Re: [RFC PATCH 1/5] x86: introduce preemption disable prefix

[Nadav Amit <namit@vmware.com>]

at 6:22 PM, Andy Lutomirski <luto@amacapital.net> wrote:

> 
>> On Oct 17, 2018, at 5:54 PM, Nadav Amit <namit@vmware.com> 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).

I agree that for super-general case this might be inappropriate.