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    SubjectRe: [PATCH -v5 12/17] x86/kprobes: Fix ordering
    On Thu, Nov 14, 2019 at 10:22:24AM -0500, Mathieu Desnoyers wrote:

    > >> > So what we do, after enabling the regular kprobe, is call
    > >> > synchronize_rcu_tasks() to wait for each task to have passed through
    > >> > schedule(). That guarantees no task is preempted inside the kprobe
    > >> > shadow (when it triggers it ensures it resumes execution at an
    > >> > instruction boundary further than 5 bytes away).
    > >>
    > >> Indeed, given that synchronize_rcu_tasks() awaits for voluntary context
    > >> switches (or user-space execution), it guarantees that no task was preempted
    > >> within the kprobe shadow.
    > >>
    > >> Considering that synchronize_rcu_tasks() is meant only for code rewriting,
    > >> I wonder if it would make sense to include the core serializing guarantees
    > >> within this RCU API ?
    > >
    > > As in have synchronize_rcu_tasks() do the IPI-sync love before doing
    > > the current wait-for-voluntary-context-switch work?
    >
    > This is what I have in mind, yes, based on the assumption that the only
    > intended use-case for synchronize_rcu_tasks() is code patching.

    I don't think that is needed. As per the patch under discussion, we
    unconditionally need that IPI-sync (even for !optimized) but we only
    need the synchonize_rcu_tasks() thing for optimized kprobes.

    Also, they really do two different things. Lets not tie them together.

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