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    SubjectRe: [patch V6 12/37] x86/entry: Provide idtentry_entry/exit_cond_rcu()
    On Tue, May 19, 2020 at 1:20 PM Thomas Gleixner <tglx@linutronix.de> wrote:
    >
    > Thomas Gleixner <tglx@linutronix.de> writes:
    > > Andy Lutomirski <luto@kernel.org> writes:
    > >> On Fri, May 15, 2020 at 5:10 PM Thomas Gleixner <tglx@linutronix.de> wrote:
    > >>> The pagefault handler cannot use the regular idtentry_enter() because that
    > >>> invokes rcu_irq_enter() if the pagefault was caused in the kernel. Not a
    > >>> problem per se, but kernel side page faults can schedule which is not
    > >>> possible without invoking rcu_irq_exit().
    > >>>
    > >>> Adding rcu_irq_exit() and a matching rcu_irq_enter() into the actual
    > >>> pagefault handling code would be possible, but not pretty either.
    > >>>
    > >>> Provide idtentry_entry/exit_cond_rcu() which calls rcu_irq_enter() only
    > >>> when RCU is not watching. The conditional RCU enabling is a correctness
    > >>> issue: A kernel page fault which hits a RCU idle reason can neither
    > >>> schedule nor is it likely to survive. But avoiding RCU warnings or RCU side
    > >>> effects is at least increasing the chance for useful debug output.
    > >>>
    > >>> The function is also useful for implementing lightweight reschedule IPI and
    > >>> KVM posted interrupt IPI entry handling later.
    > >>
    > >> Why is this conditional? That is, couldn't we do this for all
    > >> idtentry_enter() calls instead of just for page faults? Evil things
    > >> like NMI shouldn't go through this path at all.
    > >
    > > I thought about that, but then ended up with the conclusion that RCU
    > > might be unhappy, but my conclusion might be fundamentally wrong.
    >
    > It's about this:
    >
    > rcu_nmi_enter()
    > {
    > if (!rcu_is_watching()) {
    > make it watch;
    > } else if (!in_nmi()) {
    > do_magic_nohz_dyntick_muck();
    > }
    >
    > So if we do all irq/system vector entries conditional then the
    > do_magic() gets never executed. After that I got lost...

    I'm also baffled by that magic, but I'm also not suggesting doing this
    to *all* entries -- just the not-super-magic ones that use
    idtentry_enter().

    Paul, what is this code actually trying to do?

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