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 .. _development_coding:
 
 Getting the code right
 ======================
 
 While there is much to be said for a solid and community-oriented design
 process, the proof of any kernel development project is in the resulting
 code.  It is the code which will be examined by other developers and merged
 (or not) into the mainline tree.  So it is the quality of this code which
 will determine the ultimate success of the project.
 
 This section will examine the coding process.  We'll start with a look at a
 number of ways in which kernel developers can go wrong.  Then the focus
 will shift toward doing things right and the tools which can help in that
 quest.
 
 
 Pitfalls
 ---------
 
 Coding style
 ************
 
 The kernel has long had a standard coding style, described in
 :ref:`Documentation/process/coding-style.rst <codingstyle>`.  For much of
 that time, the policies described in that file were taken as being, at most,
 advisory.  As a result, there is a substantial amount of code in the kernel
 which does not meet the coding style guidelines.  The presence of that code
 leads to two independent hazards for kernel developers.
 
 The first of these is to believe that the kernel coding standards do not
 matter and are not enforced.  The truth of the matter is that adding new
 code to the kernel is very difficult if that code is not coded according to
 the standard; many developers will request that the code be reformatted
 before they will even review it.  A code base as large as the kernel
 requires some uniformity of code to make it possible for developers to
 quickly understand any part of it.  So there is no longer room for
 strangely-formatted code.
 
 Occasionally, the kernel's coding style will run into conflict with an
 employer's mandated style.  In such cases, the kernel's style will have to
 win before the code can be merged.  Putting code into the kernel means
 giving up a degree of control in a number of ways - including control over
 how the code is formatted.
 
 The other trap is to assume that code which is already in the kernel is
 urgently in need of coding style fixes.  Developers may start to generate
 reformatting patches as a way of gaining familiarity with the process, or
 as a way of getting their name into the kernel changelogs - or both.  But
 pure coding style fixes are seen as noise by the development community;
 they tend to get a chilly reception.  So this type of patch is best
 avoided.  It is natural to fix the style of a piece of code while working
 on it for other reasons, but coding style changes should not be made for
 their own sake.
 
 The coding style document also should not be read as an absolute law which
 can never be transgressed.  If there is a good reason to go against the
 style (a line which becomes far less readable if split to fit within the
 80-column limit, for example), just do it.
 
 Note that you can also use the ``clang-format`` tool to help you with
 these rules, to quickly re-format parts of your code automatically,
 and to review full files in order to spot coding style mistakes,
 typos and possible improvements. It is also handy for sorting ``#includes``,
 for aligning variables/macros, for reflowing text and other similar tasks.
 See the file :ref:`Documentation/process/clang-format.rst <clangformat>`
 for more details.
 
 
 Abstraction layers
 ******************
 
 Computer Science professors teach students to make extensive use of
 abstraction layers in the name of flexibility and information hiding.
 Certainly the kernel makes extensive use of abstraction; no project
 involving several million lines of code could do otherwise and survive.
 But experience has shown that excessive or premature abstraction can be
 just as harmful as premature optimization.  Abstraction should be used to
 the level required and no further.
 
 At a simple level, consider a function which has an argument which is
 always passed as zero by all callers.  One could retain that argument just
 in case somebody eventually needs to use the extra flexibility that it
 provides.  By that time, though, chances are good that the code which
 implements this extra argument has been broken in some subtle way which was
 never noticed - because it has never been used.  Or, when the need for
 extra flexibility arises, it does not do so in a way which matches the
 programmer's early expectation.  Kernel developers will routinely submit
 patches to remove unused arguments; they should, in general, not be added
 in the first place.
 
 Abstraction layers which hide access to hardware - often to allow the bulk
 of a driver to be used with multiple operating systems - are especially
 frowned upon.  Such layers obscure the code and may impose a performance
 penalty; they do not belong in the Linux kernel.
 
 On the other hand, if you find yourself copying significant amounts of code
 from another kernel subsystem, it is time to ask whether it would, in fact,
 make sense to pull out some of that code into a separate library or to
 implement that functionality at a higher level.  There is no value in
 replicating the same code throughout the kernel.
 
 
 #ifdef and preprocessor use in general
 **************************************
 
 The C preprocessor seems to present a powerful temptation to some C
 programmers, who see it as a way to efficiently encode a great deal of
 flexibility into a source file.  But the preprocessor is not C, and heavy
 use of it results in code which is much harder for others to read and
 harder for the compiler to check for correctness.  Heavy preprocessor use
 is almost always a sign of code which needs some cleanup work.
 
 Conditional compilation with #ifdef is, indeed, a powerful feature, and it
 is used within the kernel.  But there is little desire to see code which is
 sprinkled liberally with #ifdef blocks.  As a general rule, #ifdef use
 should be confined to header files whenever possible.
 Conditionally-compiled code can be confined to functions which, if the code
 is not to be present, simply become empty.  The compiler will then quietly
 optimize out the call to the empty function.  The result is far cleaner
 code which is easier to follow.
 
 C preprocessor macros present a number of hazards, including possible
 multiple evaluation of expressions with side effects and no type safety.
 If you are tempted to define a macro, consider creating an inline function
 instead.  The code which results will be the same, but inline functions are
 easier to read, do not evaluate their arguments multiple times, and allow
 the compiler to perform type checking on the arguments and return value.
 
 
 Inline functions
 ****************
 
 Inline functions present a hazard of their own, though.  Programmers can
 become enamored of the perceived efficiency inherent in avoiding a function
 call and fill a source file with inline functions.  Those functions,
 however, can actually reduce performance.  Since their code is replicated
 at each call site, they end up bloating the size of the compiled kernel.
 That, in turn, creates pressure on the processor's memory caches, which can
 slow execution dramatically.  Inline functions, as a rule, should be quite
 small and relatively rare.  The cost of a function call, after all, is not
 that high; the creation of large numbers of inline functions is a classic
 example of premature optimization.
 
 In general, kernel programmers ignore cache effects at their peril.  The
 classic time/space tradeoff taught in beginning data structures classes
 often does not apply to contemporary hardware.  Space *is* time, in that a
 larger program will run slower than one which is more compact.
 
 More recent compilers take an increasingly active role in deciding whether
 a given function should actually be inlined or not.  So the liberal
 placement of "inline" keywords may not just be excessive; it could also be
 irrelevant.
 
 
 Locking
 *******
 
 In May, 2006, the "Devicescape" networking stack was, with great
 fanfare, released under the GPL and made available for inclusion in the
 mainline kernel.  This donation was welcome news; support for wireless
 networking in Linux was considered substandard at best, and the Devicescape
 stack offered the promise of fixing that situation.  Yet, this code did not
 actually make it into the mainline until June, 2007 (2.6.22).  What
 happened?
 
 This code showed a number of signs of having been developed behind
 corporate doors.  But one large problem in particular was that it was not
 designed to work on multiprocessor systems.  Before this networking stack
 (now called mac80211) could be merged, a locking scheme needed to be
 retrofitted onto it.
 
 Once upon a time, Linux kernel code could be developed without thinking
 about the concurrency issues presented by multiprocessor systems.  Now,
 however, this document is being written on a dual-core laptop.  Even on
 single-processor systems, work being done to improve responsiveness will
 raise the level of concurrency within the kernel.  The days when kernel
 code could be written without thinking about locking are long past.
 
 Any resource (data structures, hardware registers, etc.) which could be
 accessed concurrently by more than one thread must be protected by a lock.
 New code should be written with this requirement in mind; retrofitting
 locking after the fact is a rather more difficult task.  Kernel developers
 should take the time to understand the available locking primitives well
 enough to pick the right tool for the job.  Code which shows a lack of
 attention to concurrency will have a difficult path into the mainline.
 
 
 Regressions
 ***********
 
 One final hazard worth mentioning is this: it can be tempting to make a
 change (which may bring big improvements) which causes something to break
 for existing users.  This kind of change is called a "regression," and
 regressions have become most unwelcome in the mainline kernel.  With few
 exceptions, changes which cause regressions will be backed out if the
 regression cannot be fixed in a timely manner.  Far better to avoid the
 regression in the first place.
 
 It is often argued that a regression can be justified if it causes things
 to work for more people than it creates problems for.  Why not make a
 change if it brings new functionality to ten systems for each one it
 breaks?  The best answer to this question was expressed by Linus in July,
 2007:
 
 ::
 
         So we don't fix bugs by introducing new problems.  That way lies
         madness, and nobody ever knows if you actually make any real
         progress at all. Is it two steps forwards, one step back, or one
         step forward and two steps back?
 
 (https://lwn.net/Articles/243460/).
 
 An especially unwelcome type of regression is any sort of change to the
 user-space ABI.  Once an interface has been exported to user space, it must
 be supported indefinitely.  This fact makes the creation of user-space
 interfaces particularly challenging: since they cannot be changed in
 incompatible ways, they must be done right the first time.  For this
 reason, a great deal of thought, clear documentation, and wide review for
 user-space interfaces is always required.
 
 
 Code checking tools
 -------------------
 
 For now, at least, the writing of error-free code remains an ideal that few
 of us can reach.  What we can hope to do, though, is to catch and fix as
 many of those errors as possible before our code goes into the mainline
 kernel.  To that end, the kernel developers have put together an impressive
 array of tools which can catch a wide variety of obscure problems in an
 automated way.  Any problem caught by the computer is a problem which will
 not afflict a user later on, so it stands to reason that the automated
 tools should be used whenever possible.
 
 The first step is simply to heed the warnings produced by the compiler.
 Contemporary versions of gcc can detect (and warn about) a large number of
 potential errors.  Quite often, these warnings point to real problems.
 Code submitted for review should, as a rule, not produce any compiler
 warnings.  When silencing warnings, take care to understand the real cause
 and try to avoid "fixes" which make the warning go away without addressing
 its cause.
 
 Note that not all compiler warnings are enabled by default.  Build the
 kernel with "make KCFLAGS=-W" to get the full set.
 
 The kernel provides several configuration options which turn on debugging
 features; most of these are found in the "kernel hacking" submenu.  Several
 of these options should be turned on for any kernel used for development or
 testing purposes.  In particular, you should turn on:
 
  - FRAME_WARN to get warnings for stack frames larger than a given amount.
    The output generated can be verbose, but one need not worry about
    warnings from other parts of the kernel.
 
  - DEBUG_OBJECTS will add code to track the lifetime of various objects
    created by the kernel and warn when things are done out of order.  If
    you are adding a subsystem which creates (and exports) complex objects
    of its own, consider adding support for the object debugging
    infrastructure.
 
  - DEBUG_SLAB can find a variety of memory allocation and use errors; it
    should be used on most development kernels.
 
  - DEBUG_SPINLOCK, DEBUG_ATOMIC_SLEEP, and DEBUG_MUTEXES will find a
    number of common locking errors.
 
 There are quite a few other debugging options, some of which will be
 discussed below.  Some of them have a significant performance impact and
 should not be used all of the time.  But some time spent learning the
 available options will likely be paid back many times over in short order.
 
 One of the heavier debugging tools is the locking checker, or "lockdep."
 This tool will track the acquisition and release of every lock (spinlock or
 mutex) in the system, the order in which locks are acquired relative to
 each other, the current interrupt environment, and more.  It can then
 ensure that locks are always acquired in the same order, that the same
 interrupt assumptions apply in all situations, and so on.  In other words,
 lockdep can find a number of scenarios in which the system could, on rare
 occasion, deadlock.  This kind of problem can be painful (for both
 developers and users) in a deployed system; lockdep allows them to be found
 in an automated manner ahead of time.  Code with any sort of non-trivial
 locking should be run with lockdep enabled before being submitted for
 inclusion.
 
 As a diligent kernel programmer, you will, beyond doubt, check the return
 status of any operation (such as a memory allocation) which can fail.  The
 fact of the matter, though, is that the resulting failure recovery paths
 are, probably, completely untested.  Untested code tends to be broken code;
 you could be much more confident of your code if all those error-handling
 paths had been exercised a few times.
 
 The kernel provides a fault injection framework which can do exactly that,
 especially where memory allocations are involved.  With fault injection
 enabled, a configurable percentage of memory allocations will be made to
 fail; these failures can be restricted to a specific range of code.
 Running with fault injection enabled allows the programmer to see how the
 code responds when things go badly.  See
 Documentation/fault-injection/fault-injection.rst for more information on
 how to use this facility.
 
 Other kinds of errors can be found with the "sparse" static analysis tool.
 With sparse, the programmer can be warned about confusion between
 user-space and kernel-space addresses, mixture of big-endian and
 small-endian quantities, the passing of integer values where a set of bit
 flags is expected, and so on.  Sparse must be installed separately (it can
 be found at https://sparse.wiki.kernel.org/index.php/Main_Page if your
 distributor does not package it); it can then be run on the code by adding
 "C=1" to your make command.
 
 The "Coccinelle" tool (http://coccinelle.lip6.fr/) is able to find a wide
 variety of potential coding problems; it can also propose fixes for those
 problems.  Quite a few "semantic patches" for the kernel have been packaged
 under the scripts/coccinelle directory; running "make coccicheck" will run
 through those semantic patches and report on any problems found.  See
 :ref:`Documentation/dev-tools/coccinelle.rst <devtools_coccinelle>`
 for more information.
 
 Other kinds of portability errors are best found by compiling your code for
 other architectures.  If you do not happen to have an S/390 system or a
 Blackfin development board handy, you can still perform the compilation
 step.  A large set of cross compilers for x86 systems can be found at
 
         https://www.kernel.org/pub/tools/crosstool/
 
 Some time spent installing and using these compilers will help avoid
 embarrassment later.
 
 
 Documentation
 -------------
 
 Documentation has often been more the exception than the rule with kernel
 development.  Even so, adequate documentation will help to ease the merging
 of new code into the kernel, make life easier for other developers, and
 will be helpful for your users.  In many cases, the addition of
 documentation has become essentially mandatory.
 
 The first piece of documentation for any patch is its associated
 changelog.  Log entries should describe the problem being solved, the form
 of the solution, the people who worked on the patch, any relevant
 effects on performance, and anything else that might be needed to
 understand the patch.  Be sure that the changelog says *why* the patch is
 worth applying; a surprising number of developers fail to provide that
 information.
 
 Any code which adds a new user-space interface - including new sysfs or
 /proc files - should include documentation of that interface which enables
 user-space developers to know what they are working with.  See
 Documentation/ABI/README for a description of how this documentation should
 be formatted and what information needs to be provided.
 
 The file :ref:`Documentation/admin-guide/kernel-parameters.rst
 <kernelparameters>` describes all of the kernel's boot-time parameters.
 Any patch which adds new parameters should add the appropriate entries to
 this file.
 
 Any new configuration options must be accompanied by help text which
 clearly explains the options and when the user might want to select them.
 
 Internal API information for many subsystems is documented by way of
 specially-formatted comments; these comments can be extracted and formatted
 in a number of ways by the "kernel-doc" script.  If you are working within
 a subsystem which has kerneldoc comments, you should maintain them and add
 them, as appropriate, for externally-available functions.  Even in areas
 which have not been so documented, there is no harm in adding kerneldoc
 comments for the future; indeed, this can be a useful activity for
 beginning kernel developers.  The format of these comments, along with some
 information on how to create kerneldoc templates can be found at
 :ref:`Documentation/doc-guide/ <doc_guide>`.
 
 Anybody who reads through a significant amount of existing kernel code will
 note that, often, comments are most notable by their absence.  Once again,
 the expectations for new code are higher than they were in the past;
 merging uncommented code will be harder.  That said, there is little desire
 for verbosely-commented code.  The code should, itself, be readable, with
 comments explaining the more subtle aspects.
 
 Certain things should always be commented.  Uses of memory barriers should
 be accompanied by a line explaining why the barrier is necessary.  The
 locking rules for data structures generally need to be explained somewhere.
 Major data structures need comprehensive documentation in general.
 Non-obvious dependencies between separate bits of code should be pointed
 out.  Anything which might tempt a code janitor to make an incorrect
 "cleanup" needs a comment saying why it is done the way it is.  And so on.
 
 
 Internal API changes
 --------------------
 
 The binary interface provided by the kernel to user space cannot be broken
 except under the most severe circumstances.  The kernel's internal
 programming interfaces, instead, are highly fluid and can be changed when
 the need arises.  If you find yourself having to work around a kernel API,
 or simply not using a specific functionality because it does not meet your
 needs, that may be a sign that the API needs to change.  As a kernel
 developer, you are empowered to make such changes.
 
 There are, of course, some catches.  API changes can be made, but they need
 to be well justified.  So any patch making an internal API change should be
 accompanied by a description of what the change is and why it is
 necessary.  This kind of change should also be broken out into a separate
 patch, rather than buried within a larger patch.
 
 The other catch is that a developer who changes an internal API is
 generally charged with the task of fixing any code within the kernel tree
 which is broken by the change.  For a widely-used function, this duty can
 lead to literally hundreds or thousands of changes - many of which are
 likely to conflict with work being done by other developers.  Needless to
 say, this can be a large job, so it is best to be sure that the
 justification is solid.  Note that the Coccinelle tool can help with
 wide-ranging API changes.
 
 When making an incompatible API change, one should, whenever possible,
 ensure that code which has not been updated is caught by the compiler.
 This will help you to be sure that you have found all in-tree uses of that
 interface.  It will also alert developers of out-of-tree code that there is
 a change that they need to respond to.  Supporting out-of-tree code is not
 something that kernel developers need to be worried about, but we also do
 not have to make life harder for out-of-tree developers than it needs to
 be.

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