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SPL(9)                     Kernel Developer's Manual                    SPL(9)

     spl, spl0, splhigh, splvm, splbio, splnet, spltty, splsched, splsoftbio,
     splsoftclock, splsoftnet, splsoftserial, splx - modify system interrupt
     priority level

     #include <sys/intr.h>












     splx(int s);

     These functions raise and lower the interrupt priority level.  They are
     used by kernel code to block interrupts in critical sections, in order to
     protect data structures.

     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

     Note that hard interrupt 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:

     splhigh(), 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(9) interfaces.

              Interrupt handlers at this level cannot acquire the global
              kernel_lock and so must be coded to ensure correct
              synchronization on multiprocessor systems.

     splsched(), 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(9) interfaces, and may use other kernel facilities that
              could cause the current LWP to sleep.

     splvm(), 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.

              The global kernel_lock is automatically acquired for interrupts
              at this level by default, in order to support device drivers
              that do not provide their own multiprocessor synchronization.
              The automatic acquisition of kernel_lock can be disabled for
              individual interrupt handlers by device drivers if supported by
              subsystem, see e.g.  pci_intr_establish(9).

              splbio(), splnet(), and spltty() are synonyms for splvm().
              Their use is deprecated; all new code should use splvm().

     splsoftserial(), 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

     splsoftnet(), IPL_SOFTNET

              Blocks soft interrupts at the IPL_SOFTNET symbolic level.

     splsoftbio(), IPL_SOFTBIO

              Blocks soft interrupts at the IPL_SOFTBIO symbolic level.

     splsoftclock(), 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:

     spl0(), 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

     condvar(9), i386/splraise(9), kpreempt(9), mutex(9), rwlock(9)

     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.

     Between NetBSD 4.0 and NetBSD 5.0, the hardware levels were reduced in
     number and a strict hierarchy defined.

NetBSD 10.99                     April 7, 2020                    NetBSD 10.99