modify system interrupt priority
These functions raise and lower the system priority level. They
are used by kernel code to block interrupts with priority less than or equal
to the named level (i.e.,
blocks interrupts of priority less than or equal to
IPL_TTY). The code may then safely access variables
and data structures which are used by kernel code that runs at an equal or
lower priority level.
spl function exists for each distinct
priority level which can exist in the system. These macros and the
corresponding priority levels are used for various defined purposes, and may
be divided into two main types: hard and soft. Hard interrupts are generated
by hardware devices, while soft interrupts are generated by callouts and
called from the kernel's periodic timer interrupt service routine.
In order of highest to lowest priority, the priority-raising macros are:
- 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.
- blocks hard interrupts from serial interfaces. Code running at this level may not access the tty subsystem.
- blocks interrupts that may access scheduler data structures. Specifically
the clock interrupt that invokes the
schedclock() function needs to be blocked. On some systems this is a separate clock; on others it is the same as the statistics clock and, on these,
splsched() must block everything that
splstatclock() does. Code running at or above this level may not call tsleep(9) or wakeup(9), nor may it post signals. Note that "running" means invoked by an interrupt handler that operates at this level or higher. Kernel code that operates in the context of a process and has called
splhigh() for blocking purposes can use tsleep(9) or wakeup(9).
- 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.
- 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 level is identical to
splclock() if there is no separate statistics clock.
- blocks hard interrupts from all devices that are allowed to use the kernel malloc(9). That includes all disk, network, and tty device interrupts.
- blocks hard interrupts from TTY devices.
- blocks soft interrupts generated by serial devices.
- blocks hard interrupts from network interfaces.
- blocks hard interrupts from disks and other mass-storage devices.
- blocks soft network interrupts.
- blocks soft clock interrupts.
Two macros lower the system priority level. They are:
- unblocks all interrupts but the soft clock interrupt.
- unblocks all interrupts.
macro blocks interrupts at the interrupt priority level specified by
restores the system priority level to the one encoded in
s, which must be a value previously returned by one of
function checks that the system is running at a certain priority level. The
argument s should be one of these constants:
function is optional and is not necessarily implemented on all architectures
nor enabled in all kernel configurations. It checks the current system
priority level to see if it's at least at the level specified in the
argument s. If possible, it also checks if it hasn't
been called from an interrupt handler with a level higher than the one
requested, which must be an error (if some code is protected from
IPL_SOFTNET interrupts, but accessed from an
IPL_NET interrupt, it must be a design error in the
The behavior of the
function is controlled by the kern.splassert
sysctl(8). Valid values for it are:
- disable error checking
- print a message if an error is detected
- print a message and a stack trace if possible
- like 2 but also drop into the kernel debugger
Any other value causes a system panic on errors.