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 byinterrupts. You can sleep, by callingschedule().

In user context, thecurrent pointer (indicating the task we arecurrently executing) is valid, andin_interrupt()(include/linux/preempt.h) is false.

Hardware Interrupts (Hard IRQs)¶

Timer ticks, network cards and keyboard are examples of real hardwarewhich produce interrupts at any time. The kernel runs interrupthandlers, which services the hardware. The kernel guarantees that thishandler is never re-entered: if the same interrupt arrives, it is queued(or dropped). Because it disables interrupts, this handler has to befast: frequently it simply acknowledges the interrupt, marks a ‘softwareinterrupt’ for execution and exits.

You can tell you are in a hardware interrupt, becausein_irq() returns true.

Software Interrupt Context: Softirqs and Tasklets¶

Whenever a system call is about to return to userspace, or a hardwareinterrupt handler exits, any ‘software interrupts’ which are markedpending (usually by hardware interrupts) are run (kernel/softirq.c).

Much of the real interrupt handling work is done here. Early in thetransition to SMP, there were only ‘bottom halves’ (BHs), which didn’ttake advantage of multiple CPUs. Shortly after we switched from wind-upcomputers made of match-sticks and snot, we abandoned this limitationand switched to ‘softirqs’.

include/linux/interrupt.h lists the different softirqs. A veryimportant softirq is the timer softirq (include/linux/timer.h): youcan register to have it call functions for you in a given length oftime.

Softirqs are often a pain to deal with, since the same softirq will runsimultaneously on more than one CPU. For this reason, tasklets(include/linux/interrupt.h) are more often used: they aredynamically-registrable (meaning you can have as many as you want), andthey also guarantee that any tasklet will only run on one CPU at anytime, although different tasklets can run simultaneously.

You can tell you are in a softirq (or tasklet) using thein_softirq() macro (include/linux/preempt.h).

printk()¶

Defined ininclude/linux/printk.h

printk() feeds kernel messages to the console, dmesg, andthe syslog daemon. It is useful for debugging and reporting errors, andcan be used inside interrupt context, but use with caution: a machinewhich has its console flooded with printk messages is unusable. It usesa format string mostly compatible with ANSI C printf, and C stringconcatenation to give it a first “priority” argument:

printk(KERN_INFO "i = %u\n", i);

Seeinclude/linux/kern_levels.h; for otherKERN_ values; these areinterpreted by syslog as the level. Special case: for printing an IPaddress use:

__be32 ipaddress;printk(KERN_INFO "my ip: %pI4\n", &ipaddress);

printk() internally uses a 1K buffer and does not catchoverruns. Make sure that will be enough.

copy_to_user() /copy_from_user() /get_user() /put_user()¶

Defined ininclude/linux/uaccess.h /asm/uaccess.h

[SLEEPS]

put_user() andget_user() are used to getand put single values (such as an int, char, or long) from and touserspace. A pointer into userspace should never be simply dereferenced:data should be copied using these routines. Both return-EFAULT or0.

copy_to_user() andcopy_from_user() aremore general: they copy an arbitrary amount of data to and fromuserspace.

[Yes, this moronic interface makes me cringe. The flamewar comes upevery year or so. –RR.]

The functions may sleep implicitly. This should never be called outsideuser context (it makes no sense), with interrupts disabled, or aspinlock held.

kmalloc()/kfree()¶

Defined ininclude/linux/slab.h

[MAY SLEEP: SEE BELOW]

These routines are used to dynamically request pointer-aligned chunks ofmemory, like malloc and free do in userspace, butkmalloc() takes an extra flag word. Important values:

GFP_KERNEL

May sleep and swap to free memory. Only allowed in user context, butis the most reliable way to allocate memory.

GFP_ATOMIC

Don’t sleep. Less reliable thanGFP_KERNEL, but may be calledfrom interrupt context. You shouldreally have a goodout-of-memory error-handling strategy.

GFP_DMA

Allocate ISA DMA lower than 16MB. If you don’t know what that is youdon’t need it. Very unreliable.

If you see a sleeping function called from invalid context warningmessage, then maybe you called a sleeping allocation function frominterrupt context withoutGFP_ATOMIC. You should really fix that.Run, don’t walk.

If you are allocating at leastPAGE_SIZE (asm/page.h orasm/page_types.h) bytes, consider using__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 priorityflag word as above.

If you are allocating more than a page worth of bytes you can usevmalloc(). It’ll allocate virtual memory in the kernelmap. This block is not contiguous in physical memory, but the MMU makesit 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 physicallycontiguous memory for some weird device, you have a problem: it ispoorly supported in Linux because after some time memory fragmentationin a running kernel makes it hard. The best way is to allocate the blockearly in the boot process via thealloc_bootmem()routine.

Before inventing your own cache of often-used objects consider using aslab cache ininclude/linux/slab.h

current()¶

Defined ininclude/asm/current.h

This global variable (really a macro) contains a pointer to the currenttask structure, so is only valid in user context. For example, when aprocess makes a system call, this will point to the task structure ofthe calling process. It isnot NULL in interrupt context.

mdelay()/udelay()¶

Defined ininclude/asm/delay.h /include/linux/delay.h

Theudelay() andndelay() functions can beused for small pauses. Do not use large values with them as you riskoverflow - the helper functionmdelay() is useful here, orconsidermsleep().

cpu_to_be32()/be32_to_cpu()/cpu_to_le32()/le32_to_cpu()¶

Defined ininclude/asm/byteorder.h

Thecpu_to_be32() family (where the “32” can be replacedby 64 or 16, and the “be” can be replaced by “le”) are the general wayto do endian conversions in the kernel: they return the converted value.All variations supply the reverse as well:be32_to_cpu(), etc.

There are two major variations of these functions: the pointervariation, such ascpu_to_be32p(), which take a pointerto the given type, and return the converted value. The other variationis the “in-situ” family, such ascpu_to_be32s(), whichconvert value referred to by the pointer, and return void.

local_irq_save()/local_irq_restore()¶

Defined ininclude/linux/irqflags.h

These routines disable hard interrupts on the local CPU, and restorethem. They are reentrant; saving the previous state in their oneunsignedlongflags argument. If you know that interrupts areenabled, you can simply uselocal_irq_disable() andlocal_irq_enable().

local_bh_disable()/local_bh_enable()¶

Defined ininclude/linux/bottom_half.h

These routines disable soft interrupts on the local CPU, and restorethem. They are reentrant; if soft interrupts were disabled before, theywill still be disabled after this pair of functions has been called.They prevent softirqs and tasklets from running on the current CPU.

smp_processor_id()¶

Defined ininclude/linux/smp.h

get_cpu() disables preemption (so you won’t suddenly getmoved to another CPU) and returns the current processor number, between0 andNR_CPUS. Note that the CPU numbers are not necessarilycontinuous. You return it again withput_cpu() when youare done.

If you know you cannot be preempted by another task (ie. you are ininterrupt context, or have preemption disabled) you can usesmp_processor_id().

__init/__exit/__initdata¶

Defined ininclude/linux/init.h

After boot, the kernel frees up a special section; functions marked with__init and data structures marked with__initdata are droppedafter boot is complete: similarly modules discard this memory afterinitialization.__exit is used to declare a function which is onlyrequired on exit: the function will be dropped if this file is notcompiled as a module. See the header file for use. Note that it makes nosense for a function marked with__init to be exported to moduleswithEXPORT_SYMBOL() orEXPORT_SYMBOL_GPL()- thiswill break.

__initcall()/module_init()¶

Defined ininclude/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 themodule_init() andmodule_exit() macros itis easy to write code without #ifdefs which can operate both as a moduleor built into the kernel.

Themodule_init() macro defines which function is to becalled 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 themodule_init() macro becomes equivalent to__initcall(), which through linker magic ensures thatthe function is called on boot.

The function can return a negative error number to cause module loadingto fail (unfortunately, this has no effect if the module is compiledinto the kernel). This function is called in user context withinterrupts enabled, so it can sleep.

module_exit()¶

Defined ininclude/linux/module.h

This macro defines the function to be called at module removal time (ornever, in the case of the file compiled into the kernel). It will onlybe called if the module usage count has reached zero. This function canalso sleep, but cannot fail: everything must be cleaned up by the timeit returns.

Note that this macro is optional: if it is not present, your module willnot be removable (except for ‘rmmod -f’).

try_module_get()/module_put()¶

Defined ininclude/linux/module.h

These manipulate the module usage count, to protect against removal (amodule also can’t be removed if another module uses one of its exportedsymbols: see below). Before calling into module code, you should calltry_module_get() on that module: if it fails, then themodule is being removed and you should act as if it wasn’t there.Otherwise, you can safely enter the module, and callmodule_put() when you’re finished.

Most registerable structures have an owner field, such as in thestructfile_operations structure.Set this field to the macroTHIS_MODULE.

Declaring¶

You declare await_queue_head_t using theDECLARE_WAIT_QUEUE_HEAD() macro, or using theinit_waitqueue_head() routine in your initializationcode.

Queuing¶

Placing yourself in the waitqueue is fairly complex, because you mustput yourself in the queue before checking the condition. There is amacro to do this:wait_event_interruptible()(include/linux/wait.h) The first argument is the wait queue head, andthe second is an expression which is evaluated; the macro returns 0 whenthis expression is true, or-ERESTARTSYS if a signal is received. Thewait_event() version ignores signals.

Waking Up Queued Tasks¶

Callwake_up() (include/linux/wait.h), which will wakeup every process in the queue. The exception is if one hasTASK_EXCLUSIVE set, in which case the remainder of the queue willnot be woken. There are other variants of this basic function availablein the same header.

EXPORT_SYMBOL()¶

Defined ininclude/linux/export.h

This is the classic method of exporting a symbol: dynamically loadedmodules will be able to use the symbol as normal.

EXPORT_SYMBOL_GPL()¶

Defined ininclude/linux/export.h

Similar toEXPORT_SYMBOL() except that the symbolsexported byEXPORT_SYMBOL_GPL() can only be seen bymodules with aMODULE_LICENSE() that specifies a GPLcompatible license. It implies that the function is considered aninternal implementation issue, and not really an interface. Somemaintainers and developers may however require EXPORT_SYMBOL_GPL()when adding any new APIs or functionality.

EXPORT_SYMBOL_NS()¶

Defined ininclude/linux/export.h

This is the variant ofEXPORT_SYMBOL() that allows specifying a symbolnamespace. Symbol Namespaces are documented inSymbol Namespaces

EXPORT_SYMBOL_NS_GPL()¶

Defined ininclude/linux/export.h

This is the variant ofEXPORT_SYMBOL_GPL() that allows specifying a symbolnamespace. Symbol Namespaces are documented inSymbol Namespaces

Double-linked listsinclude/linux/list.h¶

There used to be three sets of linked-list routines in the kernelheaders, but this one is the winner. If you don’t have some particularpressing need for a single list, it’s a good choice.

In particular,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) forfailure. This can be unintuitive at first, but it’s fairly widespread inthe kernel.

UsingERR_PTR() (include/linux/err.h) to encode anegative error number into a pointer, andIS_ERR() andPTR_ERR() to get it back out again: avoids a separatepointer parameter for the error number. Icky, but in a good way.

Breaking Compilation¶

Linus and the other developers sometimes change function or structurenames in development kernels; this is not done just to keep everyone ontheir toes: it reflects a fundamental change (eg. can no longer becalled with interrupts on, or does extra checks, or doesn’t do checkswhich were caught before). Usually this is accompanied by a fairlycomplete note to the linux-kernel mailing list; search the archive.Simply doing a global replace on the file usually makes thingsworse.

Initializing structure members¶

The preferred method of initializing structures is to use designatedinitialisers, 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 structurefields are set. You should do this because it looks cool.

GNU Extensions¶

GNU Extensions are explicitly allowed in the Linux kernel. Note thatsome of the more complex ones are not very well supported, due to lackof general use, but the following are considered standard (see the GCCinfo page section “C Extensions” for more details - Yes, really the infopage, 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 forit is horrible and worse: division and multiplication does not work oni386 because the GCC runtime functions for it are missing from thekernel environment.

C++¶

Using C++ in the kernel is usually a bad idea, because the kernel doesnot provide the necessary runtime environment and the include files arenot tested for it. It is still possible, but not recommended. If youreally want to do this, forget about exceptions at least.

#if¶

It is generally considered cleaner to use macros in header files (or atthe top of .c files) to abstract away functions rather than using `#if’pre-processor statements throughout the source code.