Merge branch 'core-rcu-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

	two jiffies.  (This is a cpp macro, not a kernel configuration

	parameter.)

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

to the following:

rcupdate.rcu_task_stall_timeout

	This boot/sysfs parameter controls the RCU-tasks stall warning

	interval.  A value of zero or less suppresses RCU-tasks stall

	warnings.  A positive value sets the stall-warning interval

	in jiffies.  An RCU-tasks stall warning starts wtih the line:

		INFO: rcu_tasks detected stalls on tasks:

	And continues with the output of sched_show_task() for each

	task stalling the current RCU-tasks grace period.

For non-RCU-tasks flavors of RCU, 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)

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	For !CONFIG_PREEMPT kernels, a CPU looping anywhere in the

	kernel without invoking schedule().  Note that cond_resched()

	does not necessarily prevent RCU CPU stall warnings.  Therefore,

	if the looping in the kernel is really expected and desirable

	behavior, you might need to replace some of the cond_resched()

	calls with calls to cond_resched_rcu_qs().

o	A CPU-bound real-time task in a CONFIG_PREEMPT kernel, which might

	happen to preempt a low-priority task in the middle of an RCU

	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 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.

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

warning.  Note that SRCU does -not- have CPU stall warnings.  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.

	lockd.nlm_udpport=M	[NFS] Assign UDP port.

			Format: <integer>

	locktorture.nreaders_stress= [KNL]

			Set the number of locking read-acquisition kthreads.

			Defaults to being automatically set based on the

			number of online CPUs.

	locktorture.nwriters_stress= [KNL]

			Set the number of locking write-acquisition kthreads.

	locktorture.onoff_holdoff= [KNL]

			Set time (s) after boot for CPU-hotplug testing.

	locktorture.onoff_interval= [KNL]

			Set time (s) between CPU-hotplug operations, or

			zero to disable CPU-hotplug testing.

	locktorture.shuffle_interval= [KNL]

			Set task-shuffle interval (jiffies).  Shuffling

			tasks allows some CPUs to go into dyntick-idle

			mode during the locktorture test.

	locktorture.shutdown_secs= [KNL]

			Set time (s) after boot system shutdown.  This

			is useful for hands-off automated testing.

	locktorture.stat_interval= [KNL]

			Time (s) between statistics printk()s.

	locktorture.stutter= [KNL]

			Time (s) to stutter testing, for example,

			specifying five seconds causes the test to run for

			five seconds, wait for five seconds, and so on.

			This tests the locking primitive's ability to

			transition abruptly to and from idle.

	locktorture.torture_runnable= [BOOT]

			Start locktorture running at boot time.

	locktorture.torture_type= [KNL]

			Specify the locking implementation to test.

	locktorture.verbose= [KNL]

			Enable additional printk() statements.

	logibm.irq=	[HW,MOUSE] Logitech Bus Mouse Driver

			Format: <irq>

			Lazy RCU callbacks are those which RCU can

			prove do nothing more than free memory.

	rcutorture.cbflood_inter_holdoff= [KNL]

			Set holdoff time (jiffies) between successive

			callback-flood tests.

	rcutorture.cbflood_intra_holdoff= [KNL]

			Set holdoff time (jiffies) between successive

			bursts of callbacks within a given callback-flood

			test.

	rcutorture.cbflood_n_burst= [KNL]

			Set the number of bursts making up a given

			callback-flood test.  Set this to zero to

			disable callback-flood testing.

	rcutorture.cbflood_n_per_burst= [KNL]

			Set the number of callbacks to be registered

			in a given burst of a callback-flood test.

	rcutorture.fqs_duration= [KNL]

			Set duration of force_quiescent_state bursts.

			Set time (s) between CPU-hotplug operations, or

			zero to disable CPU-hotplug testing.

	rcutorture.rcutorture_runnable= [BOOT]

	rcutorture.torture_runnable= [BOOT]

			Start rcutorture running at boot time.

	rcutorture.shuffle_interval= [KNL]

	rcupdate.rcu_cpu_stall_timeout= [KNL]

			Set timeout for RCU CPU stall warning messages.

	rcupdate.rcu_task_stall_timeout= [KNL]

			Set timeout in jiffies for RCU task stall warning

			messages.  Disable with a value less than or equal

			to zero.

	rdinit=		[KNL]

			Format: <full_path>

			Run specified binary instead of /init from the ramdisk,

Kernel Lock Torture Test Operation

CONFIG_LOCK_TORTURE_TEST

The CONFIG LOCK_TORTURE_TEST config option provides a kernel module

that runs torture tests on core kernel locking primitives. The kernel

module, 'locktorture', may be built after the fact on the running

kernel to be tested, if desired. The tests periodically output status

messages via printk(), which can be examined via the dmesg (perhaps

grepping for "torture").  The test is started when the module is loaded,

and stops when the module is unloaded. This program is based on how RCU

is tortured, via rcutorture.

This torture test consists of creating a number of kernel threads which

acquire the lock and hold it for specific amount of time, thus simulating

different critical region behaviors. The amount of contention on the lock

can be simulated by either enlarging this critical region hold time and/or

creating more kthreads.

MODULE PARAMETERS

This module has the following parameters:

	    ** Locktorture-specific **

nwriters_stress   Number of kernel threads that will stress exclusive lock

		  ownership (writers). The default value is twice the number

		  of online CPUs.

nreaders_stress   Number of kernel threads that will stress shared lock

		  ownership (readers). The default is the same amount of writer

		  locks. If the user did not specify nwriters_stress, then

		  both readers and writers be the amount of online CPUs.

torture_type	  Type of lock to torture. By default, only spinlocks will

		  be tortured. This module can torture the following locks,

		  with string values as follows:

		     o "lock_busted": Simulates a buggy lock implementation.

		     o "spin_lock": spin_lock() and spin_unlock() pairs.

		     o "spin_lock_irq": spin_lock_irq() and spin_unlock_irq()

					pairs.

		     o "rw_lock": read/write lock() and unlock() rwlock pairs.

		     o "rw_lock_irq": read/write lock_irq() and unlock_irq()

				      rwlock pairs.

		     o "mutex_lock": mutex_lock() and mutex_unlock() pairs.

		     o "rwsem_lock": read/write down() and up() semaphore pairs.

torture_runnable  Start locktorture at boot time in the case where the

		  module is built into the kernel, otherwise wait for

		  torture_runnable to be set via sysfs before starting.

		  By default it will begin once the module is loaded.

	    ** Torture-framework (RCU + locking) **

shutdown_secs	  The number of seconds to run the test before terminating

		  the test and powering off the system.  The default is

		  zero, which disables test termination and system shutdown.

		  This capability is useful for automated testing.

onoff_interval	  The number of seconds between each attempt to execute a

		  randomly selected CPU-hotplug operation.  Defaults

		  to zero, which disables CPU hotplugging.  In

		  CONFIG_HOTPLUG_CPU=n kernels, locktorture will silently

		  refuse to do any CPU-hotplug operations regardless of

		  what value is specified for onoff_interval.

onoff_holdoff	  The number of seconds to wait until starting CPU-hotplug

		  operations.  This would normally only be used when

		  locktorture was built into the kernel and started

		  automatically at boot time, in which case it is useful

		  in order to avoid confusing boot-time code with CPUs

		  coming and going. This parameter is only useful if

		  CONFIG_HOTPLUG_CPU is enabled.

stat_interval	  Number of seconds between statistics-related printk()s.

		  By default, locktorture will report stats every 60 seconds.

		  Setting the interval to zero causes the statistics to

		  be printed -only- when the module is unloaded, and this

		  is the default.

stutter		  The length of time to run the test before pausing for this

		  same period of time.  Defaults to "stutter=5", so as

		  to run and pause for (roughly) five-second intervals.

		  Specifying "stutter=0" causes the test to run continuously

		  without pausing, which is the old default behavior.

shuffle_interval  The number of seconds to keep the test threads affinitied

		  to a particular subset of the CPUs, defaults to 3 seconds.

		  Used in conjunction with test_no_idle_hz.

verbose		  Enable verbose debugging printing, via printk(). Enabled

		  by default. This extra information is mostly related to

		  high-level errors and reports from the main 'torture'

		  framework.

STATISTICS

Statistics are printed in the following format:

spin_lock-torture: Writes:  Total: 93746064  Max/Min: 0/0   Fail: 0

   (A)		    (B)		   (C)		  (D)	       (E)

(A): Lock type that is being tortured -- torture_type parameter.

(B): Number of writer lock acquisitions. If dealing with a read/write primitive

     a second "Reads" statistics line is printed.

(C): Number of times the lock was acquired.

(D): Min and max number of times threads failed to acquire the lock.

(E): true/false values if there were errors acquiring the lock. This should

     -only- be positive if there is a bug in the locking primitive's

     implementation. Otherwise a lock should never fail (i.e., spin_lock()).

     Of course, the same applies for (C), above. A dummy example of this is

     the "lock_busted" type.

USAGE

The following script may be used to torture locks:

	#!/bin/sh

	modprobe locktorture

	sleep 3600

	rmmod locktorture

	dmesg | grep torture:

The output can be manually inspected for the error flag of "!!!".

One could of course create a more elaborate script that automatically

checked for such errors.  The "rmmod" command forces a "SUCCESS",

"FAILURE", or "RCU_HOTPLUG" indication to be printk()ed.  The first

two are self-explanatory, while the last indicates that while there

were no locking failures, CPU-hotplug problems were detected.

Also see: Documentation/RCU/torture.txt

in the following example:

	q = ACCESS_ONCE(a);

	if (ACCESS_ONCE(q)) {

		ACCESS_ONCE(b) = p;

	}

Please note that ACCESS_ONCE() is not optional!  Without the ACCESS_ONCE(),

the compiler is within its rights to transform this example:

	q = a;

	if (q) {

		b = p;  /* BUG: Compiler can reorder!!! */

		do_something();

	} else {

		b = p;  /* BUG: Compiler can reorder!!! */

		do_something_else();

		ACCESS_ONCE(b) = p;

	}

into this, which of course defeats the ordering:

	b = p;

	q = a;

	if (q)

		do_something();

	else

		do_something_else();

Please note that ACCESS_ONCE() is not optional!  Without the

ACCESS_ONCE(), might combine the load from 'a' with other loads from

'a', and the store to 'b' with other stores to 'b', with possible highly

counterintuitive effects on ordering.

Worse yet, if the compiler is able to prove (say) that the value of

variable 'a' is always non-zero, it would be well within its rights

as follows:

	q = a;

	b = p;  /* BUG: Compiler can reorder!!! */

	do_something();

	b = p;  /* BUG: Compiler and CPU can both reorder!!! */

The solution is again ACCESS_ONCE() and barrier(), which preserves the

ordering between the load from variable 'a' and the store to variable 'b':

So don't leave out the ACCESS_ONCE().

It is tempting to try to enforce ordering on identical stores on both

branches of the "if" statement as follows:

	q = ACCESS_ONCE(a);

	if (q) {

		do_something_else();

	}

The initial ACCESS_ONCE() is required to prevent the compiler from

proving the value of 'a', and the pair of barrier() invocations are

required to prevent the compiler from pulling the two identical stores

to 'b' out from the legs of the "if" statement.

It is important to note that control dependencies absolutely require a

a conditional.  For example, the following "optimized" version of

the above example breaks ordering, which is why the barrier() invocations

are absolutely required if you have identical stores in both legs of

the "if" statement:

Unfortunately, current compilers will transform this as follows at high

optimization levels:

	q = ACCESS_ONCE(a);

	barrier();

	ACCESS_ONCE(b) = p;  /* BUG: No ordering vs. load from a!!! */

	if (q) {

		/* ACCESS_ONCE(b) = p; -- moved up, BUG!!! */

		do_something_else();

	}

It is of course legal for the prior load to be part of the conditional,

for example, as follows:

Now there is no conditional between the load from 'a' and the store to

'b', which means that the CPU is within its rights to reorder them:

The conditional is absolutely required, and must be present in the

assembly code even after all compiler optimizations have been applied.

Therefore, if you need ordering in this example, you need explicit

memory barriers, for example, smp_store_release():

	if (ACCESS_ONCE(a) > 0) {

		barrier();

		ACCESS_ONCE(b) = q / 2;

	q = ACCESS_ONCE(a);

	if (q) {

		smp_store_release(&b, p);

		do_something();

	} else {

		barrier();

		ACCESS_ONCE(b) = q / 3;

		smp_store_release(&b, p);

		do_something_else();

	}

This will again ensure that the load from variable 'a' is ordered before the

stores to variable 'b'.

In contrast, without explicit memory barriers, two-legged-if control

ordering is guaranteed only when the stores differ, for example:

	q = ACCESS_ONCE(a);

	if (q) {

		ACCESS_ONCE(b) = p;

		do_something();

	} else {

		ACCESS_ONCE(b) = r;

		do_something_else();

	}

The initial ACCESS_ONCE() is still required to prevent the compiler from

proving the value of 'a'.

In addition, you need to be careful what you do with the local variable 'q',

otherwise the compiler might be able to guess the value and again remove

	q = ACCESS_ONCE(a);

	if (q % MAX) {

		barrier();

		ACCESS_ONCE(b) = p;

		do_something();

	} else {

		barrier();

		ACCESS_ONCE(b) = p;

		ACCESS_ONCE(b) = r;

		do_something_else();

	}

	ACCESS_ONCE(b) = p;

	do_something_else();

This transformation loses the ordering between the load from variable 'a'

and the store to variable 'b'.  If you are relying on this ordering, you

should do something like the following:

Given this transformation, the CPU is not required to respect the ordering

between the load from variable 'a' and the store to variable 'b'.  It is

tempting to add a barrier(), but this does not help.  The conditional

is gone, and the barrier won't bring it back.  Therefore, if you are

relying on this ordering, you should make sure that MAX is greater than

one, perhaps as follows:

	q = ACCESS_ONCE(a);

	BUILD_BUG_ON(MAX <= 1); /* Order load from a with store to b. */

		ACCESS_ONCE(b) = p;

		do_something();

	} else {

		ACCESS_ONCE(b) = p;

		ACCESS_ONCE(b) = r;

		do_something_else();

	}

Please note once again that the stores to 'b' differ.  If they were

identical, as noted earlier, the compiler could pull this store outside

of the 'if' statement.

Finally, control dependencies do -not- provide transitivity.  This is

demonstrated by two related examples:

demonstrated by two related examples, with the initial values of

x and y both being zero:

	CPU 0                     CPU 1

	=====================     =====================

	r1 = ACCESS_ONCE(x);      r2 = ACCESS_ONCE(y);

	if (r1 >= 0)              if (r2 >= 0)

	if (r1 > 0)               if (r2 > 0)

	  ACCESS_ONCE(y) = 1;       ACCESS_ONCE(x) = 1;

	assert(!(r1 == 1 && r2 == 1));

The above two-CPU example will never trigger the assert().  However,

if control dependencies guaranteed transitivity (which they do not),

then adding the following two CPUs would guarantee a related assertion:

then adding the following CPU would guarantee a related assertion:

	CPU 2                     CPU 3

	=====================     =====================

	ACCESS_ONCE(x) = 2;       ACCESS_ONCE(y) = 2;

	CPU 2

	=====================

	ACCESS_ONCE(x) = 2;

	assert(!(r1 == 2 && r2 == 1 && x == 2)); /* FAILS!!! */

	assert(!(r1 == 2 && r2 == 2 && x == 1 && y == 1)); /* FAILS!!! */

But because control dependencies do -not- provide transitivity, the above

assertion can fail after the combined three-CPU example completes.  If you

need the three-CPU example to provide ordering, you will need smp_mb()

between the loads and stores in the CPU 0 and CPU 1 code fragments,

that is, just before or just after the "if" statements.

But because control dependencies do -not- provide transitivity, the

above assertion can fail after the combined four-CPU example completes.

If you need the four-CPU example to provide ordering, you will need

smp_mb() between the loads and stores in the CPU 0 and CPU 1 code fragments.

These two examples are the LB and WWC litmus tests from this paper:

http://www.cl.cam.ac.uk/users/pes20/ppc-supplemental/test6.pdf and this

site: https://www.cl.cam.ac.uk/~pes20/ppcmem/index.html.

In summary:

				struct file * file = xchg(&fdt->fd[i], NULL);

				if (file) {

					filp_close(file, files);

					cond_resched();

					cond_resched_rcu_qs();

				}

			}

			i++;