Software emulation of deprecated SWP instruction (CONFIG_SWP_EMULATE)

ARMv6 architecture deprecates use of the SWP/SWPB instructions, and recommends moving to the load-locked/store-conditional instructions LDREX and STREX.

ARMv7 multiprocessing extensions introduce the ability to disable these instructions, triggering an undefined instruction exception when executed. Trapped instructions are emulated using an LDREX/STREX or LDREXB/STREXB sequence. If a memory access fault (an abort) occurs, a segmentation fault is signalled to the triggering process.

/proc/cpu/swp_emulation holds some statistics/information, including the PID of the last process to trigger the emulation to be invocated. For example:

Emulated SWP:         12
Emulated SWPB:                0
Aborted SWP{B}:               1
Last process:         314

when accessing uncached shared regions, LDREX/STREX rely on an external transaction monitoring block called a global monitor to maintain update atomicity. If your system does not implement a global monitor, this option can cause programs that perform SWP operations to uncached memory to deadlock, as the STREX operation will always fail.