On Tue, Apr 23, 2019 at 01:27:29PM -0700, Linus Torvalds wrote: > On Tue, Apr 23, 2019 at 12:56 PM Peter Zijlstra <pet...@infradead.org> wrote: > > > > Unless the very next line is schedule(), or implies it, one must not use > > preempt_enable_no_resched(). It can cause a preemption to go missing and > > thereby cause arbitrary delays, breaking the PREEMPT=y invariant. > > That language may be a bit strong, or m,aybe the "implies it" might at > least be extended on. > > It doesn't need to be "schedule()" per se, it can be any of the things > that check if we _need_ to be scheduled.
I'll try and word-smith that tomorrow, brain is fried. But yes, something that ends up in schedule() 'soon'. The usage in ist_exit() is particularly 'fun'. That really should've had a comment. That relies on the return-from-interrupt path. > IOW, various variations of "if (need_resched())" exiting a loop, and > then outside the loop there's a cond_resched() or similar. That one 'funnily' doesn't actually work; cond_resched() is a no-op on PREEMPT=y.
