3While there is much to be said for a solid and community-oriented design
4process, the proof of any kernel development project is in the resulting
5code. It is the code which will be examined by other developers and merged
6(or not) into the mainline tree. So it is the quality of this code which
7will determine the ultimate success of the project.
9This section will examine the coding process. We'll start with a look at a
10number of ways in which kernel developers can go wrong. Then the focus
11will shift toward doing things right and the tools which can help in that
17* Coding style
19The kernel has long had a standard coding style, described in
20Documentation/CodingStyle. For much of that time, the policies described
21in that file were taken as being, at most, advisory. As a result, there is
22a substantial amount of code in the kernel which does not meet the coding
23style guidelines. The presence of that code leads to two independent
24hazards for kernel developers.
26The first of these is to believe that the kernel coding standards do not
27matter and are not enforced. The truth of the matter is that adding new
28code to the kernel is very difficult if that code is not coded according to
29the standard; many developers will request that the code be reformatted
30before they will even review it. A code base as large as the kernel
31requires some uniformity of code to make it possible for developers to
32quickly understand any part of it. So there is no longer room for
33strangely-formatted code.
35Occasionally, the kernel's coding style will run into conflict with an
36employer's mandated style. In such cases, the kernel's style will have to
37win before the code can be merged. Putting code into the kernel means
38giving up a degree of control in a number of ways - including control over
39how the code is formatted.
41The other trap is to assume that code which is already in the kernel is
42urgently in need of coding style fixes. Developers may start to generate
43reformatting patches as a way of gaining familiarity with the process, or
44as a way of getting their name into the kernel changelogs - or both. But
45pure coding style fixes are seen as noise by the development community;
46they tend to get a chilly reception. So this type of patch is best
47avoided. It is natural to fix the style of a piece of code while working
48on it for other reasons, but coding style changes should not be made for
49their own sake.
51The coding style document also should not be read as an absolute law which
52can never be transgressed. If there is a good reason to go against the
53style (a line which becomes far less readable if split to fit within the
5480-column limit, for example), just do it.
57* Abstraction layers
59Computer Science professors teach students to make extensive use of
60abstraction layers in the name of flexibility and information hiding.
61Certainly the kernel makes extensive use of abstraction; no project
62involving several million lines of code could do otherwise and survive.
63But experience has shown that excessive or premature abstraction can be
64just as harmful as premature optimization. Abstraction should be used to
65the level required and no further.
67At a simple level, consider a function which has an argument which is
68always passed as zero by all callers. One could retain that argument just
69in case somebody eventually needs to use the extra flexibility that it
70provides. By that time, though, chances are good that the code which
71implements this extra argument has been broken in some subtle way which was
72never noticed - because it has never been used. Or, when the need for
73extra flexibility arises, it does not do so in a way which matches the
74programmer's early expectation. Kernel developers will routinely submit
75patches to remove unused arguments; they should, in general, not be added
76in the first place.
78Abstraction layers which hide access to hardware - often to allow the bulk
79of a driver to be used with multiple operating systems - are especially
80frowned upon. Such layers obscure the code and may impose a performance
81penalty; they do not belong in the Linux kernel.
83On the other hand, if you find yourself copying significant amounts of code
84from another kernel subsystem, it is time to ask whether it would, in fact,
85make sense to pull out some of that code into a separate library or to
86implement that functionality at a higher level. There is no value in
87replicating the same code throughout the kernel.
90* #ifdef and preprocessor use in general
92The C preprocessor seems to present a powerful temptation to some C
93programmers, who see it as a way to efficiently encode a great deal of
94flexibility into a source file. But the preprocessor is not C, and heavy
95use of it results in code which is much harder for others to read and
96harder for the compiler to check for correctness. Heavy preprocessor use
97is almost always a sign of code which needs some cleanup work.
99Conditional compilation with #ifdef is, indeed, a powerful feature, and it
100is used within the kernel. But there is little desire to see code which is
101sprinkled liberally with #ifdef blocks. As a general rule, #ifdef use
102should be confined to header files whenever possible.
103Conditionally-compiled code can be confined to functions which, if the code
104is not to be present, simply become empty. The compiler will then quietly
105optimize out the call to the empty function. The result is far cleaner
106code which is easier to follow.
108C preprocessor macros present a number of hazards, including possible
109multiple evaluation of expressions with side effects and no type safety.
110If you are tempted to define a macro, consider creating an inline function
111instead. The code which results will be the same, but inline functions are
112easier to read, do not evaluate their arguments multiple times, and allow
113the compiler to perform type checking on the arguments and return value.
116* Inline functions
118Inline functions present a hazard of their own, though. Programmers can
119become enamored of the perceived efficiency inherent in avoiding a function
120call and fill a source file with inline functions. Those functions,
121however, can actually reduce performance. Since their code is replicated
122at each call site, they end up bloating the size of the compiled kernel.
123That, in turn, creates pressure on the processor's memory caches, which can
124slow execution dramatically. Inline functions, as a rule, should be quite
125small and relatively rare. The cost of a function call, after all, is not
126that high; the creation of large numbers of inline functions is a classic
127example of premature optimization.
129In general, kernel programmers ignore cache effects at their peril. The
130classic time/space tradeoff taught in beginning data structures classes
131often does not apply to contemporary hardware. Space *is* time, in that a
132larger program will run slower than one which is more compact.
135* Locking
137In May, 2006, the "Devicescape" networking stack was, with great
138fanfare, released under the GPL and made available for inclusion in the
139mainline kernel. This donation was welcome news; support for wireless
140networking in Linux was considered substandard at best, and the Devicescape
141stack offered the promise of fixing that situation. Yet, this code did not
142actually make it into the mainline until June, 2007 (2.6.22). What
145This code showed a number of signs of having been developed behind
146corporate doors. But one large problem in particular was that it was not
147designed to work on multiprocessor systems. Before this networking stack
148(now called mac80211) could be merged, a locking scheme needed to be
149retrofitted onto it.
151Once upon a time, Linux kernel code could be developed without thinking
152about the concurrency issues presented by multiprocessor systems. Now,
153however, this document is being written on a dual-core laptop. Even on
154single-processor systems, work being done to improve responsiveness will
155raise the level of concurrency within the kernel. The days when kernel
156code could be written without thinking about locking are long past.
158Any resource (data structures, hardware registers, etc.) which could be
159accessed concurrently by more than one thread must be protected by a lock.
160New code should be written with this requirement in mind; retrofitting
161locking after the fact is a rather more difficult task. Kernel developers
162should take the time to understand the available locking primitives well
163enough to pick the right tool for the job. Code which shows a lack of
164attention to concurrency will have a difficult path into the mainline.
167* Regressions
169One final hazard worth mentioning is this: it can be tempting to make a
170change (which may bring big improvements) which causes something to break
171for existing users. This kind of change is called a "regression," and
172regressions have become most unwelcome in the mainline kernel. With few
173exceptions, changes which cause regressions will be backed out if the
174regression cannot be fixed in a timely manner. Far better to avoid the
175regression in the first place.
177It is often argued that a regression can be justified if it causes things
178to work for more people than it creates problems for. Why not make a
179change if it brings new functionality to ten systems for each one it
180breaks? The best answer to this question was expressed by Linus in July,
183    So we don't fix bugs by introducing new problems. That way lies
184    madness, and nobody ever knows if you actually make any real
185    progress at all. Is it two steps forwards, one step back, or one
186    step forward and two steps back?
190An especially unwelcome type of regression is any sort of change to the
191user-space ABI. Once an interface has been exported to user space, it must
192be supported indefinitely. This fact makes the creation of user-space
193interfaces particularly challenging: since they cannot be changed in
194incompatible ways, they must be done right the first time. For this
195reason, a great deal of thought, clear documentation, and wide review for
196user-space interfaces is always required.
202For now, at least, the writing of error-free code remains an ideal that few
203of us can reach. What we can hope to do, though, is to catch and fix as
204many of those errors as possible before our code goes into the mainline
205kernel. To that end, the kernel developers have put together an impressive
206array of tools which can catch a wide variety of obscure problems in an
207automated way. Any problem caught by the computer is a problem which will
208not afflict a user later on, so it stands to reason that the automated
209tools should be used whenever possible.
211The first step is simply to heed the warnings produced by the compiler.
212Contemporary versions of gcc can detect (and warn about) a large number of
213potential errors. Quite often, these warnings point to real problems.
214Code submitted for review should, as a rule, not produce any compiler
215warnings. When silencing warnings, take care to understand the real cause
216and try to avoid "fixes" which make the warning go away without addressing
217its cause.
219Note that not all compiler warnings are enabled by default. Build the
220kernel with "make EXTRA_CFLAGS=-W" to get the full set.
222The kernel provides several configuration options which turn on debugging
223features; most of these are found in the "kernel hacking" submenu. Several
224of these options should be turned on for any kernel used for development or
225testing purposes. In particular, you should turn on:
228   extra set of warnings for problems like the use of deprecated interfaces
229   or ignoring an important return value from a function. The output
230   generated by these warnings can be verbose, but one need not worry about
231   warnings from other parts of the kernel.
233 - DEBUG_OBJECTS will add code to track the lifetime of various objects
234   created by the kernel and warn when things are done out of order. If
235   you are adding a subsystem which creates (and exports) complex objects
236   of its own, consider adding support for the object debugging
237   infrastructure.
239 - DEBUG_SLAB can find a variety of memory allocation and use errors; it
240   should be used on most development kernels.
243   number of common locking errors.
245There are quite a few other debugging options, some of which will be
246discussed below. Some of them have a significant performance impact and
247should not be used all of the time. But some time spent learning the
248available options will likely be paid back many times over in short order.
250One of the heavier debugging tools is the locking checker, or "lockdep."
251This tool will track the acquisition and release of every lock (spinlock or
252mutex) in the system, the order in which locks are acquired relative to
253each other, the current interrupt environment, and more. It can then
254ensure that locks are always acquired in the same order, that the same
255interrupt assumptions apply in all situations, and so on. In other words,
256lockdep can find a number of scenarios in which the system could, on rare
257occasion, deadlock. This kind of problem can be painful (for both
258developers and users) in a deployed system; lockdep allows them to be found
259in an automated manner ahead of time. Code with any sort of non-trivial
260locking should be run with lockdep enabled before being submitted for
263As a diligent kernel programmer, you will, beyond doubt, check the return
264status of any operation (such as a memory allocation) which can fail. The
265fact of the matter, though, is that the resulting failure recovery paths
266are, probably, completely untested. Untested code tends to be broken code;
267you could be much more confident of your code if all those error-handling
268paths had been exercised a few times.
270The kernel provides a fault injection framework which can do exactly that,
271especially where memory allocations are involved. With fault injection
272enabled, a configurable percentage of memory allocations will be made to
273fail; these failures can be restricted to a specific range of code.
274Running with fault injection enabled allows the programmer to see how the
275code responds when things go badly. See
276Documentation/fault-injection/fault-injection.text for more information on
277how to use this facility.
279Other kinds of errors can be found with the "sparse" static analysis tool.
280With sparse, the programmer can be warned about confusion between
281user-space and kernel-space addresses, mixture of big-endian and
282small-endian quantities, the passing of integer values where a set of bit
283flags is expected, and so on. Sparse must be installed separately (it can
284be found at if your
285distributor does not package it); it can then be run on the code by adding
286"C=1" to your make command.
288Other kinds of portability errors are best found by compiling your code for
289other architectures. If you do not happen to have an S/390 system or a
290Blackfin development board handy, you can still perform the compilation
291step. A large set of cross compilers for x86 systems can be found at
295Some time spent installing and using these compilers will help avoid
296embarrassment later.
301Documentation has often been more the exception than the rule with kernel
302development. Even so, adequate documentation will help to ease the merging
303of new code into the kernel, make life easier for other developers, and
304will be helpful for your users. In many cases, the addition of
305documentation has become essentially mandatory.
307The first piece of documentation for any patch is its associated
308changelog. Log entries should describe the problem being solved, the form
309of the solution, the people who worked on the patch, any relevant
310effects on performance, and anything else that might be needed to
311understand the patch.
313Any code which adds a new user-space interface - including new sysfs or
314/proc files - should include documentation of that interface which enables
315user-space developers to know what they are working with. See
316Documentation/ABI/README for a description of how this documentation should
317be formatted and what information needs to be provided.
319The file Documentation/kernel-parameters.txt describes all of the kernel's
320boot-time parameters. Any patch which adds new parameters should add the
321appropriate entries to this file.
323Any new configuration options must be accompanied by help text which
324clearly explains the options and when the user might want to select them.
326Internal API information for many subsystems is documented by way of
327specially-formatted comments; these comments can be extracted and formatted
328in a number of ways by the "kernel-doc" script. If you are working within
329a subsystem which has kerneldoc comments, you should maintain them and add
330them, as appropriate, for externally-available functions. Even in areas
331which have not been so documented, there is no harm in adding kerneldoc
332comments for the future; indeed, this can be a useful activity for
333beginning kernel developers. The format of these comments, along with some
334information on how to create kerneldoc templates can be found in the file
337Anybody who reads through a significant amount of existing kernel code will
338note that, often, comments are most notable by their absence. Once again,
339the expectations for new code are higher than they were in the past;
340merging uncommented code will be harder. That said, there is little desire
341for verbosely-commented code. The code should, itself, be readable, with
342comments explaining the more subtle aspects.
344Certain things should always be commented. Uses of memory barriers should
345be accompanied by a line explaining why the barrier is necessary. The
346locking rules for data structures generally need to be explained somewhere.
347Major data structures need comprehensive documentation in general.
348Non-obvious dependencies between separate bits of code should be pointed
349out. Anything which might tempt a code janitor to make an incorrect
350"cleanup" needs a comment saying why it is done the way it is. And so on.
355The binary interface provided by the kernel to user space cannot be broken
356except under the most severe circumstances. The kernel's internal
357programming interfaces, instead, are highly fluid and can be changed when
358the need arises. If you find yourself having to work around a kernel API,
359or simply not using a specific functionality because it does not meet your
360needs, that may be a sign that the API needs to change. As a kernel
361developer, you are empowered to make such changes.
363There are, of course, some catches. API changes can be made, but they need
364to be well justified. So any patch making an internal API change should be
365accompanied by a description of what the change is and why it is
366necessary. This kind of change should also be broken out into a separate
367patch, rather than buried within a larger patch.
369The other catch is that a developer who changes an internal API is
370generally charged with the task of fixing any code within the kernel tree
371which is broken by the change. For a widely-used function, this duty can
372lead to literally hundreds or thousands of changes - many of which are
373likely to conflict with work being done by other developers. Needless to
374say, this can be a large job, so it is best to be sure that the
375justification is solid.
377When making an incompatible API change, one should, whenever possible,
378ensure that code which has not been updated is caught by the compiler.
379This will help you to be sure that you have found all in-tree uses of that
380interface. It will also alert developers of out-of-tree code that there is
381a change that they need to respond to. Supporting out-of-tree code is not
382something that kernel developers need to be worried about, but we also do
383not have to make life harder for out-of-tree developers than it needs to

Archive Download this file