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    SubjectRe: [PATCH 2/3] livepatch/rcu: Warn when system consistency is broken in RCU code
    On Thu, May 11, 2017 at 03:52:46PM +0200, Petr Mladek wrote:
    > Anyway, a crazy idea is to use the livepatch consistency model instead
    > of RCU to protect the function stack. The model makes sure that all
    > tasks, including the idle ones, were not running any patched function
    > (and their ftrace handlers) at some point. It should be safe
    > but I am not sure if it is worth it.

    http://i3.kym-cdn.com/photos/images/original/000/173/580/Wat.jpg

    > Alternatively, it might be enough to use the probably more lightwight
    > solution that is used when ftrace handlers are deregistered, I mean:
    >
    > /*
    > * We need to do a hard force of sched synchronization.
    > * This is because we use preempt_disable() to do RCU, but
    > * the function tracers can be called where RCU is not watching
    > * (like before user_exit()). We can not rely on the RCU
    > * infrastructure to do the synchronization, thus we must do it
    > * ourselves.
    > */
    > schedule_on_each_cpu(ftrace_sync);
    >
    > /*
    > * When the kernel is preeptive, tasks can be preempted
    > * while on a ftrace trampoline. Just scheduling a task on
    > * a CPU is not good enough to flush them. Calling
    > * synchornize_rcu_tasks() will wait for those tasks to
    > * execute and either schedule voluntarily or enter user space.
    > */
    > if (IS_ENABLED(CONFIG_PREEMPT))
    > synchronize_rcu_tasks();

    I couldn't grok the first idea, but this one sounds promising...

    --
    Josh

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