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Subject: Re: [RFC PATCH for 4.18 00/14] Restartable Sequences
Thread-Topic: [RFC PATCH for 4.18 00/14] Restartable Sequences
Thread-Index: AQHT4NTlyEPvZnNLQUycRSVAXgFaHKQb0K6AgADLxwCAAAFOAIABk5qAgAAKDYCAAAiQAIAAB8+AgAHZX6A=
Date:   Fri, 4 May 2018 22:17:38 +0000
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Hey -

I think the ideas Daniel brings up here are interesting -- specifically the=
 notion that a thread could set a "pre-sleep wish" to signal it's sleeping.=
 As this conversation shows I think there's a fair bit of depth to that. Fo=
r example, the FUTEX_LOCK is an alternative approach. Another idea might be=
 using the "currently running cpu" area of rseq to tell if the process in q=
uestion was sleeping (assuming that the kernel would be modified to set thi=
s to -1 every time a process was unscheduled)

The idea discussed here seems orthogonal to the core thesis of rseq. I'm wo=
ndering if we can focus on getting rseq in, maybe with a eye for making sur=
e this use case could be handled long term. My sense is that this is possib=
le. We could use the flags setting in the per-thread rseq area, or maybe ex=
tend the meaning of the structure rseq_cs points to to signal that there wa=
s information about how to signal the sleeping of the current process. It s=
eems to me this would be a natural way to add the functionality Daniel talk=
s about if desired in the future.

Daniel - do you think there's anything we should do in the patch set today =
that would make it easier to implement your idea in the future without expa=
nding the scope of the patch today. i.e. is there anything else we need to =
do to lay the framework for your idea.

I'd really love to see us get this patch in. There's a whole host of primit=
ives this unlocks (more efficient RCU, better malloc implementations, fast =
reader-writer locks). I'm sure we'll have more ideas about APIs to provide =
once we've explored these use cases, but to me this patch is the MVP we nee=
d to ship to get that feedback. It's a solid framework that we can build up=
on, eg with the "opv" syscall or the idea in this thread if user feedback s=
hows those things are necessary.

-b=