void
spl0(
void
)
int
splhigh(
void
)
int
splsched(
void
)
int
splvm(
void
)
int
splsoftbio(
void
)
int
splsoftclock(
void
)
int
splsoftserial(
void
)
int
splsoftnet(
void
)
void
splx(
int s
)
In a multi-CPU system, these functions change the interrupt priority level on the local CPU only. In general, device drivers should not make use of these interfaces. To ensure correct synchronization, device drivers should use the condvar(9), mutex(9), and rwlock(9) interfaces.
Interrupt priorities are arranged in a strict hierarchy, although sometimes levels may be equivalent (overlap). The hierarchy means that raising the IPL to any level will block interrupts at that level, and at all lower levels. The hierarchy is used to minimize data loss due to interrupts not being serviced in a timely fashion.
The levels may be divided into two groups: 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. Software interrupts are further described by softint(9).
Note that hard interupt handlers do not possess process (thread) context and so it is not valid to use kernel facilities that may attempt to sleep from a hardware interrupt. For example, it is not possible to acquire a reader/writer lock from a hardware interrupt. Soft interrupt handlers possess limited process context and so may sleep briefly in order to acquire a reader/writer lock or adaptive mutex, but may not sleep for any other reason.
In order of highest to lowest priority, the priority-raising functions along with their counterpart symbolic tags are:
,IPL_HIGH
)Blocks all hard and soft interrupts, including the highest level I/O interrupts, such as interrupts from serial interfaces and the statistics clock (if any). It is also used for code that cannot tolerate any interrupts.
Code running at this level may not (in general) directly access
machine independent kernel services.
For example, it is illegal to call the kernel
printf()
function or to try and allocate memory.
The methods of synchronization available are: spin mutexes and
scheduling a soft interrupt.
Generally, all code run at this level must schedule additional
processing to run in a software interrupt.
Code with thread context running at this level must not use a kernel
interface that may cause the current LWP to sleep, such as the
condvar
interfaces.
Interrupt handlers at this level cannot acquire the global kernel_lock and so must be coded to ensure correct synchronization on multiprocessor systems.
,IPL_SCHED
)Blocks all medium priority hardware interrupts, such as interrupts from audio devices, and the clock interrupt.
Interrupt handlers running at this level endure the same restrictions as at IPL_HIGH, but may access scheduler interfaces, and so may awaken LWPs (light weight processes) using the condvar(9) interfaces, and may schedule callouts using the callout(9) interfaces.
Code with thread context running at this level may sleep via the
condvar
interfaces, and may use other kernel facilities that could cause the
current LWP to sleep.
,IPL_VM
)Blocks hard interrupts from 'low' priority hardware interrupts, such as interrupts from network, block I/O and tty devices.
Code running at this level endures the same restrictions as at IPL_SCHED, but may use the deprecated malloc(9) or endorsed pool_cache(9) interfaces to allocate memory.
At the time of writing, the global
kernel_lock
is automatically acquired for interrupts at this level, in order to
support device drivers that do not provide their own multiprocessor
synchronization.
A future release of the system may allow the automatic acquisition of
kernel_lock
to be disabled for individual interrupt handlers.
,IPL_SOFTSERIAL
)Blocks soft interrupts at the IPL_SOFTSERIAL symbolic level.
This is the first of the software levels. Soft interrupts at this level and lower may acquire reader/writer locks or adaptive mutexes.
,IPL_SOFTNET
)Blocks soft interrupts at the IPL_SOFTNET symbolic level.
,IPL_SOFTBIO
)Blocks soft interrupts at the IPL_SOFTBIO symbolic level.
,IPL_SOFTCLOCK
)Blocks soft interrupts at the IPL_SOFTCLOCK symbolic level.
This is the priority at which callbacks generated by the callout(9) facility runs.
One function lowers the system priority level:
,IPL_NONE
)
Unblocks all interrupts.
This should rarely be used directly;
splx()
should be used instead.
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.
)
was used to block network software interrupts.
Most device drivers used
splimp(
)
to block hardware interrupts.
To avoid unnecessarily blocking other interrupts, in
NetBSD1.1
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
NetBSD1.5
development cycle,
spllowersoftclock(
)
was introduced and the semantics of
splsoftclock(
)
were changed.
The
splimp()
call was removed from the kernel between
NetBSD1.5
and
NetBSD1.6.
The function of
splimp(
)
was replaced by
splvm(
)
and code which abused the semantics of
splimp(
)
was changed to not mix interrupt priority levels.
Between NetBSD4.0 and NetBSD5.0, the hardware levels were reduced in number and a strict hierarchy defined.