Allow modifying static keys in multi-thread environment

[Evian-Zhang]

See FAQ for reason to only allow modifying in single-thread environment. See discussions in Announcing static-keys, reimplementaion of Linux kernel’s static keys mechanism for Rust userland for difficulties in implementing such feature.

To allow modify static keys in multi-thread environment, we shall implement a text_poke in userland, which is worth investigating.

[heiher]

I think it is necessary to ensure that the 5-byte NOP and JMP instruction on x86 is contained within the same cache line. This prevents fetching the instruction from requiring two memory accesses. If fetching requires two memory accesses and a memory mapping replacement occurs almost simultaneously, unpredictable results may occur.

[Evian-Zhang]

Having been struggling with this problem for nearly a week. Found something extremely useful: https://stackoverflow.com/a/70180507/10005095. Still reading and understanding it...