Re: [PATCH v3 0/6] Static calls

[Josh Poimboeuf]

On Fri, Jan 11, 2019 at 09:36:59PM +0000, Nadav Amit wrote:
> > On Jan 11, 2019, at 1:22 PM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> > 
> > On Fri, Jan 11, 2019 at 12:46:39PM -0800, Linus Torvalds wrote:
> >> On Fri, Jan 11, 2019 at 12:31 PM Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> >>> I was referring to the fact that a single static call key update will
> >>> usually result in patching multiple call sites.  But you're right, it's
> >>> only 1-2 trampolines per text_poke_bp() invocation.  Though eventually
> >>> we may want to batch all the writes like what Daniel has proposed for
> >>> jump labels, to reduce IPIs.
> >> 
> >> Yeah, my suggestion doesn't allow for batching, since it would
> >> basically generate one trampoline for every rewritten instruction.
> > 
> > As Andy said, I think batching would still be possible, it's just that
> > we'd have to create multiple trampolines at a time.
> > 
> > Or... we could do a hybrid approach: create a single custom trampoline
> > which has the call destination patched in, but put the return address in
> > %rax -- which is always clobbered, even for callee-saved PV ops.  Like:
> > 
> > trampoline:
> > 	push %rax
> > 	call patched-dest
> > 
> > That way the batching could be done with a single trampoline
> > (particularly if using rcu-sched to avoid the sti hack).
> 
> I donât see RCU-sched solves the problem if you donât disable preemption. On
> a fully preemptable kernel, you can get preempted between the push and the
> call (jmp) or before the push. RCU-sched can then finish, and the preempted
> task may later jump to a wrong patched-dest.

Argh, I misspoke about RCU-sched.  Words are hard.

I meant synchronize_rcu_tasks(), which is a completely different animal.
My understanding is that it waits until all runnable tasks (including
preempted tasks) have gotten a chance to run.

-- 
Josh