Currently, __note_gp_changes() checks to see if the CPU has slept through
multiple grace periods.  If it has, it resynchronizes that CPU's view
of the grace-period state, which includes whether or not the current
grace period needs a quiescent state from this CPU.  The fact of this
need (or lack thereof) needs to be in two places, rdp->cpu_no_qs.b.norm
and rdp->core_needs_qs.  The former tells RCU's context-switch code to
go get a quiescent state and the latter says that it needs to be reported.
The current code unconditionally sets the former to true, but correctly
sets the latter.

This does not result in failures, but it does unnecessarily increase
the amount of work done on average at context-switch time.  This commit
therefore correctly sets both fields.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

The Kconfig currently controlling compilation of tree.c is:

init/Kconfig:config TREE_RCU
init/Kconfig:   bool

...and update.c and sync.c are "obj-y" meaning that none are ever
built as a module by anyone.

Since MODULE_ALIAS is a no-op for non-modular code, we can remove
them from these files.

We leave moduleparam.h behind since the files instantiate some boot
time configuration parameters with module_param() still.

Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Signed-off-by: NPaul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

Commit abedf8e2 ("rcu: Use simple wait queues where possible in
rcutree") converts Tree RCU's wait queues to simple wait queues,
but it incorrectly reverts the commit 2aa792e6 ("rcu: Use
rcu_gp_kthread_wake() to wake up grace period kthreads").  This can
result in redundant self-wakeups.

This commit therefore replaces the simple wait-queue wakeups with
rcu_gp_kthread_wake(), thus avoiding the redundant wakeups.
Signed-off-by: NJisheng Zhang <jszhang@marvell.com>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

Straight forward conversion to the state machine. Though the question arises
whether this needs really all these state transitions to work.
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>
Signed-off-by: NAnna-Maria Gleixner <anna-maria@linutronix.de>
Reviewed-by: NSebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: rt@linutronix.de
Link: http://lkml.kernel.org/r/20160713153337.982013161@linutronix.deSigned-off-by: NIngo Molnar <mingo@kernel.org>

In many cases in the RCU tree code, we iterate over the set of cpus for
a leaf node described by rcu_node::grplo and rcu_node::grphi, checking
per-cpu data for each cpu in this range. However, if the set of possible
cpus is sparse, some cpus described in this range are not possible, and
thus no per-cpu region will have been allocated (or initialised) for
them by the generic percpu code.

Erroneous accesses to a per-cpu area for these !possible cpus may fault
or may hit other data depending on the addressed generated when the
erroneous per cpu offset is applied. In practice, both cases have been
observed on arm64 hardware (the former being silent, but detectable with
additional patches).

To avoid issues resulting from this, we must iterate over the set of
*possible* cpus for a given leaf node. This patch add a new helper,
for_each_leaf_node_possible_cpu, to enable this. As iteration is often
intertwined with rcu_node local bitmask manipulation, a new
leaf_node_cpu_bit helper is added to make this simpler and more
consistent. The RCU tree code is made to use both of these where
appropriate.

Without this patch, running reboot at a shell can result in an oops
like:

[ 3369.075979] Unable to handle kernel paging request at virtual address ffffff8008b21b4c
[ 3369.083881] pgd = ffffffc3ecdda000
[ 3369.087270] [ffffff8008b21b4c] *pgd=00000083eca48003, *pud=00000083eca48003, *pmd=0000000000000000
[ 3369.096222] Internal error: Oops: 96000007 [#1] PREEMPT SMP
[ 3369.101781] Modules linked in:
[ 3369.104825] CPU: 2 PID: 1817 Comm: NetworkManager Tainted: G        W       4.6.0+ #3
[ 3369.121239] task: ffffffc0fa13e000 ti: ffffffc3eb940000 task.ti: ffffffc3eb940000
[ 3369.128708] PC is at sync_rcu_exp_select_cpus+0x188/0x510
[ 3369.134094] LR is at sync_rcu_exp_select_cpus+0x104/0x510
[ 3369.139479] pc : [<ffffff80081109a8>] lr : [<ffffff8008110924>] pstate: 200001c5
[ 3369.146860] sp : ffffffc3eb9435a0
[ 3369.150162] x29: ffffffc3eb9435a0 x28: ffffff8008be4f88
[ 3369.155465] x27: ffffff8008b66c80 x26: ffffffc3eceb2600
[ 3369.160767] x25: 0000000000000001 x24: ffffff8008be4f88
[ 3369.166070] x23: ffffff8008b51c3c x22: ffffff8008b66c80
[ 3369.171371] x21: 0000000000000001 x20: ffffff8008b21b40
[ 3369.176673] x19: ffffff8008b66c80 x18: 0000000000000000
[ 3369.181975] x17: 0000007fa951a010 x16: ffffff80086a30f0
[ 3369.187278] x15: 0000007fa9505590 x14: 0000000000000000
[ 3369.192580] x13: ffffff8008b51000 x12: ffffffc3eb940000
[ 3369.197882] x11: 0000000000000006 x10: ffffff8008b51b78
[ 3369.203184] x9 : 0000000000000001 x8 : ffffff8008be4000
[ 3369.208486] x7 : ffffff8008b21b40 x6 : 0000000000001003
[ 3369.213788] x5 : 0000000000000000 x4 : ffffff8008b27280
[ 3369.219090] x3 : ffffff8008b21b4c x2 : 0000000000000001
[ 3369.224406] x1 : 0000000000000001 x0 : 0000000000000140
...
[ 3369.972257] [<ffffff80081109a8>] sync_rcu_exp_select_cpus+0x188/0x510
[ 3369.978685] [<ffffff80081128b4>] synchronize_rcu_expedited+0x64/0xa8
[ 3369.985026] [<ffffff80086b987c>] synchronize_net+0x24/0x30
[ 3369.990499] [<ffffff80086ddb54>] dev_deactivate_many+0x28c/0x298
[ 3369.996493] [<ffffff80086b6bb8>] __dev_close_many+0x60/0xd0
[ 3370.002052] [<ffffff80086b6d48>] __dev_close+0x28/0x40
[ 3370.007178] [<ffffff80086bf62c>] __dev_change_flags+0x8c/0x158
[ 3370.012999] [<ffffff80086bf718>] dev_change_flags+0x20/0x60
[ 3370.018558] [<ffffff80086cf7f0>] do_setlink+0x288/0x918
[ 3370.023771] [<ffffff80086d0798>] rtnl_newlink+0x398/0x6a8
[ 3370.029158] [<ffffff80086cee84>] rtnetlink_rcv_msg+0xe4/0x220
[ 3370.034891] [<ffffff80086e274c>] netlink_rcv_skb+0xc4/0xf8
[ 3370.040364] [<ffffff80086ced8c>] rtnetlink_rcv+0x2c/0x40
[ 3370.045663] [<ffffff80086e1fe8>] netlink_unicast+0x160/0x238
[ 3370.051309] [<ffffff80086e24b8>] netlink_sendmsg+0x2f0/0x358
[ 3370.056956] [<ffffff80086a0070>] sock_sendmsg+0x18/0x30
[ 3370.062168] [<ffffff80086a21cc>] ___sys_sendmsg+0x26c/0x280
[ 3370.067728] [<ffffff80086a30ac>] __sys_sendmsg+0x44/0x88
[ 3370.073027] [<ffffff80086a3100>] SyS_sendmsg+0x10/0x20
[ 3370.078153] [<ffffff8008085e70>] el0_svc_naked+0x24/0x28
Signed-off-by: NMark Rutland <mark.rutland@arm.com>
Reported-by: NDennis Chen <dennis.chen@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Steve Capper <steve.capper@arm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Will Deacon <will.deacon@arm.com>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

It is not always easy to determine the cause of an RCU stall just by
analysing the RCU stall messages, mainly when the problem is caused
by the indirect starvation of rcu threads. For example, when preempt_rcu
is not awakened due to the starvation of a timer softirq.

We have been hard coding panic() in the RCU stall functions for
some time while testing the kernel-rt. But this is not possible in
some scenarios, like when supporting customers.

This patch implements the sysctl kernel.panic_on_rcu_stall. If
set to 1, the system will panic() when an RCU stall takes place,
enabling the capture of a vmcore. The vmcore provides a way to analyze
all kernel/tasks states, helping out to point to the culprit and the
solution for the stall.

The kernel.panic_on_rcu_stall sysctl is disabled by default.

Changes from v1:
- Fixed a typo in the git log
- The if(sysctl_panic_on_rcu_stall) panic() is in a static function
- Fixed the CONFIG_TINY_RCU compilation issue
- The var sysctl_panic_on_rcu_stall is now __read_mostly

Cc: Jonathan Corbet <corbet@lwn.net>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Acked-by: NChristian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>
Reviewed-by: NArnaldo Carvalho de Melo <acme@kernel.org>
Tested-by: N"Luis Claudio R. Goncalves" <lgoncalv@redhat.com>
Signed-off-by: NDaniel Bristot de Oliveira <bristot@redhat.com>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

People have been having some difficulty finding their way around the
RCU code.  This commit therefore pulls some of the expedited grace-period
code from tree.c to a new tree_exp.h file.  This commit is strictly code
movement, with the exception of a forward declaration that was added
for the sync_sched_exp_online_cleanup() function.

A subsequent commit will move the remaining expedited grace-period code
from tree_plugin.h to tree_exp.h.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

I think you'll find this condition is superfluous, as the whole function
is under #ifdef of that same.
Signed-off-by: NPeter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

In the past, RCU grace-period initialization excluded CPU-hotplug
operations, but this is no longer the case.  This commit therefore
removed an outdated comment in rcu_gp_init() claiming that these
are excluded.
Reported-by: NLihao Liang <lihao.liang@gmail.com>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

The comment header for rcu_scheduler_active states that it is used
to optimize synchronize_sched() at early boot.  This is incorrect.
The synchronize_sched() function instead checks the number of online
CPUs.  This commit therefore replaces the comment's synchronize_sched()
with synchronize_rcu(), which really does use rcu_scheduler_active for
this purpose.
Reported-by: NLihao Liang <lihao.liang@gmail.com>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

A while back Viro posted a number of 'interesting' mutex_is_locked()
users on IRC, one of those was RCU.

RCU seems to use mutex_is_locked() to avoid doing mutex_trylock(), the
regular load before modify pattern.

While the use isn't wrong per se, its curious in that its needed at all,
mutex_trylock() should be good enough on its own to avoid the pointless
cacheline bounces.

So fix those and remove the mutex_is_locked() (ab)use from RCU.
Reported-by: NAl Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: NPeter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: NPaul McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: NDavidlohr Bueso <dave@stgolabs.net>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Waiman Long <Waiman.Long@hpe.com>
Link: http://lkml.kernel.org/r/20160601185815.GW3190@twins.programming.kicks-ass.netSigned-off-by: NIngo Molnar <mingo@kernel.org>

This commit provides rcu_exp_batches_completed() and
rcu_exp_batches_completed_sched() functions to allow torture-test modules
to check how many expedited grace period batches have completed.
These are analogous to the existing rcu_batches_completed(),
rcu_batches_completed_bh(), and rcu_batches_completed_sched() functions.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

If it is necessary to kick the grace-period kthread, that is a good
time to dump the trace buffer in order to learn why kicking was needed.
This commit therefore does the dump.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

Recent kernels can fail to awaken the grace-period kthread for
quiescent-state forcing.  This commit is a crude hack that does
a wakeup if a scheduling-clock interrupt sees that it has been
too long since force-quiescent-state (FQS) processing.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

Currently, the force-quiescent-state (FQS) code in rcu_gp_kthread() can
advance the next FQS even if one was not executed last time.  This can
happen due timeout-duration uncertainty.  This commit therefore avoids
advancing the FQS schedule unless an FQS was just executed.  In the
corner case where an FQS was not executed, but is due now, the code does
a one-jiffy wait.

This change prepares for kthread kicking.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

Recent kernels can fail to awaken the grace-period kthread for
quiescent-state forcing.  This commit is a crude hack that does
a wakeup any time a stall is detected.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

The current expedited grace-period implementation makes subsequent grace
periods wait on wakeups for the prior grace period.  This does not fit
the dictionary definition of "expedited", so this commit allows these two
phases to overlap.  Doing this requires four waitqueues rather than two
because tasks can now be waiting on the previous, current, and next grace
periods.  The fourth waitqueue makes the bit masking work out nicely.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

This commit pulls the grace-period-start counter adjustment and tracing
from synchronize_rcu_expedited() and synchronize_sched_expedited()
into exp_funnel_lock(), thus eliminating some code duplication.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

This commit moves some duplicate code from synchronize_rcu_expedited()
and synchronize_sched_expedited() into rcu_exp_gp_seq_snap().  This
doesn't save lines of code, but does eliminate a "tell me twice" issue.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

Currently, synchronize_rcu_expedited() and rcu_sched_expedited() have
significant duplicate code.  This commit therefore consolidates some of
this code into rcu_exp_wake(), which is now renamed to rcu_exp_wait_wake()
in recognition of its added responsibilities.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

This commit speeds up the low-contention case, especially for systems
with large rcu_node trees, by attempting to directly acquire the
->exp_mutex.  This fastpath checks the leaves and root first in
order to avoid excessive memory contention on the mutex itself.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

The current mutex-based funnel-locking approach used by expedited grace
periods is subject to severe unfairness.  The problem arises when a
few tasks, making a path from leaves to root, all wake up before other
tasks do.  A new task can then follow this path all the way to the root,
which needlessly delays tasks whose grace period is done, but who do
not happen to acquire the lock quickly enough.

This commit avoids this problem by maintaining per-rcu_node wait queues,
along with a per-rcu_node counter that tracks the latest grace period
sought by an earlier task to visit this node.  If that grace period
would satisfy the current task, instead of proceeding up the tree,
it waits on the current rcu_node structure using a pair of wait queues
provided for that purpose.  This decouples awakening of old tasks from
the arrival of new tasks.

If the wakeups prove to be a bottleneck, additional kthreads can be
brought to bear for that purpose.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

Just a name change to save a few lines and a bit of typing.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

The cpu_online() function can return values other than 0 and 1, which
can result in subscript overflow when applied to a two-element array.
This commit allows for this behavior by using "!!" on the return value
from cpu_online() when used as a subscript.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

Commit #cdacbe1f ("rcu: Add fastpath bypassing funnel locking")
turns out to be a pessimization at high load because it forces a tree
full of tasks to wait for an expedited grace period that they probably
do not need.  This commit therefore removes this optimization.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

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

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

Although cond_resched_rcu_qs() supplies quiescent states to all flavors
of normal RCU grace periods, it does nothing for expedited RCU-sched
grace periods.  This commit therefore adds a check for a need for a
quiescent state from the current CPU by an expedited RCU-sched grace
period, and invokes rcu_sched_qs() to supply that quiescent state if so.

Note that the check is racy in that we might be migrated to some other
CPU just after checking the per-CPU variable.  This is OK because the
act of migration will do a context switch, which will supply the needed
quiescent state.  The only downside is that we might do an unnecessary
call to rcu_sched_qs(), but the probability is low and the overhead
is small.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

Currently, synchronize_sched_expedited_wait() simply sets the ndetected
variable to the rcu_print_task_exp_stall() return value.  This means
that if the last rcu_node structure has no stalled tasks, record of
any stalled tasks in previous rcu_node structures is lost, which can
in turn result in failure to dump out the blocking rcu_node structures.
Or could, had the test been correct.

This commit therefore adds the return value of rcu_print_task_exp_stall()
to ndetected and corrects the later test for ndetected.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

Currently, sync_sched_exp_handler() will force a reschedule unless
this CPU has already checked in or unless a reschedule has already
been called for.  This is clearly wasteful if sync_sched_exp_handler()
interrupted an idle CPU, so this commit immediately reports the
quiescent state in that case.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

This commit consolidates a couple definitions and several calls for
single-shot ftrace-buffer dumping.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

Make the RCU CPU_DYING_IDLE callback an explicit function call, so it gets
invoked at the proper place.
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>
Cc: linux-arch@vger.kernel.org
Cc: Rik van Riel <riel@redhat.com>
Cc: Rafael Wysocki <rafael.j.wysocki@intel.com>
Cc: "Srivatsa S. Bhat" <srivatsa@mit.edu>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: Sebastian Siewior <bigeasy@linutronix.de>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul Turner <pjt@google.com>
Link: http://lkml.kernel.org/r/20160226182341.870167933@linutronix.deSigned-off-by: NThomas Gleixner <tglx@linutronix.de>

As of commit dae6e64d ("rcu: Introduce proper blocking to no-CBs kthreads
GP waits") the RCU subsystem started making use of wait queues.

Here we convert all additions of RCU wait queues to use simple wait queues,
since they don't need the extra overhead of the full wait queue features.

Originally this was done for RT kernels[1], since we would get things like...

  BUG: sleeping function called from invalid context at kernel/rtmutex.c:659
  in_atomic(): 1, irqs_disabled(): 1, pid: 8, name: rcu_preempt
  Pid: 8, comm: rcu_preempt Not tainted
  Call Trace:
   [<ffffffff8106c8d0>] __might_sleep+0xd0/0xf0
   [<ffffffff817d77b4>] rt_spin_lock+0x24/0x50
   [<ffffffff8106fcf6>] __wake_up+0x36/0x70
   [<ffffffff810c4542>] rcu_gp_kthread+0x4d2/0x680
   [<ffffffff8105f910>] ? __init_waitqueue_head+0x50/0x50
   [<ffffffff810c4070>] ? rcu_gp_fqs+0x80/0x80
   [<ffffffff8105eabb>] kthread+0xdb/0xe0
   [<ffffffff8106b912>] ? finish_task_switch+0x52/0x100
   [<ffffffff817e0754>] kernel_thread_helper+0x4/0x10
   [<ffffffff8105e9e0>] ? __init_kthread_worker+0x60/0x60
   [<ffffffff817e0750>] ? gs_change+0xb/0xb

...and hence simple wait queues were deployed on RT out of necessity
(as simple wait uses a raw lock), but mainline might as well take
advantage of the more streamline support as well.

[1] This is a carry forward of work from v3.10-rt; the original conversion
was by Thomas on an earlier -rt version, and Sebastian extended it to
additional post-3.10 added RCU waiters; here I've added a commit log and
unified the RCU changes into one, and uprev'd it to match mainline RCU.
Signed-off-by: NDaniel Wagner <daniel.wagner@bmw-carit.de>
Acked-by: NPeter Zijlstra (Intel) <peterz@infradead.org>
Cc: linux-rt-users@vger.kernel.org
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Link: http://lkml.kernel.org/r/1455871601-27484-6-git-send-email-wagi@monom.orgSigned-off-by: NThomas Gleixner <tglx@linutronix.de>

rcu_nocb_gp_cleanup() is called while holding rnp->lock. Currently,
this is okay because the wake_up_all() in rcu_nocb_gp_cleanup() will
not enable the IRQs. lockdep is happy.

By switching over using swait this is not true anymore. swake_up_all()
enables the IRQs while processing the waiters. __do_softirq() can now
run and will eventually call rcu_process_callbacks() which wants to
grap nrp->lock.

Let's move the rcu_nocb_gp_cleanup() call outside the lock before we
switch over to swait.

If we would hold the rnp->lock and use swait, lockdep reports
following:

 =================================
 [ INFO: inconsistent lock state ]
 4.2.0-rc5-00025-g9a73ba0 #136 Not tainted
 ---------------------------------
 inconsistent {IN-SOFTIRQ-W} -> {SOFTIRQ-ON-W} usage.
 rcu_preempt/8 [HC0[0]:SC0[0]:HE1:SE1] takes:
  (rcu_node_1){+.?...}, at: [<ffffffff811387c7>] rcu_gp_kthread+0xb97/0xeb0
 {IN-SOFTIRQ-W} state was registered at:
   [<ffffffff81109b9f>] __lock_acquire+0xd5f/0x21e0
   [<ffffffff8110be0f>] lock_acquire+0xdf/0x2b0
   [<ffffffff81841cc9>] _raw_spin_lock_irqsave+0x59/0xa0
   [<ffffffff81136991>] rcu_process_callbacks+0x141/0x3c0
   [<ffffffff810b1a9d>] __do_softirq+0x14d/0x670
   [<ffffffff810b2214>] irq_exit+0x104/0x110
   [<ffffffff81844e96>] smp_apic_timer_interrupt+0x46/0x60
   [<ffffffff81842e70>] apic_timer_interrupt+0x70/0x80
   [<ffffffff810dba66>] rq_attach_root+0xa6/0x100
   [<ffffffff810dbc2d>] cpu_attach_domain+0x16d/0x650
   [<ffffffff810e4b42>] build_sched_domains+0x942/0xb00
   [<ffffffff821777c2>] sched_init_smp+0x509/0x5c1
   [<ffffffff821551e3>] kernel_init_freeable+0x172/0x28f
   [<ffffffff8182cdce>] kernel_init+0xe/0xe0
   [<ffffffff8184231f>] ret_from_fork+0x3f/0x70
 irq event stamp: 76
 hardirqs last  enabled at (75): [<ffffffff81841330>] _raw_spin_unlock_irq+0x30/0x60
 hardirqs last disabled at (76): [<ffffffff8184116f>] _raw_spin_lock_irq+0x1f/0x90
 softirqs last  enabled at (0): [<ffffffff810a8df2>] copy_process.part.26+0x602/0x1cf0
 softirqs last disabled at (0): [<          (null)>]           (null)
 other info that might help us debug this:
  Possible unsafe locking scenario:
        CPU0
        ----
   lock(rcu_node_1);
   <Interrupt>
     lock(rcu_node_1);
  *** DEADLOCK ***
 1 lock held by rcu_preempt/8:
  #0:  (rcu_node_1){+.?...}, at: [<ffffffff811387c7>] rcu_gp_kthread+0xb97/0xeb0
 stack backtrace:
 CPU: 0 PID: 8 Comm: rcu_preempt Not tainted 4.2.0-rc5-00025-g9a73ba0 #136
 Hardware name: Dell Inc. PowerEdge R820/066N7P, BIOS 2.0.20 01/16/2014
  0000000000000000 000000006d7e67d8 ffff881fb081fbd8 ffffffff818379e0
  0000000000000000 ffff881fb0812a00 ffff881fb081fc38 ffffffff8110813b
  0000000000000000 0000000000000001 ffff881f00000001 ffffffff8102fa4f
 Call Trace:
  [<ffffffff818379e0>] dump_stack+0x4f/0x7b
  [<ffffffff8110813b>] print_usage_bug+0x1db/0x1e0
  [<ffffffff8102fa4f>] ? save_stack_trace+0x2f/0x50
  [<ffffffff811087ad>] mark_lock+0x66d/0x6e0
  [<ffffffff81107790>] ? check_usage_forwards+0x150/0x150
  [<ffffffff81108898>] mark_held_locks+0x78/0xa0
  [<ffffffff81841330>] ? _raw_spin_unlock_irq+0x30/0x60
  [<ffffffff81108a28>] trace_hardirqs_on_caller+0x168/0x220
  [<ffffffff81108aed>] trace_hardirqs_on+0xd/0x10
  [<ffffffff81841330>] _raw_spin_unlock_irq+0x30/0x60
  [<ffffffff810fd1c7>] swake_up_all+0xb7/0xe0
  [<ffffffff811386e1>] rcu_gp_kthread+0xab1/0xeb0
  [<ffffffff811089bf>] ? trace_hardirqs_on_caller+0xff/0x220
  [<ffffffff81841341>] ? _raw_spin_unlock_irq+0x41/0x60
  [<ffffffff81137c30>] ? rcu_barrier+0x20/0x20
  [<ffffffff810d2014>] kthread+0x104/0x120
  [<ffffffff81841330>] ? _raw_spin_unlock_irq+0x30/0x60
  [<ffffffff810d1f10>] ? kthread_create_on_node+0x260/0x260
  [<ffffffff8184231f>] ret_from_fork+0x3f/0x70
  [<ffffffff810d1f10>] ? kthread_create_on_node+0x260/0x260
Signed-off-by: NDaniel Wagner <daniel.wagner@bmw-carit.de>
Acked-by: NPeter Zijlstra (Intel) <peterz@infradead.org>
Cc: linux-rt-users@vger.kernel.org
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Link: http://lkml.kernel.org/r/1455871601-27484-5-git-send-email-wagi@monom.orgSigned-off-by: NThomas Gleixner <tglx@linutronix.de>

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

In patch:

"rcu: Add transitivity to remaining rcu_node ->lock acquisitions"

All locking operations on rcu_node::lock are replaced with the wrappers
because of the need of transitivity, which indicates we should never
write code using LOCK primitives alone(i.e. without a proper barrier
following) on rcu_node::lock outside those wrappers. We could detect
this kind of misuses on rcu_node::lock in the future by adding __private
modifier on rcu_node::lock.

To privatize rcu_node::lock, unlock wrappers are also needed. Replacing
spinlock unlocks with these wrappers not only privatizes rcu_node::lock
but also makes it easier to figure out critical sections of rcu_node.

This patch adds __private modifier to rcu_node::lock and makes every
access to it wrapped by ACCESS_PRIVATE(). Besides, unlock wrappers are
added and raw_spin_unlock(&rnp->lock) and its friends are replaced with
those wrappers.
Signed-off-by: NBoqun Feng <boqun.feng@gmail.com>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

The related warning from gcc 6.0:

  In file included from kernel/rcu/tree.c:4630:0:
  kernel/rcu/tree_plugin.h:810:40: warning: ‘rcu_data_p’ defined but not used [-Wunused-const-variable]
   static struct rcu_data __percpu *const rcu_data_p = &rcu_sched_data;
                                          ^~~~~~~~~~

Also remove always redundant rcu_data_p in tree.c.
Signed-off-by: NChen Gang <gang.chen.5i5j@gmail.com>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

Commit #e3663b10 ("rcu: Handle gpnum/completed wrap while dyntick
idle") sets rdp->gpwrap on the wrong side of the "if" statement in
dyntick_save_progress_counter(), that is, it sets it when the CPU is
not idle instead of when it is idle.  Of course, if the CPU is not idle,
its rdp->gpnum won't be lagging beind the global rsp->gpnum, which means
that rdp->gpwrap will never be set.

This commit therefore moves this code to the proper leg of that "if"
statement.  This change means that the "else" cause is just "return 0"
and the "then" clause ends with "return 1", so also move the "return 0"
to follow the "if", dropping the "else" clause.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

Commit 4a81e832 ("Reduce overhead of cond_resched() checks for RCU")
handles the error case where a nohz_full loops indefinitely in the kernel
with the scheduling-clock interrupt disabled.  However, this handling
includes IPIing the CPU running the offending loop, which is not what
we want for real-time workloads.  And there are starting to be real-time
CPU-bound in-kernel workloads, and these must be handled without IPIing
the CPU, at least not in the common case.  Therefore, this situation can
no longer be dismissed as an error case.

This commit therefore splits the handling out, so that the setting of
bits in the per-CPU rcu_sched_qs_mask variable is done relatively early,
but if the problem persists, resched_cpu() is eventually used to IPI the
CPU containing the offending loop.  Assuming that in-kernel CPU-bound
loops used by real-time tasks contain frequent calls cond_resched_rcu_qs()
(as in more than once per few tens of milliseconds), the real-time tasks
will never be IPIed.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>

The header comment for rcu_report_qs_rsp() was obsolete, dating well
before the advent of RCU grace-period kthreads.  This commit therefore
brings this comment back into alignment with current reality.
Reported-by: NLihao Liang <lihao.liang@cs.ox.ac.uk>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>