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

[Nadav Amit <namit@vmware.com>]

at 8:11 PM, Nadav Amit <namit@vmware.com> wrote:

> 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.

Ignore this nonsense that I wrote. I’ll try to come up with a decent
solution.

>> 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.