[PATCH v2] arm64: implement support for static call trampolines

Peter Zijlstra peterz at infradead.org
Thu Oct 29 07:46:05 EDT 2020


On Thu, Oct 29, 2020 at 11:58:52AM +0100, Ard Biesheuvel wrote:
> On Thu, 29 Oct 2020 at 11:40, Peter Zijlstra <peterz at infradead.org> wrote:

> > Would it make things easier if your trampoline consisted of two complete
> > slots, between which you can flip?
> >
> > Something like:
> >
> >         0x00    B 0x24 / NOP
> >         0x04    < slot 1 >
> >                 ....
> >         0x20
> >         0x24    < slot 2 >
> >                 ....
> >         0x40
> >
> > Then each (20 byte) slot can contain any of the variants above and you
> > can write the unused slot without stop-machine. Then, when the unused
> > slot is populated, flip the initial instruction (like a static-branch),
> > issue synchronize_rcu_tasks() and flip to using the other slot for next
> > time.
> >
> 
> Once we've populated a slot and activated it, we have to assume that
> it is live and we can no longer modify it freely.

Urhm how so? Once you pass through synchronize_rcu_tasks() nobody should
still be using the old one.



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