linux/Documentation/arm/swp_emulation
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   1Software emulation of deprecated SWP instruction (CONFIG_SWP_EMULATE)
   2---------------------------------------------------------------------
   3
   4ARMv6 architecture deprecates use of the SWP/SWPB instructions, and recommeds
   5moving to the load-locked/store-conditional instructions LDREX and STREX.
   6
   7ARMv7 multiprocessing extensions introduce the ability to disable these
   8instructions, triggering an undefined instruction exception when executed.
   9Trapped instructions are emulated using an LDREX/STREX or LDREXB/STREXB
  10sequence. If a memory access fault (an abort) occurs, a segmentation fault is
  11signalled to the triggering process.
  12
  13/proc/cpu/swp_emulation holds some statistics/information, including the PID of
  14the last process to trigger the emulation to be invocated. For example:
  15---
  16Emulated SWP:           12
  17Emulated SWPB:          0
  18Aborted SWP{B}:         1
  19Last process:           314
  20---
  21
  22NOTE: when accessing uncached shared regions, LDREX/STREX rely on an external
  23transaction monitoring block called a global monitor to maintain update
  24atomicity. If your system does not implement a global monitor, this option can
  25cause programs that perform SWP operations to uncached memory to deadlock, as
  26the STREX operation will always fail.
  27
  28