On 4/23/19 12:27 PM, Linus Torvalds wrote: > On Tue, Apr 23, 2019 at 7:17 AM Peter Zijlstra <pet...@infradead.org> wrote: >> I'm not aware of an architecture where disabling interrupts is faster >> than disabling preemption. > I don't thin kit ever is, but I'd worry a bit about the > preempt_enable() just because it also checks if need_resched() is true > when re-enabling preemption. > > So doing preempt_enable() as part of rwsem_read_trylock() might cause > us to schedule in *exactly* the wrong place,
You are right on that. However, there is a variant called preempt_enable_no_resched() that doesn't have this side effect. So I am going to use that one instead. > So if we play preemption games, I wonder if we should make them more > explicit than hiding them in that helper function, because > particularly for the slow path case, I think we'd be much better off > just avoiding the busy-loop in the slow path, rather than first > scheduling due to preempt_enable(), and then starting to look at the > slow path onlly afterwards. > > IOW, I get the feeling that the preemption-off area might be better > off being potentially much bigger, and covering the whole (or a large > portion) of the semaphore operation, rather than just the > rwsem_read_trylock() fastpath. > > Hmm? That is true in general, but doing preempt_disable/enable across function boundary is ugly and prone to further problems down the road. Cheers, Longman > Linus