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+4: 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.
+
+
+4.1: PITFALLS
+
+* Coding style
+
+The kernel has long had a standard coding style, described in
+Documentation/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.
+
+
+* 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?
+
+(http://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.
+
+
+
+4.2: 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 EXTRA_CFLAGS=-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:
+
+ - ENABLE_WARN_DEPRECATED, ENABLE_MUST_CHECK, and FRAME_WARN to get an
+ extra set of warnings for problems like the use of deprecated interfaces
+ or ignoring an important return value from a function. The output
+ generated by these warnings 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.txt 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
+Documentation/coccinelle.txt 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
+
+ http://www.kernel.org/pub/tools/crosstool/
+
+Some time spent installing and using these compilers will help avoid
+embarrassment later.
+
+
+4.3: 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 Documentation/kernel-parameters.txt 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 in the file
+Documentation/kernel-doc-nano-HOWTO.txt.
+
+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.
+
+
+4.4: 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.