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

Lai Jiangshan
2014-11-27T06:59:45Z

>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
>

Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>

> 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;
}

> + 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);
>
> --
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at http://vger.kernel.org/majordomo-info.html
> Please read the FAQ at http://www.tux.org/lkml/
> .
>