rcu: ->signaled better named ->fqs_state

The ->signaled field was named before complications in the form of
dyntick-idle mode and offlined CPUs.  These complications have required
that force_quiescent_state() be implemented as a state machine, instead
of simply unconditionally sending reschedule IPIs.  Therefore, this
commit renames ->signaled to ->fqs_state to catch up with the new
force_quiescent_state() reality.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
(cherry picked from commit af446b702c58b700cc5fa99f6edc78b99e55b995)
rcu: Avoid RCU-preempt expedited grace-period botch

Because rcu_read_unlock_special() samples rcu_preempted_readers_exp(rnp)
after dropping rnp->lock, the following sequence of events is possible:

1.	Task A exits its RCU read-side critical section, and removes
	itself from the ->blkd_tasks list, releases rnp->lock, and is
	then preempted.  Task B remains on the ->blkd_tasks list, and
	blocks the current expedited grace period.

2.	Task B exits from its RCU read-side critical section and removes
	itself from the ->blkd_tasks list.  Because it is the last task
	blocking the current expedited grace period, it ends that
	expedited grace period.

3.	Task A resumes, and samples rcu_preempted_readers_exp(rnp) which
	of course indicates that nothing is blocking the nonexistent
	expedited grace period. Task A is again preempted.

4.	Some other CPU starts an expedited grace period.  There are several
	tasks blocking this expedited grace period queued on the
	same rcu_node structure that Task A was using in step 1 above.

5.	Task A examines its state and incorrectly concludes that it was
	the last task blocking the expedited grace period on the current
	rcu_node structure.  It therefore reports completion up the
	rcu_node tree.

6.	The expedited grace period can then incorrectly complete before
	the tasks blocked on this same rcu_node structure exit their
	RCU read-side critical sections.  Arbitrarily bad things happen.

This commit therefore takes a snapshot of rcu_preempted_readers_exp(rnp)
prior to dropping the lock, so that only the last task thinks that it is
the last task, thus avoiding the failure scenario laid out above.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
(cherry picked from commit 389abd48efe1ceacb141b2fd151263b1bc432dbc)
rcu: Make synchronize_sched_expedited() better at work sharing

When synchronize_sched_expedited() takes its second and subsequent
snapshots of sync_sched_expedited_started, it subtracts 1.  This
means that the concurrent caller of synchronize_sched_expedited()
that incremented to that value sees our successful completion, it
will not be able to take advantage of it.  This restriction is
pointless, given that our full expedited grace period would have
happened after the other guy started, and thus should be able to
serve as a proxy for the other guy successfully executing
try_stop_cpus().

This commit therefore removes the subtraction of 1.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
(cherry picked from commit 7077714ec4940a6c5b1189c3afb4f47bf49ad877)

Conflicts:

	kernel/rcutree_plugin.h
lockdep: Update documentation for lock-class leak detection

There are a number of bugs that can leak or overuse lock classes,
which can cause the maximum number of lock classes (currently 8191)
to be exceeded.  However, the documentation does not tell you how to
track down these problems.  This commit addresses this shortcoming.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
(cherry picked from commit b804cb9e91c6c304959c69d4f9daeef4ffdba71c)
rcu: Track idleness independent of idle tasks

Earlier versions of RCU used the scheduling-clock tick to detect idleness
by checking for the idle task, but handled idleness differently for
CONFIG_NO_HZ=y.  But there are now a number of uses of RCU read-side
critical sections in the idle task, for example, for tracing.  A more
fine-grained detection of idleness is therefore required.

This commit presses the old dyntick-idle code into full-time service,
so that rcu_idle_enter(), previously known as rcu_enter_nohz(), is
always invoked at the beginning of an idle loop iteration.  Similarly,
rcu_idle_exit(), previously known as rcu_exit_nohz(), is always invoked
at the end of an idle-loop iteration.  This allows the idle task to
use RCU everywhere except between consecutive rcu_idle_enter() and
rcu_idle_exit() calls, in turn allowing architecture maintainers to
specify exactly where in the idle loop that RCU may be used.

Because some of the userspace upcall uses can result in what looks
to RCU like half of an interrupt, it is not possible to expect that
the irq_enter() and irq_exit() hooks will give exact counts.  This
patch therefore expands the ->dynticks_nesting counter to 64 bits
and uses two separate bitfields to count process/idle transitions
and interrupt entry/exit transitions.  It is presumed that userspace
upcalls do not happen in the idle loop or from usermode execution
(though usermode might do a system call that results in an upcall).
The counter is hard-reset on each process/idle transition, which
avoids the interrupt entry/exit error from accumulating.  Overflow
is avoided by the 64-bitness of the ->dyntick_nesting counter.

This commit also adds warnings if a non-idle task asks RCU to enter
idle state (and these checks will need some adjustment before applying
Frederic's OS-jitter patches (http://lkml.org/lkml/2011/10/7/246).
In addition, validation of ->dynticks and ->dynticks_nesting is added.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
(cherry picked from commit 9b2e4f1880b789be1f24f9684f7a54b90310b5c0)

Conflicts:

	kernel/rcutree.c
	kernel/rcutree.h
trace: Allow ftrace_dump() to be called from modules

Add an EXPORT_SYMBOL_GPL() so that rcutorture can dump the trace buffer
upon detection of an RCU error.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
(cherry picked from commit a8eecf2248a45bf69f0625b23c003ad2ccd765ee)
rcu: Add failure tracing to rcutorture

Trace the rcutorture RCU accesses and dump the trace buffer when the
first failure is detected.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
(cherry picked from commit 91afaf300269aa99a4d646969b3258b74294ac4d)
rcu: Document failing tick as cause of RCU CPU stall warning

One of lclaudio's systems was seeing RCU CPU stall warnings from idle.
These turned out to be caused by a bug that stopped scheduling-clock
tick interrupts from being sent to a given CPU for several hundred seconds.
This commit therefore updates the documentation to call this out as a
possible cause for RCU CPU stall warnings.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
(cherry picked from commit 2c01531f08f8ba663a20d8472a3032f6df133b6e)
rcu: Disable preemption in rcu_is_cpu_idle()

Because rcu_is_cpu_idle() is to be used to check for extended quiescent
states in RCU-preempt read-side critical sections, it cannot assume that
preemption is disabled.  And preemption must be disabled when accessing
the dyntick-idle state, because otherwise the following sequence of events
could occur:

1.	Task A on CPU 1 enters rcu_is_cpu_idle() and picks up the pointer
	to CPU 1's per-CPU variables.

2.	Task B preempts Task A and starts running on CPU 1.

3.	Task A migrates to CPU 2.

4.	Task B blocks, leaving CPU 1 idle.

5.	Task A continues execution on CPU 2, accessing CPU 1's dyntick-idle
	information using the pointer fetched in step 1 above, and finds
	that CPU 1 is idle.

6.	Task A therefore incorrectly concludes that it is executing in
	an extended quiescent state, possibly issuing a spurious splat.

Therefore, this commit disables preemption within the rcu_is_cpu_idle()
function.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
(cherry picked from commit 34240697d619c439c55f21989680024dcb604aab)
rcu: Omit self-awaken when setting up expedited grace period

When setting up an expedited grace period, if there were no readers, the
task will awaken itself.  This commit removes this useless self-awakening.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
(cherry picked from commit b40d293eb36ba40cd428b6d178db911174689702)
rcu: Remove redundant return from rcu_report_exp_rnp()

Empty void functions do not need "return", so this commit removes it
from rcu_report_exp_rnp().

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
(cherry picked from commit a0f8eefb127f5be07628954f310a7fc8c82b2fc3)
rcu: Detect illegal rcu dereference in extended quiescent state

Report that none of the rcu read lock maps are held while in an RCU
extended quiescent state (the section between rcu_idle_enter()
and rcu_idle_exit()). This helps detect any use of rcu_dereference()
and friends from within the section in idle where RCU is not allowed.

This way we can guarantee an extended quiescent window where the CPU
can be put in dyntick idle mode or can simply aoid to be part of any
global grace period completion while in the idle loop.

Uses of RCU from such mode are totally ignored by RCU, hence the
importance of these checks.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
(cherry picked from commit e6b80a3b0994ea6c3d876d72464f2debbfcfeb05)
rcu: Inform the user about extended quiescent state on PROVE_RCU warning

Inform the user if an RCU usage error is detected by lockdep while in
an extended quiescent state (in this case, the RCU-free window in idle).
This is accomplished by adding a line to the RCU lockdep splat indicating
whether or not the splat occurred in extended quiescent state.

Uses of RCU from within extended quiescent state mode are totally ignored
by RCU, hence the importance of this diagnostic.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
(cherry picked from commit 0464e937485f15d2add78e3b0f498469f4e6600d)
rcu: Warn when rcu_read_lock() is used in extended quiescent state

We are currently able to detect uses of rcu_dereference_check() inside
extended quiescent states (such as the RCU-free window in idle).
But rcu_read_lock() and friends can be used without rcu_dereference(),
so that the earlier commit checking for use of rcu_dereference() and
friends while in RCU idle mode miss some error conditions.  This commit
therefore adds extended quiescent state checking to rcu_read_lock() and
friends.

Uses of RCU from within RCU-idle mode are totally ignored by
RCU, hence the importance of these checks.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
(cherry picked from commit 00f49e5729af602deb559b0cf293a00b625e8636)
rcu: Remove one layer of abstraction from PROVE_RCU checking

Simplify things a bit by substituting the definitions of the single-line
rcu_read_acquire(), rcu_read_release(), rcu_read_acquire_bh(),
rcu_read_release_bh(), rcu_read_acquire_sched(), and
rcu_read_release_sched() functions at their call points.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
(cherry picked from commit d8ab29f8be918b34a1ccd174569a53f0eb04b0a5)
rcu: Warn when srcu_read_lock() is used in an extended quiescent state

Catch SRCU up to the other variants of RCU by making PROVE_RCU
complain if either srcu_read_lock() or srcu_read_lock_held() are
used from within RCU-idle mode.

Frederic reworked this to allow for the new versions of his patches
that check for extended quiescent states.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
(cherry picked from commit ff195cb69ba8d2af9b891be3a26db95fe1999d43)
rcu: Make srcu_read_lock_held() call common lockdep-enabled function

A common debug_lockdep_rcu_enabled() function is used to check whether
RCU lockdep splats should be reported, but srcu_read_lock() does not
use it.  This commit therefore brings srcu_read_lock_held() up to date.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
(cherry picked from commit 867f236bd12f5091df6dc7cc75f94d7fd982d78a)