 n 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.
p 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 n 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.
p 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 n 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. .Sh IMPLEMENTATION NOTES .Nm 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.
p 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 .Xr 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, .Xr 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 .Xr malloc 9 call to create an array of the largest slot size and all subsequent calls to .Fn osd_set will proceed without any .Xr realloc 9 calls.
p The .Nm API is geared towards slot identifiers storing pointers to the same underlying data structure type for a given .Nm type identifier. This is not a requirement, and .Xr khelp 9 for example stores completely different data types in slots under the OSD_KHELP type identifier. .Ss Locking .Nm internally uses a mix of .Xr mutex 9 , .Xr rmlock 9 and .Xr sx 9 locks to protect its internal data structures and state.
p 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 .Nm API.
p .Fn osd_get only acquires an .Xr rmlock in read mode, therefore making it safe to use in the majority of contexts within the kernel including most fast paths. .Sh RETURN VALUES .Fn osd_register returns the slot identifier for the newly registered data type.
p .Fn osd_set returns zero on success or ENOMEM if the specified type/slot identifier pair triggered an internal .Xr realloc 9 which failed.
p .Fn osd_get returns the data pointer for the specified type/slot identifier pair, or NULL if the slot has not been initialised yet.
p .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. .Sh SEE ALSO .Xr khelp 9 .Sh HISTORY The Object Specific Data (OSD) facility first appeared in .Fx 8.0 . .Sh AUTHORS .An -nosplit The .Nm facility was written by .An Pawel Jakub Dawidek Aq pjd@FreeBSD.org .
p This manual page was written by .An Lawrence Stewart Aq lstewart@FreeBSD.org .