Documentation/arm/swp_emulation

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