Re: [PATCH v4 2/5] x86, traps: Track entry into and exit from IST context

Paul E. McKenney
2014-12-01T16:46:58Z

On Thu, Nov 27, 2014 at 03:03:22PM +0800, Lai Jiangshan wrote:
>
> >
> > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
>
> Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>

Thank you, recorded!

> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 8749f43f3f05..fc0236992655 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -759,39 +759,71 @@ void rcu_irq_enter(void)
> > /**
> > * rcu_nmi_enter - inform RCU of entry to NMI context
> > *
> > - * If the CPU was idle with dynamic ticks active, and there is no
> > - * irq handler running, this updates rdtp->dynticks_nmi to let the
> > - * RCU grace-period handling know that the CPU is active.
> > + * If the CPU was idle from RCU's viewpoint, update rdtp->dynticks and
> > + * rdtp->dynticks_nmi_nesting to let the RCU grace-period handling know
> > + * that the CPU is active. This implementation permits nested NMIs, as
> > + * long as the nesting level does not overflow an int. (You will probably
> > + * run out of stack space first.)
> > */
> > void rcu_nmi_enter(void)
> > {
> > struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
> > + int incby = 2;
> >
> > - if (rdtp->dynticks_nmi_nesting == 0 &&
> > - (atomic_read(&rdtp->dynticks) & 0x1))
> > - return;
> > - rdtp->dynticks_nmi_nesting++;
> > - smp_mb__before_atomic(); /* Force delay from prior write. */
> > - atomic_inc(&rdtp->dynticks);
> > - /* CPUs seeing atomic_inc() must see later RCU read-side crit sects */
> > - smp_mb__after_atomic(); /* See above. */
> > - WARN_ON_ONCE(!(atomic_read(&rdtp->dynticks) & 0x1));
> > + /* Complain about underflow. */
> > + WARN_ON_ONCE(rdtp->dynticks_nmi_nesting < 0);
> > +
> > + /*
> > + * If idle from RCU viewpoint, atomically increment ->dynticks
> > + * to mark non-idle and increment ->dynticks_nmi_nesting by one.
> > + * Otherwise, increment ->dynticks_nmi_nesting by two. This means
> > + * if ->dynticks_nmi_nesting is equal to one, we are guaranteed
> > + * to be in the outermost NMI handler that interrupted an RCU-idle
> > + * period (observation due to Andy Lutomirski).
> > + */
> > + if (!(atomic_read(&rdtp->dynticks) & 0x1)) {
> > + smp_mb__before_atomic(); /* Force delay from prior write. */
> > + atomic_inc(&rdtp->dynticks);
> > + /* atomic_inc() before later RCU read-side crit sects */
> > + smp_mb__after_atomic(); /* See above. */
> > + WARN_ON_ONCE(!(atomic_read(&rdtp->dynticks) & 0x1));
> > + incby = 1;
> > + }
> > + rdtp->dynticks_nmi_nesting += incby;
>
> I prefer a "else" branch here.
>
> if (!(atomic_read(&rdtp->dynticks) & 0x1)) {
> ...
> WARN_ON_ONCE(rdtp->dynticks_nmi_nesting); /* paired with "rdtp->dynticks_nmi_nesting = 0" */
> rdtp->dynticks_nmi_nesting = 1; /* paired with "if (rdtp->dynticks_nmi_nesting != 1) {" */
> } else {
> rdtp->dynticks_nmi_nesting += 2;
> }

I avoided this approach due to the extra line.

Thanx, Paul

> > + barrier();
> > }
> >
> > /**
> > * rcu_nmi_exit - inform RCU of exit from NMI context
> > *
> > - * If the CPU was idle with dynamic ticks active, and there is no
> > - * irq handler running, this updates rdtp->dynticks_nmi to let the
> > - * RCU grace-period handling know that the CPU is no longer active.
> > + * If we are returning from the outermost NMI handler that interrupted an
> > + * RCU-idle period, update rdtp->dynticks and rdtp->dynticks_nmi_nesting
> > + * to let the RCU grace-period handling know that the CPU is back to
> > + * being RCU-idle.
> > */
> > void rcu_nmi_exit(void)
> > {
> > struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
> >
> > - if (rdtp->dynticks_nmi_nesting == 0 ||
> > - --rdtp->dynticks_nmi_nesting != 0)
> > + /*
> > + * Check for ->dynticks_nmi_nesting underflow and bad ->dynticks.
> > + * (We are exiting an NMI handler, so RCU better be paying attention
> > + * to us!)
> > + */
> > + WARN_ON_ONCE(rdtp->dynticks_nmi_nesting <= 0);
> > + WARN_ON_ONCE(!(atomic_read(&rdtp->dynticks) & 0x1));
> > +
> > + /*
> > + * If the nesting level is not 1, the CPU wasn't RCU-idle, so
> > + * leave it in non-RCU-idle state.
> > + */
> > + if (rdtp->dynticks_nmi_nesting != 1) {
> > + rdtp->dynticks_nmi_nesting -= 2;
> > return;
> > + }
> > +
> > + /* This NMI interrupted an RCU-idle CPU, restore RCU-idleness. */
> > + rdtp->dynticks_nmi_nesting = 0;
> > /* CPUs seeing atomic_inc() must see prior RCU read-side crit sects */
> > smp_mb__before_atomic(); /* See above. */
> > atomic_inc(&rdtp->dynticks);
> >
> > --
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