Merge branch 'core-rcu-for-linus' of...

The CONFIG_RCU_CPU_STALL_DETECTOR kernel config parameter enables

RCU's CPU stall detector, which detects conditions that unduly delay

RCU grace periods.  The stall detector's idea of what constitutes

"unduly delayed" is controlled by a pair of C preprocessor macros:

"unduly delayed" is controlled by a set of C preprocessor macros:

RCU_SECONDS_TILL_STALL_CHECK

	This macro defines the period of time that RCU will wait from

	the beginning of a grace period until it issues an RCU CPU

	stall warning.	It is normally ten seconds.

	stall warning.	This time period is normally ten seconds.

RCU_SECONDS_TILL_STALL_RECHECK

	This macro defines the period of time that RCU will wait after

	issuing a stall warning until it issues another stall warning.

	It is normally set to thirty seconds.

	issuing a stall warning until it issues another stall warning

	for the same stall.  This time period is normally set to thirty

	seconds.

RCU_STALL_RAT_DELAY

	The CPU stall detector tries to make the offending CPU rat on itself,

	as this often gives better-quality stack traces.  However, if

	the offending CPU does not detect its own stall in the number

	of jiffies specified by RCU_STALL_RAT_DELAY, then other CPUs will

	complain.  This is normally set to two jiffies.

	The CPU stall detector tries to make the offending CPU print its

	own warnings, as this often gives better-quality stack traces.

	However, if the offending CPU does not detect its own stall in

	the number of jiffies specified by RCU_STALL_RAT_DELAY, then

	some other CPU will complain.  This delay is normally set to

	two jiffies.

The following problems can result in an RCU CPU stall warning:

When a CPU detects that it is stalling, it will print a message similar

to the following:

INFO: rcu_sched_state detected stall on CPU 5 (t=2500 jiffies)

This message indicates that CPU 5 detected that it was causing a stall,

and that the stall was affecting RCU-sched.  This message will normally be

followed by a stack dump of the offending CPU.  On TREE_RCU kernel builds,

RCU and RCU-sched are implemented by the same underlying mechanism,

while on TREE_PREEMPT_RCU kernel builds, RCU is instead implemented

by rcu_preempt_state.

On the other hand, if the offending CPU fails to print out a stall-warning

message quickly enough, some other CPU will print a message similar to

the following:

INFO: rcu_bh_state detected stalls on CPUs/tasks: { 3 5 } (detected by 2, 2502 jiffies)

This message indicates that CPU 2 detected that CPUs 3 and 5 were both

causing stalls, and that the stall was affecting RCU-bh.  This message

will normally be followed by stack dumps for each CPU.  Please note that

TREE_PREEMPT_RCU builds can be stalled by tasks as well as by CPUs,

and that the tasks will be indicated by PID, for example, "P3421".

It is even possible for a rcu_preempt_state stall to be caused by both

CPUs -and- tasks, in which case the offending CPUs and tasks will all

be called out in the list.

Finally, if the grace period ends just as the stall warning starts

printing, there will be a spurious stall-warning message:

INFO: rcu_bh_state detected stalls on CPUs/tasks: { } (detected by 4, 2502 jiffies)

This is rare, but does happen from time to time in real life.

So your kernel printed an RCU CPU stall warning.  The next question is

"What caused it?"  The following problems can result in RCU CPU stall

warnings:

o	A CPU looping in an RCU read-side critical section.

o	A CPU looping with interrupts disabled.

o	A CPU looping with interrupts disabled.  This condition can

	result in RCU-sched and RCU-bh stalls.

o	A CPU looping with preemption disabled.

o	A CPU looping with preemption disabled.  This condition can

	result in RCU-sched stalls and, if ksoftirqd is in use, RCU-bh

	stalls.

o	A CPU looping with bottom halves disabled.  This condition can

	result in RCU-sched and RCU-bh stalls.

o	For !CONFIG_PREEMPT kernels, a CPU looping anywhere in the kernel

	without invoking schedule().

o	A bug in the RCU implementation.

o	A hardware failure.  This is quite unlikely, but has occurred

	at least once in a former life.  A CPU failed in a running system,

	at least once in real life.  A CPU failed in a running system,

	becoming unresponsive, but not causing an immediate crash.

	This resulted in a series of RCU CPU stall warnings, eventually

	leading the realization that the CPU had failed.

The RCU, RCU-sched, and RCU-bh implementations have CPU stall warning.

SRCU does not do so directly, but its calls to synchronize_sched() will

result in RCU-sched detecting any CPU stalls that might be occurring.

To diagnose the cause of the stall, inspect the stack traces.  The offending

function will usually be near the top of the stack.  If you have a series

of stall warnings from a single extended stall, comparing the stack traces

can often help determine where the stall is occurring, which will usually

be in the function nearest the top of the stack that stays the same from

trace to trace.

The RCU, RCU-sched, and RCU-bh implementations have CPU stall

warning.  SRCU does not have its own CPU stall warnings, but its

calls to synchronize_sched() will result in RCU-sched detecting

RCU-sched-related CPU stalls.  Please note that RCU only detects

CPU stalls when there is a grace period in progress.  No grace period,

no CPU stall warnings.

To diagnose the cause of the stall, inspect the stack traces.

The offending function will usually be near the top of the stack.

If you have a series of stall warnings from a single extended stall,

comparing the stack traces can often help determine where the stall

is occurring, which will usually be in the function nearest the top of

that portion of the stack which remains the same from trace to trace.

If you can reliably trigger the stall, ftrace can be quite helpful.

RCU bugs can often be debugged with the help of CONFIG_RCU_TRACE.

The output of "cat rcu/rcu_pending" looks as follows:

rcu_sched:

  0 np=255892 qsp=53936 cbr=0 cng=14417 gpc=10033 gps=24320 nf=6445 nn=146741

  1 np=261224 qsp=54638 cbr=0 cng=25723 gpc=16310 gps=2849 nf=5912 nn=155792

  2 np=237496 qsp=49664 cbr=0 cng=2762 gpc=45478 gps=1762 nf=1201 nn=136629

  3 np=236249 qsp=48766 cbr=0 cng=286 gpc=48049 gps=1218 nf=207 nn=137723

  4 np=221310 qsp=46850 cbr=0 cng=26 gpc=43161 gps=4634 nf=3529 nn=123110

  5 np=237332 qsp=48449 cbr=0 cng=54 gpc=47920 gps=3252 nf=201 nn=137456

  6 np=219995 qsp=46718 cbr=0 cng=50 gpc=42098 gps=6093 nf=4202 nn=120834

  7 np=249893 qsp=49390 cbr=0 cng=72 gpc=38400 gps=17102 nf=41 nn=144888

  0 np=255892 qsp=53936 rpq=85 cbr=0 cng=14417 gpc=10033 gps=24320 nf=6445 nn=146741

  1 np=261224 qsp=54638 rpq=33 cbr=0 cng=25723 gpc=16310 gps=2849 nf=5912 nn=155792

  2 np=237496 qsp=49664 rpq=23 cbr=0 cng=2762 gpc=45478 gps=1762 nf=1201 nn=136629

  3 np=236249 qsp=48766 rpq=98 cbr=0 cng=286 gpc=48049 gps=1218 nf=207 nn=137723

  4 np=221310 qsp=46850 rpq=7 cbr=0 cng=26 gpc=43161 gps=4634 nf=3529 nn=123110

  5 np=237332 qsp=48449 rpq=9 cbr=0 cng=54 gpc=47920 gps=3252 nf=201 nn=137456

  6 np=219995 qsp=46718 rpq=12 cbr=0 cng=50 gpc=42098 gps=6093 nf=4202 nn=120834

  7 np=249893 qsp=49390 rpq=42 cbr=0 cng=72 gpc=38400 gps=17102 nf=41 nn=144888

rcu_bh:

  0 np=146741 qsp=1419 cbr=0 cng=6 gpc=0 gps=0 nf=2 nn=145314

  1 np=155792 qsp=12597 cbr=0 cng=0 gpc=4 gps=8 nf=3 nn=143180

  2 np=136629 qsp=18680 cbr=0 cng=0 gpc=7 gps=6 nf=0 nn=117936

  3 np=137723 qsp=2843 cbr=0 cng=0 gpc=10 gps=7 nf=0 nn=134863

  4 np=123110 qsp=12433 cbr=0 cng=0 gpc=4 gps=2 nf=0 nn=110671

  5 np=137456 qsp=4210 cbr=0 cng=0 gpc=6 gps=5 nf=0 nn=133235

  6 np=120834 qsp=9902 cbr=0 cng=0 gpc=6 gps=3 nf=2 nn=110921

  7 np=144888 qsp=26336 cbr=0 cng=0 gpc=8 gps=2 nf=0 nn=118542

  0 np=146741 qsp=1419 rpq=6 cbr=0 cng=6 gpc=0 gps=0 nf=2 nn=145314

  1 np=155792 qsp=12597 rpq=3 cbr=0 cng=0 gpc=4 gps=8 nf=3 nn=143180

  2 np=136629 qsp=18680 rpq=1 cbr=0 cng=0 gpc=7 gps=6 nf=0 nn=117936

  3 np=137723 qsp=2843 rpq=0 cbr=0 cng=0 gpc=10 gps=7 nf=0 nn=134863

  4 np=123110 qsp=12433 rpq=0 cbr=0 cng=0 gpc=4 gps=2 nf=0 nn=110671

  5 np=137456 qsp=4210 rpq=1 cbr=0 cng=0 gpc=6 gps=5 nf=0 nn=133235

  6 np=120834 qsp=9902 rpq=2 cbr=0 cng=0 gpc=6 gps=3 nf=2 nn=110921

  7 np=144888 qsp=26336 rpq=0 cbr=0 cng=0 gpc=8 gps=2 nf=0 nn=118542

As always, this is once again split into "rcu_sched" and "rcu_bh"

portions, with CONFIG_TREE_PREEMPT_RCU kernels having an additional

o	"qsp" is the number of times that the RCU was waiting for a

	quiescent state from this CPU.

o	"rpq" is the number of times that the CPU had passed through

	a quiescent state, but not yet reported it to RCU.

o	"cbr" is the number of times that this CPU had RCU callbacks

	that had passed through a grace period, and were thus ready

	to be invoked.

 * struct debug_obj - representaion of an tracked object

 * @node:	hlist node to link the object into the tracker list

 * @state:	tracked object state

 * @astate:	current active state

 * @object:	pointer to the real object

 * @descr:	pointer to an object type specific debug description structure

 */

struct debug_obj {

	struct hlist_node	node;

	enum debug_obj_state	state;

	unsigned int		astate;

	void			*object;

	struct debug_obj_descr	*descr;

};

extern void debug_object_destroy   (void *addr, struct debug_obj_descr *descr);

extern void debug_object_free      (void *addr, struct debug_obj_descr *descr);

/*

 * Active state:

 * - Set at 0 upon initialization.

 * - Must return to 0 before deactivation.

 */

extern void

debug_object_active_state(void *addr, struct debug_obj_descr *descr,

			  unsigned int expect, unsigned int next);

extern void debug_objects_early_init(void);

extern void debug_objects_mem_init(void);

#else

		{ .first = &init_task.pids[PIDTYPE_PGID].node },	\

		{ .first = &init_task.pids[PIDTYPE_SID].node },		\

	},								\

	.rcu		= RCU_HEAD_INIT,				\

	.level		= 0,						\

	.numbers	= { {						\

		.nr		= 0,					\

};

/* Exported common interfaces */

extern void synchronize_rcu_bh(void);

extern void synchronize_sched(void);

extern void rcu_barrier(void);

extern void rcu_barrier_bh(void);

extern void rcu_barrier_sched(void);

/* Internal to kernel */

extern void rcu_init(void);

extern int rcu_scheduler_active;

extern void rcu_scheduler_starting(void);

#if defined(CONFIG_TREE_RCU) || defined(CONFIG_TREE_PREEMPT_RCU)

#include <linux/rcutree.h>

       (ptr)->next = NULL; (ptr)->func = NULL; \

} while (0)

static inline void init_rcu_head_on_stack(struct rcu_head *head)

{

}

static inline void destroy_rcu_head_on_stack(struct rcu_head *head)

{

}

#ifdef CONFIG_DEBUG_LOCK_ALLOC

extern struct lockdep_map rcu_lock_map;

/**

 * rcu_read_lock_held - might we be in RCU read-side critical section?

 *

 * If CONFIG_PROVE_LOCKING is selected and enabled, returns nonzero iff in

 * an RCU read-side critical section.  In absence of CONFIG_PROVE_LOCKING,

 * If CONFIG_DEBUG_LOCK_ALLOC is selected, returns nonzero iff in an RCU

 * read-side critical section.  In absence of CONFIG_DEBUG_LOCK_ALLOC,

 * this assumes we are in an RCU read-side critical section unless it can

 * prove otherwise.

 *

 * Check rcu_scheduler_active to prevent false positives during boot.

 * Check debug_lockdep_rcu_enabled() to prevent false positives during boot

 * and while lockdep is disabled.

 */

static inline int rcu_read_lock_held(void)

{

/**

 * rcu_read_lock_sched_held - might we be in RCU-sched read-side critical section?

 *

 * If CONFIG_PROVE_LOCKING is selected and enabled, returns nonzero iff in an

 * RCU-sched read-side critical section.  In absence of CONFIG_PROVE_LOCKING,

 * this assumes we are in an RCU-sched read-side critical section unless it

 * can prove otherwise.  Note that disabling of preemption (including

 * disabling irqs) counts as an RCU-sched read-side critical section.

 * If CONFIG_DEBUG_LOCK_ALLOC is selected, returns nonzero iff in an

 * RCU-sched read-side critical section.  In absence of

 * CONFIG_DEBUG_LOCK_ALLOC, this assumes we are in an RCU-sched read-side

 * critical section unless it can prove otherwise.  Note that disabling

 * of preemption (including disabling irqs) counts as an RCU-sched

 * read-side critical section.

 *

 * Check rcu_scheduler_active to prevent false positives during boot.

 * Check debug_lockdep_rcu_enabled() to prevent false positives during boot

 * and while lockdep is disabled.

 */

#ifdef CONFIG_PREEMPT

static inline int rcu_read_lock_sched_held(void)

#ifdef CONFIG_PREEMPT

static inline int rcu_read_lock_sched_held(void)

{

	return !rcu_scheduler_active || preempt_count() != 0 || irqs_disabled();

	return preempt_count() != 0 || irqs_disabled();

}

#else /* #ifdef CONFIG_PREEMPT */

static inline int rcu_read_lock_sched_held(void)

extern int rcu_my_thread_group_empty(void);

#define __do_rcu_dereference_check(c)					\

	do {								\

		static bool __warned;					\

		if (debug_lockdep_rcu_enabled() && !__warned && !(c)) {	\

			__warned = true;				\

			lockdep_rcu_dereference(__FILE__, __LINE__);	\

		}							\

	} while (0)

/**

 * rcu_dereference_check - rcu_dereference with debug checking

 * @p: The pointer to read, prior to dereferencing

 */

#define rcu_dereference_check(p, c) \

	({ \

		if (debug_lockdep_rcu_enabled() && !(c)) \

			lockdep_rcu_dereference(__FILE__, __LINE__); \

		__do_rcu_dereference_check(c); \

		rcu_dereference_raw(p); \

	})

 */

#define rcu_dereference_protected(p, c) \

	({ \

		if (debug_lockdep_rcu_enabled() && !(c)) \

			lockdep_rcu_dereference(__FILE__, __LINE__); \

		__do_rcu_dereference_check(c); \

		(p); \

	})