80 lines
2.9 KiB
ReStructuredText
80 lines
2.9 KiB
ReStructuredText
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.. SPDX-License-Identifier: CC-BY-SA-4.0
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=============
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ID Allocation
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=============
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:Author: Matthew Wilcox
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Overview
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========
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A common problem to solve is allocating identifiers (IDs); generally
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small numbers which identify a thing. Examples include file descriptors,
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process IDs, packet identifiers in networking protocols, SCSI tags
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and device instance numbers. The IDR and the IDA provide a reasonable
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solution to the problem to avoid everybody inventing their own. The IDR
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provides the ability to map an ID to a pointer, while the IDA provides
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only ID allocation, and as a result is much more memory-efficient.
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IDR usage
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=========
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Start by initialising an IDR, either with :c:func:`DEFINE_IDR`
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for statically allocated IDRs or :c:func:`idr_init` for dynamically
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allocated IDRs.
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You can call :c:func:`idr_alloc` to allocate an unused ID. Look up
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the pointer you associated with the ID by calling :c:func:`idr_find`
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and free the ID by calling :c:func:`idr_remove`.
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If you need to change the pointer associated with an ID, you can call
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:c:func:`idr_replace`. One common reason to do this is to reserve an
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ID by passing a ``NULL`` pointer to the allocation function; initialise the
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object with the reserved ID and finally insert the initialised object
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into the IDR.
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Some users need to allocate IDs larger than ``INT_MAX``. So far all of
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these users have been content with a ``UINT_MAX`` limit, and they use
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:c:func:`idr_alloc_u32`. If you need IDs that will not fit in a u32,
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we will work with you to address your needs.
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If you need to allocate IDs sequentially, you can use
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:c:func:`idr_alloc_cyclic`. The IDR becomes less efficient when dealing
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with larger IDs, so using this function comes at a slight cost.
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To perform an action on all pointers used by the IDR, you can
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either use the callback-based :c:func:`idr_for_each` or the
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iterator-style :c:func:`idr_for_each_entry`. You may need to use
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:c:func:`idr_for_each_entry_continue` to continue an iteration. You can
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also use :c:func:`idr_get_next` if the iterator doesn't fit your needs.
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When you have finished using an IDR, you can call :c:func:`idr_destroy`
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to release the memory used by the IDR. This will not free the objects
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pointed to from the IDR; if you want to do that, use one of the iterators
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to do it.
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You can use :c:func:`idr_is_empty` to find out whether there are any
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IDs currently allocated.
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If you need to take a lock while allocating a new ID from the IDR,
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you may need to pass a restrictive set of GFP flags, which can lead
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to the IDR being unable to allocate memory. To work around this,
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you can call :c:func:`idr_preload` before taking the lock, and then
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:c:func:`idr_preload_end` after the allocation.
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.. kernel-doc:: include/linux/idr.h
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:doc: idr sync
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IDA usage
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=========
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.. kernel-doc:: lib/idr.c
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:doc: IDA description
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Functions and structures
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========================
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.. kernel-doc:: include/linux/idr.h
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.. kernel-doc:: lib/idr.c
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