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LoongArch only support 32-bit/64-bit xchg/cmpxchg in native. But percpu operation, qspinlock and some drivers need 8-bit/16-bit xchg/cmpxchg. We add subword xchg/cmpxchg emulation in this patch because the emulation has better performance than the generic implementation (on NUMA system), and it can fix some build errors meanwhile [1]. LoongArch's guarantee for forward progress (avoid many ll/sc happening at the same time and no one succeeds): We have the "exclusive access (with timeout) of ll" feature to avoid simultaneous ll (which also blocks other memory load/store on the same address), and the "random delay of sc" feature to avoid simultaneous sc. It is a mandatory requirement for multi-core LoongArch processors to implement such features, only except those single-core and dual-core processors (they also don't support multi-chip interconnection). Feature bits are introduced in CPUCFG3, bit 3 and bit 4 [2]. [1] https://lore.kernel.org/loongarch/CAAhV-H6vvkuOzy8OemWdYK3taj5Jn3bFX0ZTwE=twM8ywpBUYA@mail.gmail.com/T/#t [2] https://loongson.github.io/LoongArch-Documentation/LoongArch-Vol1-EN.html#_cpucfg Reported-by: Sudip Mukherjee (Codethink) <sudipm.mukherjee@gmail.com> Suggested-by: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Rui Wang <wangrui@loongson.cn> Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.