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SubjectRe: [PATCH v3 0/6] Static calls
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> On Jan 11, 2019, at 1:22 PM, Josh Poimboeuf <jpoimboe@redhat.com> 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@redhat.com> 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 don’t see RCU-sched solves the problem if you don’t 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.

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