SPL(9)

NAME
spl, spl0, splaudio, splbio, splclock, splhigh, splvm, spllock, spllowersoftclock, splnet, splsched, splserial, splsoftclock, splsoftnet, splsoftserial, splstatclock, spltty, splvm, splx - modify system interrupt priority level

SYNOPSIS
#include <sys/param.h>
int
splaudio(void);
int
splhigh(void);
int
spllock(void);
int
splserial(void);
int
splsched(void);
int
splclock(void);
int
splstatclock(void);
int
splvm(void);
int
spltty(void);
int
splsoftserial(void);
int
splnet(void);
int
splbio(void);
int
splsoftnet(void);
int
splsoftclock(void);
void
spllowersoftclock(void);
void
spl0(void);
void
splx(int s);

DESCRIPTION
These functions raise and lower the system priority level. They are used by kernel code to block interrupts in critical sections, in order to protect data structures (much like a locking primitive) or to ensure uninterrupted access to hardware devices which are sensitive to timing.

Interrupt priorities are not arranged in a strict hierarchy, although interrupt hardware sometimes is. For this reason the priorities listed here are arranged from “highest” to “lowest”. In other words, if a platform's hardware interrupts are arranged in a hierarchical manner, a priority level should also block all of the levels listed below it.

Note that a strict hierarchy is not required. For example, splnet() is not required to block disk controller interrupts, as they do not access the same data structures. However, the priorities are presented as a hierarchy in order to minimize data loss due to blocked interrupts, or interrupts not being serviced in a timely fashion.

A spl function exists for each distinct priority level which can exist in the system, as well as for some special priority levels that are designed to be used in conjunction with multiprocessor-safe locking primitives. These levels may be divided into two main types: hard and soft. Hard interrupts are generated by hardware devices. Soft interrupts are a way of deferring hardware interrupts to do more expensive processing at a lower interrupt priority, and are explicitly scheduled by the higher-level interrupt handler. The most common use of this is in the networking code, where network interface drivers defer the more expensive TCP/IP processing in order to avoid dropping additional incoming packets. Software interrupts are further described by softintr(9).

In order of highest to lowest priority, the priority-raising functions are:

splhigh()
    blocks all hard and soft interrupts. It is used for code that cannot tolerate any interrupts, like hardware context switching code and the ddb(4) in-kernel debugger.

spllock()
    blocks all hard and soft interrupts that can acquire a simple lock. This is provided as a distinct level from splhigh() as some platforms may need to make use of extremely high priority interrupts while locks are spinning, which would be blocked by splhigh().

splserial()
    blocks hard interrupts from serial interfaces (IPL_SERIAL). Code running at this level may not access the tty subsystem. Generally, all code run at this level must schedule additional processing to run in a software interrupt. Note that code running at this priority is not blocked by splvm() (described below), and is therefore prohibited from using the kernel memory allocators.

splsched()
    blocks all hard and soft interrupts that may access scheduler data structures. Code running at or above this level may not call sleep(), tsleep(), ltsleep(), or wakeup(), nor may it post signals.

splclock()
    blocks the hardware clock interrupt. It is used by hardclock() to update kernel and process times, and must be used by any other code that accesses time-related data, specifically the time and mono_time global variables. This level also protects the callout(9) data structures, and nothing running at or above this level may schedule, cancel, or otherwise access any callout-related data structures.

splstatclock()
    blocks the hardware statistics clock interrupt. It is used by statclock() to update kernel profiling and other statistics, and must be used by any code that accesses that data. This is the clock that drives scheduling. This level is identical to splclock() if there is no separate statistics clock.

splvm()
    blocks hard interrupts from all devices that are allowed to use the kernel malloc(9), or any virtual memory operations. That includes all disk, network, and tty device interrupts. The temptation to abuse the semantics of splvm() should be avoided; if you feel as if you need to block more than one class of interrupts at a time, use software interrupts instead.

spltty()
    blocks hard and soft interrupts from TTY devices (IPL_TTY). This must also block soft serial interrupts.

splaudio()
    blocks hard interrupts generated by audio devices (IPL_AUDIO).

splsoftserial()
    blocks soft interrupts generated by serial devices (IPL_SOFTSERIAL).

splnet()
    blocks hard interrupts from network interfaces (IPL_NET).

splbio()
    blocks hard interrupts from disks and other mass-storage devices (IPL_BIO).

splsoftnet()
    blocks soft network interrupts (IPL_SOFTNET). Soft interrupts blocked by this priority are also blocked by all of the priorities listed above.

splsoftclock()
    blocks soft clock interrupts. This is the priority at which the callout(9) facility runs. Soft interrupts blocked by this priority are also blocked by all of the priorities listed above. In particular, splsoftnet() must be a higher priority than splsoftclock().

Two functions lower the system priority level. They are:

spllowersoftclock()
    unblocks all interrupts but the soft clock interrupt.

spl0()
    unblocks all interrupts.

The splx() function restores the system priority level to the one encoded in s, which must be a value previously returned by one of the other spl functions.

Note that the functions which lower the system priority level ( spllowersoftclock(), spl0(), and splx()) do not return a value. They should only be used in places where the system priority level is being decreased permanently. It is inappropriate to attempt to use them where the system priority level is being decreased temporarily, and would need to be restored to a previous value before continuing.

HISTORY
In 4.4BSD, splnet() was used to block network software interrupts. Most device drivers used splimp() to block hardware interrupts. To avoid unnecessarily blocking other interrupts, in NetBSD 1.1 a new function was added that blocks only network hardware interrupts. For consistency with other spl functions, the old splnet() function was renamed to splsoftnet(), and the new function was named splnet().

Originally, splsoftclock() lowered the system priority level. During the NetBSD 1.5 development cycle, spllowersoftclock() was introduced and the semantics of splsoftclock() were changed.

The splimp() call was removed from the kernel between NetBSD 1.5 and NetBSD 1.6. The function of splimp() was replaced by splvm() and code which abused the semantics of splimp() was changed to not mix interrupt priority levels.