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rcu: Make rcu_nmi_enter() handle nesting 

The x86 architecture has multiple types of NMI-like interrupts: real
NMIs, machine checks, and, for some values of NMI-like, debugging
and breakpoint interrupts.  These interrupts can nest inside each
other.  Andy Lutomirski is adding RCU support to these interrupts,
so rcu_nmi_enter() and rcu_nmi_exit() must now correctly handle nesting.

This commit therefore introduces nesting, using a clever NMI-coordination
algorithm suggested by Andy.  The trick is to atomically increment
->dynticks (if needed) before manipulating ->dynticks_nmi_nesting on entry
(and, accordingly, after on exit).  In addition, ->dynticks_nmi_nesting
is incremented by one if ->dynticks was incremented and by two otherwise.
This means that when rcu_nmi_exit() sees ->dynticks_nmi_nesting equal
to one, it knows that ->dynticks must be atomically incremented.

This NMI-coordination algorithms has been validated by the following
Promela model:

------------------------------------------------------------------------

/*
 * Promela model for Andy Lutomirski's suggested change to rcu_nmi_enter()
 * that allows nesting.
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, you can access it online at
 * http://www.gnu.org/licenses/gpl-2.0.html.
 *
 * Copyright IBM Corporation, 2014
 *
 * Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
 */

byte dynticks_nmi_nesting = 0;
byte dynticks = 0;

/*
 * Promela verision of rcu_nmi_enter().
 */
inline rcu_nmi_enter()
{
	byte incby;
	byte tmp;

	incby = BUSY_INCBY;
	assert(dynticks_nmi_nesting >= 0);
	if
	:: (dynticks & 1) == 0 ->
		atomic {
			dynticks = dynticks + 1;
		}
		assert((dynticks & 1) == 1);
		incby = 1;
	:: else ->
		skip;
	fi;
	tmp = dynticks_nmi_nesting;
	tmp = tmp + incby;
	dynticks_nmi_nesting = tmp;
	assert(dynticks_nmi_nesting >= 1);
}

/*
 * Promela verision of rcu_nmi_exit().
 */
inline rcu_nmi_exit()
{
	byte tmp;

	assert(dynticks_nmi_nesting > 0);
	assert((dynticks & 1) != 0);
	if
	:: dynticks_nmi_nesting != 1 ->
		tmp = dynticks_nmi_nesting;
		tmp = tmp - BUSY_INCBY;
		dynticks_nmi_nesting = tmp;
	:: else ->
		dynticks_nmi_nesting = 0;
		atomic {
			dynticks = dynticks + 1;
		}
		assert((dynticks & 1) == 0);
	fi;
}

/*
 * Base-level NMI runs non-atomically.  Crudely emulates process-level
 * dynticks-idle entry/exit.
 */
proctype base_NMI()
{
	byte busy;

	busy = 0;
	do
	::	/* Emulate base-level dynticks and not. */
		if
		:: 1 ->	atomic {
				dynticks = dynticks + 1;
			}
			busy = 1;
		:: 1 ->	skip;
		fi;

		/* Verify that we only sometimes have base-level dynticks. */
		if
		:: busy == 0 -> skip;
		:: busy == 1 -> skip;
		fi;

		/* Model RCU's NMI entry and exit actions. */
		rcu_nmi_enter();
		assert((dynticks & 1) == 1);
		rcu_nmi_exit();

		/* Emulated re-entering base-level dynticks and not. */
		if
		:: !busy -> skip;
		:: busy ->
			atomic {
				dynticks = dynticks + 1;
			}
			busy = 0;
		fi;

		/* We had better now be in dyntick-idle mode. */
		assert((dynticks & 1) == 0);
	od;
}

/*
 * Nested NMI runs atomically to emulate interrupting base_level().
 */
proctype nested_NMI()
{
	do
	::	/*
		 * Use an atomic section to model a nested NMI.  This is
		 * guaranteed to interleave into base_NMI() between a pair
		 * of base_NMI() statements, just as a nested NMI would.
		 */
		atomic {
			/* Verify that we only sometimes are in dynticks. */
			if
			:: (dynticks & 1) == 0 -> skip;
			:: (dynticks & 1) == 1 -> skip;
			fi;

			/* Model RCU's NMI entry and exit actions. */
			rcu_nmi_enter();
			assert((dynticks & 1) == 1);
			rcu_nmi_exit();
		}
	od;
}

init {
	run base_NMI();
	run nested_NMI();
}

------------------------------------------------------------------------

The following script can be used to run this model if placed in
rcu_nmi.spin:

------------------------------------------------------------------------

if ! spin -a rcu_nmi.spin
then
	echo Spin errors!!!
	exit 1
fi
if ! cc -DSAFETY -o pan pan.c
then
	echo Compilation errors!!!
	exit 1
fi
./pan -m100000

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
This commit is contained in:
Paul E. McKenney 2014-11-21 14:45:12 -08:00
parent 97bf6af1f9
commit 734d168013
1 changed files with 49 additions and 17 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

58
kernel/rcu/tree.c
View File

@ -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++;
/* 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);
/* CPUs seeing atomic_inc() must see later RCU read-side crit sects */
/* 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;
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);