1.. _development_coding: 2 3Getting the code right 4====================== 5 6While there is much to be said for a solid and community-oriented design 7process, the proof of any kernel development project is in the resulting 8code. It is the code which will be examined by other developers and merged 9(or not) into the mainline tree. So it is the quality of this code which 10will determine the ultimate success of the project. 11 12This section will examine the coding process. We'll start with a look at a 13number of ways in which kernel developers can go wrong. Then the focus 14will shift toward doing things right and the tools which can help in that 15quest. 16 17 18Pitfalls 19--------- 20 21Coding style 22************ 23 24The kernel has long had a standard coding style, described in 25:ref:`Documentation/process/coding-style.rst <codingstyle>`. For much of 26that time, the policies described in that file were taken as being, at most, 27advisory. As a result, there is a substantial amount of code in the kernel 28which does not meet the coding style guidelines. The presence of that code 29leads to two independent hazards for kernel developers. 30 31The first of these is to believe that the kernel coding standards do not 32matter and are not enforced. The truth of the matter is that adding new 33code to the kernel is very difficult if that code is not coded according to 34the standard; many developers will request that the code be reformatted 35before they will even review it. A code base as large as the kernel 36requires some uniformity of code to make it possible for developers to 37quickly understand any part of it. So there is no longer room for 38strangely-formatted code. 39 40Occasionally, the kernel's coding style will run into conflict with an 41employer's mandated style. In such cases, the kernel's style will have to 42win before the code can be merged. Putting code into the kernel means 43giving up a degree of control in a number of ways - including control over 44how the code is formatted. 45 46The other trap is to assume that code which is already in the kernel is 47urgently in need of coding style fixes. Developers may start to generate 48reformatting patches as a way of gaining familiarity with the process, or 49as a way of getting their name into the kernel changelogs - or both. But 50pure coding style fixes are seen as noise by the development community; 51they tend to get a chilly reception. So this type of patch is best 52avoided. It is natural to fix the style of a piece of code while working 53on it for other reasons, but coding style changes should not be made for 54their own sake. 55 56The coding style document also should not be read as an absolute law which 57can never be transgressed. If there is a good reason to go against the 58style (a line which becomes far less readable if split to fit within the 5980-column limit, for example), just do it. 60 61 62Abstraction layers 63****************** 64 65Computer Science professors teach students to make extensive use of 66abstraction layers in the name of flexibility and information hiding. 67Certainly the kernel makes extensive use of abstraction; no project 68involving several million lines of code could do otherwise and survive. 69But experience has shown that excessive or premature abstraction can be 70just as harmful as premature optimization. Abstraction should be used to 71the level required and no further. 72 73At a simple level, consider a function which has an argument which is 74always passed as zero by all callers. One could retain that argument just 75in case somebody eventually needs to use the extra flexibility that it 76provides. By that time, though, chances are good that the code which 77implements this extra argument has been broken in some subtle way which was 78never noticed - because it has never been used. Or, when the need for 79extra flexibility arises, it does not do so in a way which matches the 80programmer's early expectation. Kernel developers will routinely submit 81patches to remove unused arguments; they should, in general, not be added 82in the first place. 83 84Abstraction layers which hide access to hardware - often to allow the bulk 85of a driver to be used with multiple operating systems - are especially 86frowned upon. Such layers obscure the code and may impose a performance 87penalty; they do not belong in the Linux kernel. 88 89On the other hand, if you find yourself copying significant amounts of code 90from another kernel subsystem, it is time to ask whether it would, in fact, 91make sense to pull out some of that code into a separate library or to 92implement that functionality at a higher level. There is no value in 93replicating the same code throughout the kernel. 94 95 96#ifdef and preprocessor use in general 97************************************** 98 99The C preprocessor seems to present a powerful temptation to some C 100programmers, who see it as a way to efficiently encode a great deal of 101flexibility into a source file. But the preprocessor is not C, and heavy 102use of it results in code which is much harder for others to read and 103harder for the compiler to check for correctness. Heavy preprocessor use 104is almost always a sign of code which needs some cleanup work. 105 106Conditional compilation with #ifdef is, indeed, a powerful feature, and it 107is used within the kernel. But there is little desire to see code which is 108sprinkled liberally with #ifdef blocks. As a general rule, #ifdef use 109should be confined to header files whenever possible. 110Conditionally-compiled code can be confined to functions which, if the code 111is not to be present, simply become empty. The compiler will then quietly 112optimize out the call to the empty function. The result is far cleaner 113code which is easier to follow. 114 115C preprocessor macros present a number of hazards, including possible 116multiple evaluation of expressions with side effects and no type safety. 117If you are tempted to define a macro, consider creating an inline function 118instead. The code which results will be the same, but inline functions are 119easier to read, do not evaluate their arguments multiple times, and allow 120the compiler to perform type checking on the arguments and return value. 121 122 123Inline functions 124**************** 125 126Inline functions present a hazard of their own, though. Programmers can 127become enamored of the perceived efficiency inherent in avoiding a function 128call and fill a source file with inline functions. Those functions, 129however, can actually reduce performance. Since their code is replicated 130at each call site, they end up bloating the size of the compiled kernel. 131That, in turn, creates pressure on the processor's memory caches, which can 132slow execution dramatically. Inline functions, as a rule, should be quite 133small and relatively rare. The cost of a function call, after all, is not 134that high; the creation of large numbers of inline functions is a classic 135example of premature optimization. 136 137In general, kernel programmers ignore cache effects at their peril. The 138classic time/space tradeoff taught in beginning data structures classes 139often does not apply to contemporary hardware. Space *is* time, in that a 140larger program will run slower than one which is more compact. 141 142More recent compilers take an increasingly active role in deciding whether 143a given function should actually be inlined or not. So the liberal 144placement of "inline" keywords may not just be excessive; it could also be 145irrelevant. 146 147 148Locking 149******* 150 151In May, 2006, the "Devicescape" networking stack was, with great 152fanfare, released under the GPL and made available for inclusion in the 153mainline kernel. This donation was welcome news; support for wireless 154networking in Linux was considered substandard at best, and the Devicescape 155stack offered the promise of fixing that situation. Yet, this code did not 156actually make it into the mainline until June, 2007 (2.6.22). What 157happened? 158 159This code showed a number of signs of having been developed behind 160corporate doors. But one large problem in particular was that it was not 161designed to work on multiprocessor systems. Before this networking stack 162(now called mac80211) could be merged, a locking scheme needed to be 163retrofitted onto it. 164 165Once upon a time, Linux kernel code could be developed without thinking 166about the concurrency issues presented by multiprocessor systems. Now, 167however, this document is being written on a dual-core laptop. Even on 168single-processor systems, work being done to improve responsiveness will 169raise the level of concurrency within the kernel. The days when kernel 170code could be written without thinking about locking are long past. 171 172Any resource (data structures, hardware registers, etc.) which could be 173accessed concurrently by more than one thread must be protected by a lock. 174New code should be written with this requirement in mind; retrofitting 175locking after the fact is a rather more difficult task. Kernel developers 176should take the time to understand the available locking primitives well 177enough to pick the right tool for the job. Code which shows a lack of 178attention to concurrency will have a difficult path into the mainline. 179 180 181Regressions 182*********** 183 184One final hazard worth mentioning is this: it can be tempting to make a 185change (which may bring big improvements) which causes something to break 186for existing users. This kind of change is called a "regression," and 187regressions have become most unwelcome in the mainline kernel. With few 188exceptions, changes which cause regressions will be backed out if the 189regression cannot be fixed in a timely manner. Far better to avoid the 190regression in the first place. 191 192It is often argued that a regression can be justified if it causes things 193to work for more people than it creates problems for. Why not make a 194change if it brings new functionality to ten systems for each one it 195breaks? The best answer to this question was expressed by Linus in July, 1962007: 197 198:: 199 200 So we don't fix bugs by introducing new problems. That way lies 201 madness, and nobody ever knows if you actually make any real 202 progress at all. Is it two steps forwards, one step back, or one 203 step forward and two steps back? 204 205(http://lwn.net/Articles/243460/). 206 207An especially unwelcome type of regression is any sort of change to the 208user-space ABI. Once an interface has been exported to user space, it must 209be supported indefinitely. This fact makes the creation of user-space 210interfaces particularly challenging: since they cannot be changed in 211incompatible ways, they must be done right the first time. For this 212reason, a great deal of thought, clear documentation, and wide review for 213user-space interfaces is always required. 214 215 216Code checking tools 217------------------- 218 219For now, at least, the writing of error-free code remains an ideal that few 220of us can reach. What we can hope to do, though, is to catch and fix as 221many of those errors as possible before our code goes into the mainline 222kernel. To that end, the kernel developers have put together an impressive 223array of tools which can catch a wide variety of obscure problems in an 224automated way. Any problem caught by the computer is a problem which will 225not afflict a user later on, so it stands to reason that the automated 226tools should be used whenever possible. 227 228The first step is simply to heed the warnings produced by the compiler. 229Contemporary versions of gcc can detect (and warn about) a large number of 230potential errors. Quite often, these warnings point to real problems. 231Code submitted for review should, as a rule, not produce any compiler 232warnings. When silencing warnings, take care to understand the real cause 233and try to avoid "fixes" which make the warning go away without addressing 234its cause. 235 236Note that not all compiler warnings are enabled by default. Build the 237kernel with "make EXTRA_CFLAGS=-W" to get the full set. 238 239The kernel provides several configuration options which turn on debugging 240features; most of these are found in the "kernel hacking" submenu. Several 241of these options should be turned on for any kernel used for development or 242testing purposes. In particular, you should turn on: 243 244 - ENABLE_WARN_DEPRECATED, ENABLE_MUST_CHECK, and FRAME_WARN to get an 245 extra set of warnings for problems like the use of deprecated interfaces 246 or ignoring an important return value from a function. The output 247 generated by these warnings can be verbose, but one need not worry about 248 warnings from other parts of the kernel. 249 250 - DEBUG_OBJECTS will add code to track the lifetime of various objects 251 created by the kernel and warn when things are done out of order. If 252 you are adding a subsystem which creates (and exports) complex objects 253 of its own, consider adding support for the object debugging 254 infrastructure. 255 256 - DEBUG_SLAB can find a variety of memory allocation and use errors; it 257 should be used on most development kernels. 258 259 - DEBUG_SPINLOCK, DEBUG_ATOMIC_SLEEP, and DEBUG_MUTEXES will find a 260 number of common locking errors. 261 262There are quite a few other debugging options, some of which will be 263discussed below. Some of them have a significant performance impact and 264should not be used all of the time. But some time spent learning the 265available options will likely be paid back many times over in short order. 266 267One of the heavier debugging tools is the locking checker, or "lockdep." 268This tool will track the acquisition and release of every lock (spinlock or 269mutex) in the system, the order in which locks are acquired relative to 270each other, the current interrupt environment, and more. It can then 271ensure that locks are always acquired in the same order, that the same 272interrupt assumptions apply in all situations, and so on. In other words, 273lockdep can find a number of scenarios in which the system could, on rare 274occasion, deadlock. This kind of problem can be painful (for both 275developers and users) in a deployed system; lockdep allows them to be found 276in an automated manner ahead of time. Code with any sort of non-trivial 277locking should be run with lockdep enabled before being submitted for 278inclusion. 279 280As a diligent kernel programmer, you will, beyond doubt, check the return 281status of any operation (such as a memory allocation) which can fail. The 282fact of the matter, though, is that the resulting failure recovery paths 283are, probably, completely untested. Untested code tends to be broken code; 284you could be much more confident of your code if all those error-handling 285paths had been exercised a few times. 286 287The kernel provides a fault injection framework which can do exactly that, 288especially where memory allocations are involved. With fault injection 289enabled, a configurable percentage of memory allocations will be made to 290fail; these failures can be restricted to a specific range of code. 291Running with fault injection enabled allows the programmer to see how the 292code responds when things go badly. See 293Documentation/fault-injection/fault-injection.txt for more information on 294how to use this facility. 295 296Other kinds of errors can be found with the "sparse" static analysis tool. 297With sparse, the programmer can be warned about confusion between 298user-space and kernel-space addresses, mixture of big-endian and 299small-endian quantities, the passing of integer values where a set of bit 300flags is expected, and so on. Sparse must be installed separately (it can 301be found at https://sparse.wiki.kernel.org/index.php/Main_Page if your 302distributor does not package it); it can then be run on the code by adding 303"C=1" to your make command. 304 305The "Coccinelle" tool (http://coccinelle.lip6.fr/) is able to find a wide 306variety of potential coding problems; it can also propose fixes for those 307problems. Quite a few "semantic patches" for the kernel have been packaged 308under the scripts/coccinelle directory; running "make coccicheck" will run 309through those semantic patches and report on any problems found. See 310Documentation/dev-tools/coccinelle.rst for more information. 311 312Other kinds of portability errors are best found by compiling your code for 313other architectures. If you do not happen to have an S/390 system or a 314Blackfin development board handy, you can still perform the compilation 315step. A large set of cross compilers for x86 systems can be found at 316 317 http://www.kernel.org/pub/tools/crosstool/ 318 319Some time spent installing and using these compilers will help avoid 320embarrassment later. 321 322 323Documentation 324------------- 325 326Documentation has often been more the exception than the rule with kernel 327development. Even so, adequate documentation will help to ease the merging 328of new code into the kernel, make life easier for other developers, and 329will be helpful for your users. In many cases, the addition of 330documentation has become essentially mandatory. 331 332The first piece of documentation for any patch is its associated 333changelog. Log entries should describe the problem being solved, the form 334of the solution, the people who worked on the patch, any relevant 335effects on performance, and anything else that might be needed to 336understand the patch. Be sure that the changelog says *why* the patch is 337worth applying; a surprising number of developers fail to provide that 338information. 339 340Any code which adds a new user-space interface - including new sysfs or 341/proc files - should include documentation of that interface which enables 342user-space developers to know what they are working with. See 343Documentation/ABI/README for a description of how this documentation should 344be formatted and what information needs to be provided. 345 346The file :ref:`Documentation/admin-guide/kernel-parameters.rst 347<kernelparameters>` describes all of the kernel's boot-time parameters. 348Any patch which adds new parameters should add the appropriate entries to 349this file. 350 351Any new configuration options must be accompanied by help text which 352clearly explains the options and when the user might want to select them. 353 354Internal API information for many subsystems is documented by way of 355specially-formatted comments; these comments can be extracted and formatted 356in a number of ways by the "kernel-doc" script. If you are working within 357a subsystem which has kerneldoc comments, you should maintain them and add 358them, as appropriate, for externally-available functions. Even in areas 359which have not been so documented, there is no harm in adding kerneldoc 360comments for the future; indeed, this can be a useful activity for 361beginning kernel developers. The format of these comments, along with some 362information on how to create kerneldoc templates can be found at 363:ref:`Documentation/doc-guide/ <doc_guide>`. 364 365Anybody who reads through a significant amount of existing kernel code will 366note that, often, comments are most notable by their absence. Once again, 367the expectations for new code are higher than they were in the past; 368merging uncommented code will be harder. That said, there is little desire 369for verbosely-commented code. The code should, itself, be readable, with 370comments explaining the more subtle aspects. 371 372Certain things should always be commented. Uses of memory barriers should 373be accompanied by a line explaining why the barrier is necessary. The 374locking rules for data structures generally need to be explained somewhere. 375Major data structures need comprehensive documentation in general. 376Non-obvious dependencies between separate bits of code should be pointed 377out. Anything which might tempt a code janitor to make an incorrect 378"cleanup" needs a comment saying why it is done the way it is. And so on. 379 380 381Internal API changes 382-------------------- 383 384The binary interface provided by the kernel to user space cannot be broken 385except under the most severe circumstances. The kernel's internal 386programming interfaces, instead, are highly fluid and can be changed when 387the need arises. If you find yourself having to work around a kernel API, 388or simply not using a specific functionality because it does not meet your 389needs, that may be a sign that the API needs to change. As a kernel 390developer, you are empowered to make such changes. 391 392There are, of course, some catches. API changes can be made, but they need 393to be well justified. So any patch making an internal API change should be 394accompanied by a description of what the change is and why it is 395necessary. This kind of change should also be broken out into a separate 396patch, rather than buried within a larger patch. 397 398The other catch is that a developer who changes an internal API is 399generally charged with the task of fixing any code within the kernel tree 400which is broken by the change. For a widely-used function, this duty can 401lead to literally hundreds or thousands of changes - many of which are 402likely to conflict with work being done by other developers. Needless to 403say, this can be a large job, so it is best to be sure that the 404justification is solid. Note that the Coccinelle tool can help with 405wide-ranging API changes. 406 407When making an incompatible API change, one should, whenever possible, 408ensure that code which has not been updated is caught by the compiler. 409This will help you to be sure that you have found all in-tree uses of that 410interface. It will also alert developers of out-of-tree code that there is 411a change that they need to respond to. Supporting out-of-tree code is not 412something that kernel developers need to be worried about, but we also do 413not have to make life harder for out-of-tree developers than it needs to 414be. 415