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    SubjectRe: [PATCH 2/9] rcu: Fixup noinstr warnings
    On Wed, Jun 03, 2020 at 09:46:00AM -0700, Paul E. McKenney wrote:

    > > --- a/kernel/rcu/tree.c
    > > +++ b/kernel/rcu/tree.c
    > > @@ -250,7 +250,7 @@ static noinstr void rcu_dynticks_eqs_ent
    > > * next idle sojourn.
    > > */
    > > rcu_dynticks_task_trace_enter(); // Before ->dynticks update!
    > > - seq = atomic_add_return(RCU_DYNTICK_CTRL_CTR, &rdp->dynticks);
    > > + seq = arch_atomic_add_return(RCU_DYNTICK_CTRL_CTR, &rdp->dynticks);
    >
    > To preserve KCSAN's ability to see this, there would be something like
    > instrument_atomic_write(&rdp->dynticks, sizeof(rdp->dynticks)) prior
    > to the instrumentation_end() invoked before rcu_dynticks_eqs_enter()
    > in each of rcu_eqs_enter() and rcu_nmi_exit(), correct?

    Yes.

    > > // RCU is no longer watching. Better be in extended quiescent state!
    > > WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
    > > (seq & RCU_DYNTICK_CTRL_CTR));
    > > @@ -274,13 +274,13 @@ static noinstr void rcu_dynticks_eqs_exi
    > > * and we also must force ordering with the next RCU read-side
    > > * critical section.
    > > */
    > > - seq = atomic_add_return(RCU_DYNTICK_CTRL_CTR, &rdp->dynticks);
    > > + seq = arch_atomic_add_return(RCU_DYNTICK_CTRL_CTR, &rdp->dynticks);
    >
    > And same here, but after the instrumentation_begin() following
    > rcu_dynticks_eqs_exit() in both rcu_eqs_exit() and rcu_nmi_enter(),
    > correct?

    Yep.

    > > // RCU is now watching. Better not be in an extended quiescent state!
    > > rcu_dynticks_task_trace_exit(); // After ->dynticks update!
    > > WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
    > > !(seq & RCU_DYNTICK_CTRL_CTR));
    > > if (seq & RCU_DYNTICK_CTRL_MASK) {
    > > - atomic_andnot(RCU_DYNTICK_CTRL_MASK, &rdp->dynticks);
    > > + arch_atomic_andnot(RCU_DYNTICK_CTRL_MASK, &rdp->dynticks);
    >
    > This one is gone in -rcu.

    Good, because that would make things 'complicated' with the external
    instrumentation call. And is actually the reason I didn't even attempt
    it this time around.

    > > smp_mb__after_atomic(); /* _exit after clearing mask. */
    > > }
    > > }
    > > @@ -313,7 +313,7 @@ static __always_inline bool rcu_dynticks
    > > {
    > > struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
    > >
    > > - return !(atomic_read(&rdp->dynticks) & RCU_DYNTICK_CTRL_CTR);
    > > + return !(arch_atomic_read(&rdp->dynticks) & RCU_DYNTICK_CTRL_CTR);

    The above is actually instrumented by KCSAN, due to arch_atomic_read()
    being a READ_ONCE() and it now understanding volatile.

    > Also instrument_atomic_write(&rdp->dynticks, sizeof(rdp->dynticks)) as
    > follows:
    >
    > o rcu_nmi_exit(): After each following instrumentation_begin().

    Yes

    > o In theory in rcu_irq_exit_preempt(), but as this generates code
    > only in lockdep builds, it might not be worth worrying about.
    >
    > o Ditto for rcu_irq_exit_check_preempt().
    >
    > o Ditto for __rcu_irq_enter_check_tick().

    Not these, afaict they're all the above arch_atomic_read(), which is
    instrumented due to volatile in these cases.

    > o rcu_nmi_enter(): After each following instrumentation_begin().

    Yes

    > o __rcu_is_watching() is itself noinstr:
    >
    > o idtentry_enter_cond_rcu(): After each following
    > instrumentation_begin().
    >
    > o rcu_is_watching(): Either before or after the call to
    > rcu_dynticks_curr_cpu_in_eqs().

    Something like that yes.

    > > }
    > >
    > > /*
    > > @@ -692,6 +692,7 @@ noinstr void rcu_nmi_exit(void)
    > > {
    > > struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
    > >
    > > + instrumentation_begin();
    > > /*
    > > * Check for ->dynticks_nmi_nesting underflow and bad ->dynticks.
    > > * (We are exiting an NMI handler, so RCU better be paying attention
    > > @@ -705,7 +706,6 @@ noinstr void rcu_nmi_exit(void)
    > > * leave it in non-RCU-idle state.
    > > */
    > > if (rdp->dynticks_nmi_nesting != 1) {
    > > - instrumentation_begin();
    > > trace_rcu_dyntick(TPS("--="), rdp->dynticks_nmi_nesting, rdp->dynticks_nmi_nesting - 2,
    > > atomic_read(&rdp->dynticks));
    > > WRITE_ONCE(rdp->dynticks_nmi_nesting, /* No store tearing. */
    > > @@ -714,7 +714,6 @@ noinstr void rcu_nmi_exit(void)
    > > return;
    > > }
    > >
    > > - instrumentation_begin();
    > > /* This NMI interrupted an RCU-idle CPU, restore RCU-idleness. */
    > > trace_rcu_dyntick(TPS("Startirq"), rdp->dynticks_nmi_nesting, 0, atomic_read(&rdp->dynticks));
    > > WRITE_ONCE(rdp->dynticks_nmi_nesting, 0); /* Avoid store tearing. */
    >
    > This one looks to be having no effect on instrumentation of atomics, but
    > rather coalescing a pair of instrumentation_begin() into one.
    >
    > Do I understand correctly?

    Almost, it puts the WARN_ON_ONCE()s under instrumentation_begin() too,
    and that makes a differnce, iirc it was the
    rcu_dynticks_curr_cpu_in_eqs() call that stood out. But that could've
    been before I switched it to arch_atomic_read(). In any case, I find
    this form a lot clearer.

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