spl (short for set priority level, after the PDP-11 assembler instruction of the same name[1]) is the name for a collection of Unix kernel routines or macros used to change the interrupt priority level.[2][3] This was historically needed to synchronize critical sections of kernel code that should not be interrupted.[4] Newer Unix variants which support symmetric multiprocessing now mostly use mutexes for this purpose, which is a more general solution, so multiple processors can execute kernel code at the same time.[5][1]

On older PDP-11 versions of Unix, there were eight of these routines, ranging from spl0 to spl7, each corresponding to one PDP-11 interrupt priority level,[3] in addition to splx, which restores a previous priority level (returned by one of the other routines).[2] On BSD Unix and its derivatives, these are called splhigh, splserial, splsched, splclock, splstatclock, splvm, spltty, splsofttty, splnet, splbio, splsoftnet, splsoftclock, spllowersoftclock, spl0, and splx.[2]

As of March 2019, the spl family of primitives is still heavily used in OpenBSD[6] and NetBSD,[7] which is evidenced by the plentiful calls to splnet() within the networking code;[6][7] whereas FreeBSD and DragonFly BSD use more modern concepts; for example, in DragonFly, LWKT tokens may be used in place of spl.

See also

References

  1. 1 2 Lehey, Greg (2001), Improving the FreeBSD SMP implementation, retrieved 11 May 2018
  2. 1 2 3 "spl(9) - OpenBSD manual pages". Retrieved 11 May 2018.
  3. 1 2 Lions, John (1976). Lions' Commentary on UNIX 6th Edition, with Source Code. p. 43.
  4. ↑ Lions, John (1976). Lions' Commentary on UNIX 6th Edition, with Source Code. p. 41.
  5. ↑ McKusick, Marshall Kirk; et al. (authors) (2004). The Design and Implementation of the FreeBSD Operating System. Addison-Wesley. p. 93.
  6. 1 2 "/sys/net/if.c". BSD Cross Reference. OpenBSD. 2019-03-01. Retrieved 2019-03-05. s = splnet();
  7. 1 2 "/sys/net/if.c". BSD Cross Reference. NetBSD. 2019-03-01. Retrieved 2019-03-05. s = splnet();


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