[$] Restartable sequences, TCMalloc, and Hyrum's Law

Hyrum's Law states that anyobservable behavior of a system will eventually be depended upon bysomebody. The kernel community is currently contending with a cleardemonstration of that principle. The recent work to address some restartable-sequencesperformance problems in the 6.19 release maintained the documented APIin all respects, but that was not enough; Google's TCMalloclibrary, as it turns out, violates the documented API, prevents other codefrom using restartable features, and breaks with 6.19. But the kernel'sno-regressions rule is forcing developers to find a way to accommodateTCMalloc's behavior.