Messages in this thread | | | Date | Wed, 3 Jun 2020 19:13:20 +0200 | From | Peter Zijlstra <> | Subject | Re: [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.
| |