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diff --git a/Documentation/kernel-hacking/hacking.rst b/Documentation/kernel-hacking/hacking.rst new file mode 100644 index 000000000000..daf3883b2694 --- /dev/null +++ b/Documentation/kernel-hacking/hacking.rst @@ -0,0 +1,811 @@ +============================================ +Unreliable Guide To Hacking The Linux Kernel +============================================ + +:Author: Rusty Russell + +Introduction +============ + +Welcome, gentle reader, to Rusty's Remarkably Unreliable Guide to Linux +Kernel Hacking. This document describes the common routines and general +requirements for kernel code: its goal is to serve as a primer for Linux +kernel development for experienced C programmers. I avoid implementation +details: that's what the code is for, and I ignore whole tracts of +useful routines. + +Before you read this, please understand that I never wanted to write +this document, being grossly under-qualified, but I always wanted to +read it, and this was the only way. I hope it will grow into a +compendium of best practice, common starting points and random +information. + +The Players +=========== + +At any time each of the CPUs in a system can be: + +- not associated with any process, serving a hardware interrupt; + +- not associated with any process, serving a softirq or tasklet; + +- running in kernel space, associated with a process (user context); + +- running a process in user space. + +There is an ordering between these. The bottom two can preempt each +other, but above that is a strict hierarchy: each can only be preempted +by the ones above it. For example, while a softirq is running on a CPU, +no other softirq will preempt it, but a hardware interrupt can. However, +any other CPUs in the system execute independently. + +We'll see a number of ways that the user context can block interrupts, +to become truly non-preemptable. + +User Context +------------ + +User context is when you are coming in from a system call or other trap: +like userspace, you can be preempted by more important tasks and by +interrupts. You can sleep, by calling :c:func:`schedule()`. + +.. note:: + + You are always in user context on module load and unload, and on + operations on the block device layer. + +In user context, the ``current`` pointer (indicating the task we are +currently executing) is valid, and :c:func:`in_interrupt()` +(``include/linux/preempt.h``) is false. + +.. warning:: + + Beware that if you have preemption or softirqs disabled (see below), + :c:func:`in_interrupt()` will return a false positive. + +Hardware Interrupts (Hard IRQs) +------------------------------- + +Timer ticks, network cards and keyboard are examples of real hardware +which produce interrupts at any time. The kernel runs interrupt +handlers, which services the hardware. The kernel guarantees that this +handler is never re-entered: if the same interrupt arrives, it is queued +(or dropped). Because it disables interrupts, this handler has to be +fast: frequently it simply acknowledges the interrupt, marks a 'software +interrupt' for execution and exits. + +You can tell you are in a hardware interrupt, because +:c:func:`in_irq()` returns true. + +.. warning:: + + Beware that this will return a false positive if interrupts are + disabled (see below). + +Software Interrupt Context: Softirqs and Tasklets +------------------------------------------------- + +Whenever a system call is about to return to userspace, or a hardware +interrupt handler exits, any 'software interrupts' which are marked +pending (usually by hardware interrupts) are run (``kernel/softirq.c``). + +Much of the real interrupt handling work is done here. Early in the +transition to SMP, there were only 'bottom halves' (BHs), which didn't +take advantage of multiple CPUs. Shortly after we switched from wind-up +computers made of match-sticks and snot, we abandoned this limitation +and switched to 'softirqs'. + +``include/linux/interrupt.h`` lists the different softirqs. A very +important softirq is the timer softirq (``include/linux/timer.h``): you +can register to have it call functions for you in a given length of +time. + +Softirqs are often a pain to deal with, since the same softirq will run +simultaneously on more than one CPU. For this reason, tasklets +(``include/linux/interrupt.h``) are more often used: they are +dynamically-registrable (meaning you can have as many as you want), and +they also guarantee that any tasklet will only run on one CPU at any +time, although different tasklets can run simultaneously. + +.. warning:: + + The name 'tasklet' is misleading: they have nothing to do with + 'tasks', and probably more to do with some bad vodka Alexey + Kuznetsov had at the time. + +You can tell you are in a softirq (or tasklet) using the +:c:func:`in_softirq()` macro (``include/linux/preempt.h``). + +.. warning:: + + Beware that this will return a false positive if a + :ref:`botton half lock <local_bh_disable>` is held. + +Some Basic Rules +================ + +No memory protection + If you corrupt memory, whether in user context or interrupt context, + the whole machine will crash. Are you sure you can't do what you + want in userspace? + +No floating point or MMX + The FPU context is not saved; even in user context the FPU state + probably won't correspond with the current process: you would mess + with some user process' FPU state. If you really want to do this, + you would have to explicitly save/restore the full FPU state (and + avoid context switches). It is generally a bad idea; use fixed point + arithmetic first. + +A rigid stack limit + Depending on configuration options the kernel stack is about 3K to + 6K for most 32-bit architectures: it's about 14K on most 64-bit + archs, and often shared with interrupts so you can't use it all. + Avoid deep recursion and huge local arrays on the stack (allocate + them dynamically instead). + +The Linux kernel is portable + Let's keep it that way. Your code should be 64-bit clean, and + endian-independent. You should also minimize CPU specific stuff, + e.g. inline assembly should be cleanly encapsulated and minimized to + ease porting. Generally it should be restricted to the + architecture-dependent part of the kernel tree. + +ioctls: Not writing a new system call +===================================== + +A system call generally looks like this:: + + asmlinkage long sys_mycall(int arg) + { + return 0; + } + + +First, in most cases you don't want to create a new system call. You +create a character device and implement an appropriate ioctl for it. +This is much more flexible than system calls, doesn't have to be entered +in every architecture's ``include/asm/unistd.h`` and +``arch/kernel/entry.S`` file, and is much more likely to be accepted by +Linus. + +If all your routine does is read or write some parameter, consider +implementing a :c:func:`sysfs()` interface instead. + +Inside the ioctl you're in user context to a process. When a error +occurs you return a negated errno (see +``include/uapi/asm-generic/errno-base.h``, +``include/uapi/asm-generic/errno.h`` and ``include/linux/errno.h``), +otherwise you return 0. + +After you slept you should check if a signal occurred: the Unix/Linux +way of handling signals is to temporarily exit the system call with the +``-ERESTARTSYS`` error. The system call entry code will switch back to +user context, process the signal handler and then your system call will +be restarted (unless the user disabled that). So you should be prepared +to process the restart, e.g. if you're in the middle of manipulating +some data structure. + +:: + + if (signal_pending(current)) + return -ERESTARTSYS; + + +If you're doing longer computations: first think userspace. If you +**really** want to do it in kernel you should regularly check if you need +to give up the CPU (remember there is cooperative multitasking per CPU). +Idiom:: + + cond_resched(); /* Will sleep */ + + +A short note on interface design: the UNIX system call motto is "Provide +mechanism not policy". + +Recipes for Deadlock +==================== + +You cannot call any routines which may sleep, unless: + +- You are in user context. + +- You do not own any spinlocks. + +- You have interrupts enabled (actually, Andi Kleen says that the + scheduling code will enable them for you, but that's probably not + what you wanted). + +Note that some functions may sleep implicitly: common ones are the user +space access functions (\*_user) and memory allocation functions +without ``GFP_ATOMIC``. + +You should always compile your kernel ``CONFIG_DEBUG_ATOMIC_SLEEP`` on, +and it will warn you if you break these rules. If you **do** break the +rules, you will eventually lock up your box. + +Really. + +Common Routines +=============== + +:c:func:`printk()` +------------------ + +Defined in ``include/linux/printk.h`` + +:c:func:`printk()` feeds kernel messages to the console, dmesg, and +the syslog daemon. It is useful for debugging and reporting errors, and +can be used inside interrupt context, but use with caution: a machine +which has its console flooded with printk messages is unusable. It uses +a format string mostly compatible with ANSI C printf, and C string +concatenation to give it a first "priority" argument:: + + printk(KERN_INFO "i = %u\n", i); + + +See ``include/linux/kern_levels.h``; for other ``KERN_`` values; these are +interpreted by syslog as the level. Special case: for printing an IP +address use:: + + __be32 ipaddress; + printk(KERN_INFO "my ip: %pI4\n", &ipaddress); + + +:c:func:`printk()` internally uses a 1K buffer and does not catch +overruns. Make sure that will be enough. + +.. note:: + + You will know when you are a real kernel hacker when you start + typoing printf as printk in your user programs :) + +.. note:: + + Another sidenote: the original Unix Version 6 sources had a comment + on top of its printf function: "Printf should not be used for + chit-chat". You should follow that advice. + +:c:func:`copy_to_user()` / :c:func:`copy_from_user()` / :c:func:`get_user()` / :c:func:`put_user()` +--------------------------------------------------------------------------------------------------- + +Defined in ``include/linux/uaccess.h`` / ``asm/uaccess.h`` + +**[SLEEPS]** + +:c:func:`put_user()` and :c:func:`get_user()` are used to get +and put single values (such as an int, char, or long) from and to +userspace. A pointer into userspace should never be simply dereferenced: +data should be copied using these routines. Both return ``-EFAULT`` or +0. + +:c:func:`copy_to_user()` and :c:func:`copy_from_user()` are +more general: they copy an arbitrary amount of data to and from +userspace. + +.. warning:: + + Unlike :c:func:`put_user()` and :c:func:`get_user()`, they + return the amount of uncopied data (ie. 0 still means success). + +[Yes, this moronic interface makes me cringe. The flamewar comes up +every year or so. --RR.] + +The functions may sleep implicitly. This should never be called outside +user context (it makes no sense), with interrupts disabled, or a +spinlock held. + +:c:func:`kmalloc()`/:c:func:`kfree()` +------------------------------------- + +Defined in ``include/linux/slab.h`` + +**[MAY SLEEP: SEE BELOW]** + +These routines are used to dynamically request pointer-aligned chunks of +memory, like malloc and free do in userspace, but +:c:func:`kmalloc()` takes an extra flag word. Important values: + +``GFP_KERNEL`` + May sleep and swap to free memory. Only allowed in user context, but + is the most reliable way to allocate memory. + +``GFP_ATOMIC`` + Don't sleep. Less reliable than ``GFP_KERNEL``, but may be called + from interrupt context. You should **really** have a good + out-of-memory error-handling strategy. + +``GFP_DMA`` + Allocate ISA DMA lower than 16MB. If you don't know what that is you + don't need it. Very unreliable. + +If you see a sleeping function called from invalid context warning +message, then maybe you called a sleeping allocation function from +interrupt context without ``GFP_ATOMIC``. You should really fix that. +Run, don't walk. + +If you are allocating at least ``PAGE_SIZE`` (``asm/page.h`` or +``asm/page_types.h``) bytes, consider using :c:func:`__get_free_pages()` +(``include/linux/gfp.h``). It takes an order argument (0 for page sized, +1 for double page, 2 for four pages etc.) and the same memory priority +flag word as above. + +If you are allocating more than a page worth of bytes you can use +:c:func:`vmalloc()`. It'll allocate virtual memory in the kernel +map. This block is not contiguous in physical memory, but the MMU makes +it look like it is for you (so it'll only look contiguous to the CPUs, +not to external device drivers). If you really need large physically +contiguous memory for some weird device, you have a problem: it is +poorly supported in Linux because after some time memory fragmentation +in a running kernel makes it hard. The best way is to allocate the block +early in the boot process via the :c:func:`alloc_bootmem()` +routine. + +Before inventing your own cache of often-used objects consider using a +slab cache in ``include/linux/slab.h`` + +:c:func:`current()` +------------------- + +Defined in ``include/asm/current.h`` + +This global variable (really a macro) contains a pointer to the current +task structure, so is only valid in user context. For example, when a +process makes a system call, this will point to the task structure of +the calling process. It is **not NULL** in interrupt context. + +:c:func:`mdelay()`/:c:func:`udelay()` +------------------------------------- + +Defined in ``include/asm/delay.h`` / ``include/linux/delay.h`` + +The :c:func:`udelay()` and :c:func:`ndelay()` functions can be +used for small pauses. Do not use large values with them as you risk +overflow - the helper function :c:func:`mdelay()` is useful here, or +consider :c:func:`msleep()`. + +:c:func:`cpu_to_be32()`/:c:func:`be32_to_cpu()`/:c:func:`cpu_to_le32()`/:c:func:`le32_to_cpu()` +----------------------------------------------------------------------------------------------- + +Defined in ``include/asm/byteorder.h`` + +The :c:func:`cpu_to_be32()` family (where the "32" can be replaced +by 64 or 16, and the "be" can be replaced by "le") are the general way +to do endian conversions in the kernel: they return the converted value. +All variations supply the reverse as well: +:c:func:`be32_to_cpu()`, etc. + +There are two major variations of these functions: the pointer +variation, such as :c:func:`cpu_to_be32p()`, which take a pointer +to the given type, and return the converted value. The other variation +is the "in-situ" family, such as :c:func:`cpu_to_be32s()`, which +convert value referred to by the pointer, and return void. + +:c:func:`local_irq_save()`/:c:func:`local_irq_restore()` +-------------------------------------------------------- + +Defined in ``include/linux/irqflags.h`` + +These routines disable hard interrupts on the local CPU, and restore +them. They are reentrant; saving the previous state in their one +``unsigned long flags`` argument. If you know that interrupts are +enabled, you can simply use :c:func:`local_irq_disable()` and +:c:func:`local_irq_enable()`. + +.. _local_bh_disable: + +:c:func:`local_bh_disable()`/:c:func:`local_bh_enable()` +-------------------------------------------------------- + +Defined in ``include/linux/bottom_half.h`` + + +These routines disable soft interrupts on the local CPU, and restore +them. They are reentrant; if soft interrupts were disabled before, they +will still be disabled after this pair of functions has been called. +They prevent softirqs and tasklets from running on the current CPU. + +:c:func:`smp_processor_id()` +---------------------------- + +Defined in ``include/linux/smp.h`` + +:c:func:`get_cpu()` disables preemption (so you won't suddenly get +moved to another CPU) and returns the current processor number, between +0 and ``NR_CPUS``. Note that the CPU numbers are not necessarily +continuous. You return it again with :c:func:`put_cpu()` when you +are done. + +If you know you cannot be preempted by another task (ie. you are in +interrupt context, or have preemption disabled) you can use +smp_processor_id(). + +``__init``/``__exit``/``__initdata`` +------------------------------------ + +Defined in ``include/linux/init.h`` + +After boot, the kernel frees up a special section; functions marked with +``__init`` and data structures marked with ``__initdata`` are dropped +after boot is complete: similarly modules discard this memory after +initialization. ``__exit`` is used to declare a function which is only +required on exit: the function will be dropped if this file is not +compiled as a module. See the header file for use. Note that it makes no +sense for a function marked with ``__init`` to be exported to modules +with :c:func:`EXPORT_SYMBOL()` or :c:func:`EXPORT_SYMBOL_GPL()`- this +will break. + +:c:func:`__initcall()`/:c:func:`module_init()` +---------------------------------------------- + +Defined in ``include/linux/init.h`` / ``include/linux/module.h`` + +Many parts of the kernel are well served as a module +(dynamically-loadable parts of the kernel). Using the +:c:func:`module_init()` and :c:func:`module_exit()` macros it +is easy to write code without #ifdefs which can operate both as a module +or built into the kernel. + +The :c:func:`module_init()` macro defines which function is to be +called at module insertion time (if the file is compiled as a module), +or at boot time: if the file is not compiled as a module the +:c:func:`module_init()` macro becomes equivalent to +:c:func:`__initcall()`, which through linker magic ensures that +the function is called on boot. + +The function can return a negative error number to cause module loading +to fail (unfortunately, this has no effect if the module is compiled +into the kernel). This function is called in user context with +interrupts enabled, so it can sleep. + +:c:func:`module_exit()` +----------------------- + + +Defined in ``include/linux/module.h`` + +This macro defines the function to be called at module removal time (or +never, in the case of the file compiled into the kernel). It will only +be called if the module usage count has reached zero. This function can +also sleep, but cannot fail: everything must be cleaned up by the time +it returns. + +Note that this macro is optional: if it is not present, your module will +not be removable (except for 'rmmod -f'). + +:c:func:`try_module_get()`/:c:func:`module_put()` +------------------------------------------------- + +Defined in ``include/linux/module.h`` + +These manipulate the module usage count, to protect against removal (a +module also can't be removed if another module uses one of its exported +symbols: see below). Before calling into module code, you should call +:c:func:`try_module_get()` on that module: if it fails, then the +module is being removed and you should act as if it wasn't there. +Otherwise, you can safely enter the module, and call +:c:func:`module_put()` when you're finished. + +Most registerable structures have an owner field, such as in the +:c:type:`struct file_operations <file_operations>` structure. +Set this field to the macro ``THIS_MODULE``. + +Wait Queues ``include/linux/wait.h`` +==================================== + +**[SLEEPS]** + +A wait queue is used to wait for someone to wake you up when a certain +condition is true. They must be used carefully to ensure there is no +race condition. You declare a :c:type:`wait_queue_head_t`, and then processes +which want to wait for that condition declare a :c:type:`wait_queue_entry_t` +referring to themselves, and place that in the queue. + +Declaring +--------- + +You declare a ``wait_queue_head_t`` using the +:c:func:`DECLARE_WAIT_QUEUE_HEAD()` macro, or using the +:c:func:`init_waitqueue_head()` routine in your initialization +code. + +Queuing +------- + +Placing yourself in the waitqueue is fairly complex, because you must +put yourself in the queue before checking the condition. There is a +macro to do this: :c:func:`wait_event_interruptible()` +(``include/linux/wait.h``) The first argument is the wait queue head, and +the second is an expression which is evaluated; the macro returns 0 when +this expression is true, or ``-ERESTARTSYS`` if a signal is received. The +:c:func:`wait_event()` version ignores signals. + +Waking Up Queued Tasks +---------------------- + +Call :c:func:`wake_up()` (``include/linux/wait.h``);, which will wake +up every process in the queue. The exception is if one has +``TASK_EXCLUSIVE`` set, in which case the remainder of the queue will +not be woken. There are other variants of this basic function available +in the same header. + +Atomic Operations +================= + +Certain operations are guaranteed atomic on all platforms. The first +class of operations work on :c:type:`atomic_t` (``include/asm/atomic.h``); +this contains a signed integer (at least 32 bits long), and you must use +these functions to manipulate or read :c:type:`atomic_t` variables. +:c:func:`atomic_read()` and :c:func:`atomic_set()` get and set +the counter, :c:func:`atomic_add()`, :c:func:`atomic_sub()`, +:c:func:`atomic_inc()`, :c:func:`atomic_dec()`, and +:c:func:`atomic_dec_and_test()` (returns true if it was +decremented to zero). + +Yes. It returns true (i.e. != 0) if the atomic variable is zero. + +Note that these functions are slower than normal arithmetic, and so +should not be used unnecessarily. + +The second class of atomic operations is atomic bit operations on an +``unsigned long``, defined in ``include/linux/bitops.h``. These +operations generally take a pointer to the bit pattern, and a bit +number: 0 is the least significant bit. :c:func:`set_bit()`, +:c:func:`clear_bit()` and :c:func:`change_bit()` set, clear, +and flip the given bit. :c:func:`test_and_set_bit()`, +:c:func:`test_and_clear_bit()` and +:c:func:`test_and_change_bit()` do the same thing, except return +true if the bit was previously set; these are particularly useful for +atomically setting flags. + +It is possible to call these operations with bit indices greater than +``BITS_PER_LONG``. The resulting behavior is strange on big-endian +platforms though so it is a good idea not to do this. + +Symbols +======= + +Within the kernel proper, the normal linking rules apply (ie. unless a +symbol is declared to be file scope with the ``static`` keyword, it can +be used anywhere in the kernel). However, for modules, a special +exported symbol table is kept which limits the entry points to the +kernel proper. Modules can also export symbols. + +:c:func:`EXPORT_SYMBOL()` +------------------------- + +Defined in ``include/linux/export.h`` + +This is the classic method of exporting a symbol: dynamically loaded +modules will be able to use the symbol as normal. + +:c:func:`EXPORT_SYMBOL_GPL()` +----------------------------- + +Defined in ``include/linux/export.h`` + +Similar to :c:func:`EXPORT_SYMBOL()` except that the symbols +exported by :c:func:`EXPORT_SYMBOL_GPL()` can only be seen by +modules with a :c:func:`MODULE_LICENSE()` that specifies a GPL +compatible license. It implies that the function is considered an +internal implementation issue, and not really an interface. Some +maintainers and developers may however require EXPORT_SYMBOL_GPL() +when adding any new APIs or functionality. + +Routines and Conventions +======================== + +Double-linked lists ``include/linux/list.h`` +-------------------------------------------- + +There used to be three sets of linked-list routines in the kernel +headers, but this one is the winner. If you don't have some particular +pressing need for a single list, it's a good choice. + +In particular, :c:func:`list_for_each_entry()` is useful. + +Return Conventions +------------------ + +For code called in user context, it's very common to defy C convention, +and return 0 for success, and a negative error number (eg. ``-EFAULT``) for +failure. This can be unintuitive at first, but it's fairly widespread in +the kernel. + +Using :c:func:`ERR_PTR()` (``include/linux/err.h``) to encode a +negative error number into a pointer, and :c:func:`IS_ERR()` and +:c:func:`PTR_ERR()` to get it back out again: avoids a separate +pointer parameter for the error number. Icky, but in a good way. + +Breaking Compilation +-------------------- + +Linus and the other developers sometimes change function or structure +names in development kernels; this is not done just to keep everyone on +their toes: it reflects a fundamental change (eg. can no longer be +called with interrupts on, or does extra checks, or doesn't do checks +which were caught before). Usually this is accompanied by a fairly +complete note to the linux-kernel mailing list; search the archive. +Simply doing a global replace on the file usually makes things **worse**. + +Initializing structure members +------------------------------ + +The preferred method of initializing structures is to use designated +initialisers, as defined by ISO C99, eg:: + + static struct block_device_operations opt_fops = { + .open = opt_open, + .release = opt_release, + .ioctl = opt_ioctl, + .check_media_change = opt_media_change, + }; + + +This makes it easy to grep for, and makes it clear which structure +fields are set. You should do this because it looks cool. + +GNU Extensions +-------------- + +GNU Extensions are explicitly allowed in the Linux kernel. Note that +some of the more complex ones are not very well supported, due to lack +of general use, but the following are considered standard (see the GCC +info page section "C Extensions" for more details - Yes, really the info +page, the man page is only a short summary of the stuff in info). + +- Inline functions + +- Statement expressions (ie. the ({ and }) constructs). + +- Declaring attributes of a function / variable / type + (__attribute__) + +- typeof + +- Zero length arrays + +- Macro varargs + +- Arithmetic on void pointers + +- Non-Constant initializers + +- Assembler Instructions (not outside arch/ and include/asm/) + +- Function names as strings (__func__). + +- __builtin_constant_p() + +Be wary when using long long in the kernel, the code gcc generates for +it is horrible and worse: division and multiplication does not work on +i386 because the GCC runtime functions for it are missing from the +kernel environment. + +C++ +--- + +Using C++ in the kernel is usually a bad idea, because the kernel does +not provide the necessary runtime environment and the include files are +not tested for it. It is still possible, but not recommended. If you +really want to do this, forget about exceptions at least. + +NUMif +----- + +It is generally considered cleaner to use macros in header files (or at +the top of .c files) to abstract away functions rather than using \`#if' +pre-processor statements throughout the source code. + +Putting Your Stuff in the Kernel +================================ + +In order to get your stuff into shape for official inclusion, or even to +make a neat patch, there's administrative work to be done: + +- Figure out whose pond you've been pissing in. Look at the top of the + source files, inside the ``MAINTAINERS`` file, and last of all in the + ``CREDITS`` file. You should coordinate with this person to make sure + you're not duplicating effort, or trying something that's already + been rejected. + + Make sure you put your name and EMail address at the top of any files + you create or mangle significantly. This is the first place people + will look when they find a bug, or when **they** want to make a change. + +- Usually you want a configuration option for your kernel hack. Edit + ``Kconfig`` in the appropriate directory. The Config language is + simple to use by cut and paste, and there's complete documentation in + ``Documentation/kbuild/kconfig-language.txt``. + + In your description of the option, make sure you address both the + expert user and the user who knows nothing about your feature. + Mention incompatibilities and issues here. **Definitely** end your + description with “if in doubt, say N” (or, occasionally, \`Y'); this + is for people who have no idea what you are talking about. + +- Edit the ``Makefile``: the CONFIG variables are exported here so you + can usually just add a "obj-$(CONFIG_xxx) += xxx.o" line. The syntax + is documented in ``Documentation/kbuild/makefiles.txt``. + +- Put yourself in ``CREDITS`` if you've done something noteworthy, + usually beyond a single file (your name should be at the top of the + source files anyway). ``MAINTAINERS`` means you want to be consulted + when changes are made to a subsystem, and hear about bugs; it implies + a more-than-passing commitment to some part of the code. + +- Finally, don't forget to read + ``Documentation/process/submitting-patches.rst`` and possibly + ``Documentation/process/submitting-drivers.rst``. + +Kernel Cantrips +=============== + +Some favorites from browsing the source. Feel free to add to this list. + +``arch/x86/include/asm/delay.h``:: + + #define ndelay(n) (__builtin_constant_p(n) ? \ + ((n) > 20000 ? __bad_ndelay() : __const_udelay((n) * 5ul)) : \ + __ndelay(n)) + + +``include/linux/fs.h``:: + + /* + * Kernel pointers have redundant information, so we can use a + * scheme where we can return either an error code or a dentry + * pointer with the same return value. + * + * This should be a per-architecture thing, to allow different + * error and pointer decisions. + */ + #define ERR_PTR(err) ((void *)((long)(err))) + #define PTR_ERR(ptr) ((long)(ptr)) + #define IS_ERR(ptr) ((unsigned long)(ptr) > (unsigned long)(-1000)) + +``arch/x86/include/asm/uaccess_32.h:``:: + + #define copy_to_user(to,from,n) \ + (__builtin_constant_p(n) ? \ + __constant_copy_to_user((to),(from),(n)) : \ + __generic_copy_to_user((to),(from),(n))) + + +``arch/sparc/kernel/head.S:``:: + + /* + * Sun people can't spell worth damn. "compatability" indeed. + * At least we *know* we can't spell, and use a spell-checker. + */ + + /* Uh, actually Linus it is I who cannot spell. Too much murky + * Sparc assembly will do this to ya. + */ + C_LABEL(cputypvar): + .asciz "compatibility" + + /* Tested on SS-5, SS-10. Probably someone at Sun applied a spell-checker. */ + .align 4 + C_LABEL(cputypvar_sun4m): + .asciz "compatible" + + +``arch/sparc/lib/checksum.S:``:: + + /* Sun, you just can't beat me, you just can't. Stop trying, + * give up. I'm serious, I am going to kick the living shit + * out of you, game over, lights out. + */ + + +Thanks +====== + +Thanks to Andi Kleen for the idea, answering my questions, fixing my +mistakes, filling content, etc. Philipp Rumpf for more spelling and +clarity fixes, and some excellent non-obvious points. Werner Almesberger +for giving me a great summary of :c:func:`disable_irq()`, and Jes +Sorensen and Andrea Arcangeli added caveats. Michael Elizabeth Chastain +for checking and adding to the Configure section. Telsa Gwynne for +teaching me DocBook. |