On Wed, Oct 17, 2018 at 8:12 PM Nadav Amit <[email protected]> wrote: > > at 6:22 PM, Andy Lutomirski <[email protected]> wrote: > > > > >> On Oct 17, 2018, at 5:54 PM, Nadav Amit <[email protected]> wrote: > >> > >> It is sometimes beneficial to prevent preemption for very few > >> instructions, or prevent preemption for some instructions that precede > >> a branch (this latter case will be introduced in the next patches). > >> > >> To provide such functionality on x86-64, we use an empty REX-prefix > >> (opcode 0x40) as an indication that preemption is disabled for the > >> following instruction. > > > > Nifty! > > > > That being said, I think you have a few bugs. First, you can’t just ignore > > a rescheduling interrupt, as you introduce unbounded latency when this > > happens — you’re effectively emulating preempt_enable_no_resched(), which > > is not a drop-in replacement for preempt_enable(). To fix this, you may > > need to jump to a slow-path trampoline that calls schedule() at the end or > > consider rewinding one instruction instead. Or use TF, which is only a > > little bit terrifying… > > Yes, I didn’t pay enough attention here. For my use-case, I think that the > easiest solution would be to make synchronize_sched() ignore preemptions > that happen while the prefix is detected. It would slightly change the > meaning of the prefix. > > > You also aren’t accounting for the case where you get an exception that > > is, in turn, preempted. > > Hmm.. Can you give me an example for such an exception in my use-case? I > cannot think of an exception that might be preempted (assuming #BP, #MC > cannot be preempted). >
Look for cond_local_irq_enable(). --Andy
