Re: [RFC] in-kernel rseq

[David Laight]

On Mon, 23 Feb 2026 22:54:36 +0100
Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:

> On Mon, Feb 23, 2026 at 01:22:18PM -0500, Mathieu Desnoyers wrote:
> 
> > > I think it would be better as the address of the instruction after
> > > the 'store'.  
> > 
> > That's indeed what we do for userspace rseq.  
> 
> Either works I suppose. The only think to be careful about is that you
> must not restart once the store has happened.
> 
> > > You probably don't need separate 'begin' and 'restart' addresses.  
> > 
> > It's not needed as long as the abort behavior is only restart. It
> > becomes useful if another behavior is wanted on abort. But since
> > this is kernel code and not ABI, it can change if the need arise.  
> 
> Right, didn't want to limit to restart, although that is what is used
> here.
> 
> > > It might be enough to save the 'restart' address and a byte length
> > > directly in 'current', much simpler code.  
> > 
> > That would make it two stores to the task struct. Those would not be
> > single-instruction, so we'd have to deal with preemption coming between
> > those two stores. Also this would be more code: two stores compared
> > to a single pointer store to the task struct to begin the critical
> > section. AFAIU Peter's proposed approach is more efficient.  
> 
> Must indeed be a single store. Either we have it set in full, or we
> don't.

Not really, you can do two stores (to the task struct) provided you
check the second one - remember the data is being looked at by the
cpu that did the writes.

> > We could turn the end address into a length if we want, this would
> > make it more alike the userspace rseq ABI counterpart.  
> 
> I find 3 distinct addresses easier to fill out, but again it doesn't
> matter.

Actually if you save the end address you only need to check if the
current %pc is less that that address, if it is you back it up to
the start of the sequence.

> 
> > > How much it helps is another matter.
> > > I'm sure I remember something about per-cpu data being used for something
> > > because it was faster then using 'current' - not sure of the context.  
> > 
> > The problem with per-cpu data for this is how to handle migration ?
> > The whole point of this is to replace preempt disable.  
> 
> This; it cannot be a per-cpu address, if you need it to implement
> per-cpu ops.

Sorry yes, you are replacing a per-cpu data operation with a per-task one.
But I'm sure I remember something where the opposite was done because it
was unexpectedly faster to use per-cpu data.
I'm not sure where arm gets 'current' from, x86 'has it easy' because
of %fg and %gs.
(If current is loaded from per-cpu data that might explain why using
per-cpu data is faster.)

That makes me think (a bad sign)...
Are the compilers 'clever' enough to use %fs for current->member while
current()->member uses a #define to get the actual address?

preempt_disable() itself can be implemented using per-cpu or per-task
data. I think it varies between architectures, not sure which asm uses.

> > > The real problem with rseq is they don't scale.  
> > 
> > Not sure what you mean. They don't scale with respect to what ?  
> 
> He might be talking about forward progress instead of scaling. There are
> indeed foward progress concerns with rseq -- as there are with trivial
> LL/SC. But given the length of a slice vs the length of a rseq section,
> this should be a non-issue.

No scaling, in this case it is fine to add the rseq just before needing it.
But if they have to be set in advance then you start getting a long list
to check - I'm sure that must happen with userspace rseq?

> 
> Doing the restart on interrupt would be a bigger issue. Although even
> there I think that since the operations we're talking about are but a
> few instructions, it should all just work well enough.
> 
...

> > > I think that is just unlocked RMW of a per-cpu/thread variable.  
> 
> That's missing the point entirely. He might be stuck in x86_64 or
> something.

Not entirely, it doesn't matter if code is preempted between the read
and write in preempt_disable() because that can only happen when the
count is changing from 0 to 1.
What does matter is that the 1 is written to the correct place.

	David