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    SubjectRe: [patch V4 part 3 11/29] rcu: Provide rcu_irq_exit_preempt()
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    Joel,

    Joel Fernandes <joel@joelfernandes.org> writes:
    > On Tue, May 05, 2020 at 03:44:05PM +0200, Thomas Gleixner wrote:
    > Could you let me know which patch or part in the multi-part series is
    > using it?

    You found it :)
    >> +void rcu_irq_exit_preempt(void)
    >> +{
    >> + lockdep_assert_irqs_disabled();
    >> + rcu_nmi_exit();
    >> +
    >> + RCU_LOCKDEP_WARN(__this_cpu_read(rcu_data.dynticks_nesting) <= 0,
    >> + "RCU dynticks_nesting counter underflow/zero!");
    >
    > Makes sense.
    >
    >> + RCU_LOCKDEP_WARN(__this_cpu_read(rcu_data.dynticks_nmi_nesting) <= 0,
    >> + "RCU dynticks_nmi_nesting counter underflow/zero!");
    >
    > This new function will be called only from the outer-most IRQ that
    > interrupted kernel mode (process context). Right? If so, a better (more
    > specific) check for the second RCU_LOCKDEP_WARN above is:
    >
    > RCU_LOCKDEP_WARN(__this_cpu_read(rcu_data.dynticks_nmi_nesting) != DYNTICK_IRQ_NONIDLE,
    > "Bad RCU dynticks_nmi_nesting counter\n");
    >
    > That will make sure, it is only called from outer-most rcu_irq_exit() and
    > interrupting kernel mode.

    Makes sense.

    > Or, if [1] is merged, then we could just combine the checks into one check.
    > RCU_LOCKDEP_WARN(__this_cpu_read(rcu_data.dynticks_nesting) != 1,
    > "Bad RCU dynticks_nmi_nesting counter\n");
    >
    >> + RCU_LOCKDEP_WARN(rcu_dynticks_curr_cpu_in_eqs(),
    >> + "RCU in extended quiescent state!");
    >
    > Makes sense.
    >
    > BTW, I wonder if a better place to do this "don't enter scheduler while RCU
    > is not watching" is rcu_note_context_switch()...

    I actually want to catch even the case where we don't schedule, i.e.

    if (ret_to_kernel) {
    if (interrupts_on_after_return((regs)) {
    if (IS_ENABLED(CONFIG_PREEMPTION)) {
    if (!preempt_count()) {
    /* Preemption is possible ... */
    rcu_irq_exit_preempt();
    if (need_resched())
    schedule_preempt_irq();

    that catches any exit where preemption is possible and RCU is not
    watching after rcu_irq_exit().

    It does not matter whether need-resched is set here or not. Any
    interrupt/exception could set it.

    Yes, I'm paranoid :)

    Thanks,

    tglx

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