Restartable sequences in glibc

A small correction about the sentence "Immediately prior to entering its critical section, a thread should store the address of its rseq_cs structure into the rseq_cs field of the global rseq structure; it should reset that field to NULL on exit." in the article.

Whereas it is OK to set rseq_cs to NULL when exiting a rseq critical section, it is not actually needed. The only requirement is that the rseq_cs pointer is set to NULL at some point after exiting the rseq critical section, but before reclaim of the memory holding the rseq_cs structure and the code it points to (e.g. dlclose(3) of a shared library).

Removing the requirement for setting the rseq_cs pointer to NULL on exit from a rseq critical section is a significant performance improvement considering that the entire critical section is implemented with very few instructions, which is achieved by letting the kernel detect when it returns to user-space over an instruction pointer which is outside of the range of the rseq critical section. When this is detected, the kernel simply clears the rseq_cs pointer.