Commit e31d28b6 ("trace: Eliminate cond_resched_rcu_qs() in favor
of cond_resched()") substituted cond_resched() for the earlier call
to cond_resched_rcu_qs().  However, the new-age cond_resched() does
not do anything to help RCU-tasks grace periods because (1) RCU-tasks
is only enabled when CONFIG_PREEMPT=y and (2) cond_resched() is a
complete no-op when preemption is enabled.  This situation results
in hangs when running the trace benchmarks.

A number of potential fixes were discussed on LKML
(https://lkml.kernel.org/r/20180224151240.0d63a059@vmware.local.home),
including making cond_resched() not be a no-op; making cond_resched()
not be a no-op, but only when running tracing benchmarks; reverting
the aforementioned commit (which works because cond_resched_rcu_qs()
does provide an RCU-tasks quiescent state; and adding a call to the
scheduler/RCU rcu_note_voluntary_context_switch() function.  All were
deemed unsatisfactory, either due to added cond_resched() overhead or
due to magic functions inviting cargo culting.

This commit renames cond_resched_rcu_qs() to cond_resched_tasks_rcu_qs(),
which provides a clear hint as to what this function is doing and
why and where it should be used, and then replaces the call to
cond_resched() with cond_resched_tasks_rcu_qs() in the trace benchmark's
benchmark_event_kthread() function.
Reported-by: NSteven Rostedt <rostedt@goodmis.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: NNicholas Piggin <npiggin@gmail.com>

RCU's expedited grace periods can participate in out-of-memory deadlocks
due to all available system_wq kthreads being blocked and there not being
memory available to create more.  This commit prevents such deadlocks
by allocating an RCU-specific workqueue_struct at early boot time, and
providing it with a rescuer to ensure forward progress.  This uses the
shiny new init_rescuer() function provided by Tejun (but indirectly).

This commit also causes SRCU to use this new RCU-specific
workqueue_struct.  Note that SRCU's use of workqueues never blocks them
waiting for readers, so this should be safe from a forward-progress
viewpoint.  Note that this moves SRCU from system_power_efficient_wq
to a normal workqueue.  In the unlikely event that this results in
measurable degradation, a separate power-efficient workqueue will be
creates for SRCU.
Reported-by: NPrateek Sood <prsood@codeaurora.org>
Reported-by: NTejun Heo <tj@kernel.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: NTejun Heo <tj@kernel.org>

Commits c0b334c5 and ea9b0c8a introduced new sparse warnings
by accessing rcu_node->lock directly and ignoring the __private
marker.  Introduce a new wrapper and use it.  Also fix a similar problem
in srcutree.c introduced by a3883df3.
Signed-off-by: NMatthew Wilcox <mawilcox@microsoft.com>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

It is not always obvious that the stack dump from a starved grace-period
kthread isn't instead that of a CPU stalling the current grace period.
This commit therefore adds a pr_err() flagging these dumps.
Reported-by: NPeter Zijlstra <peterz@infradead.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

The debugfs interface displayed statistics on RCU-pending checks but
this interface has since been removed.  This commit therefore removes the
no-longer-used rcu_state structure's ->n_force_qs_lh and ->n_force_qs_ngp
fields along with their updates.  (Though the ->n_force_qs_ngp field
was actually not used at all, embarrassingly enough.)

If this information proves necessary in the future, the corresponding
event traces will be added.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

The debugfs interface displayed statistics on RCU-pending checks
but this interface has since been removed.  This commit therefore
removes the no-longer-used rcu_data structure's ->n_rcu_pending,
->n_rp_core_needs_qs, ->n_rp_report_qs, ->n_rp_cb_ready,
->n_rp_cpu_needs_gp, ->n_rp_gp_completed, ->n_rp_gp_started,
->n_rp_nocb_defer_wakeup, and ->n_rp_need_nothing fields along with
their updates.

If this information proves necessary in the future, the corresponding
event traces will be added.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

The debugfs interface displayed statistics on RCU callback invocation but
this interface has since been removed.  This commit therefore removes the
no-longer-used rcu_data structure's ->n_cbs_invoked and ->n_nocbs_invoked
fields along with their updates.

If this information proves necessary in the future, the corresponding
event traces will be added.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

Since rcu_boot_init_percpu_data() is only called at boot time,
there is no data race and spinlock is not needed.
Signed-off-by: NLihao Liang <lianglihao@huawei.com>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

The following statement has for some reason proven non-intuitive:

	WARN_ON_ONCE(rcu_segcblist_empty(&rdp->cblist) != (count == 0));

This commit therefore adds a comment that states that this warning
usually triggers in response to a double call_rcu(), which is sort
of like a double free.  The comment also suggests building with
CONFIG_DEBUG_OBJECTS_RCU_HEAD=y to track down the double call_rcu().
Reported-by: NDavid Howells <dhowells@redhat.com>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

The code that checks for non-idle non-nohz_idle-usermode tasks invoking
rcu_eqs_enter() and rcu_eqs_exit() prints a considerable quantity of
helpful information.  However, these checks fire rarely, so the extra
complexity is no longer worth it.  This commit therefore replaces this
debug code with simple WARN_ON_ONCE() statements.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

There is now only one call to rcu_eqs_exit_common() and there is no other
reason to keep it separate.  This commit therefore inlines it into its
sole call site, saving a few lines of code in the process.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

There is now only one call to rcu_eqs_enter_common() and there is no other
reason to keep it separate.  This commit therefore inlines it into its
sole call site, saving a few lines of code in the process.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

Although ->dynticks_nesting is updated only by process level, it is
accessed from hardirq to check for interrupt-from-idle quiescent states.
Store tearing is thus possible, so this commit applies WRITE_ONCE()
to ->dynticks_nesting stores.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

The three RCU_LOCKDEP_WARN() calls in rcu_eqs_enter_common() are
redundant with other lockdep checks, so this commit removes them.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

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

Because the ->dynticks_nesting field now only contains the process-based
nesting level instead of a value encoding both the process nesting level
and the irq "nesting" level, we no longer need a long long, even on
32-bit systems.  This commit therefore changes both the ->dynticks_nesting
and ->dynticks_nmi_nesting fields to long.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

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

Now that the irq path uses the rcu_nmi_{enter,exit}() algorithm,
rcu_irq_enter() and rcu_irq_exit() may be used from any context.  There is
thus no need for rcu_irq_enter_disabled() and for the checks using it.
This commit therefore eliminates rcu_irq_enter_disabled().
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

Now that ->dynticks_nesting counts only process-level dyntick-idle
entry and exit, there is no need for the elaborate segmented counter
with its guard fields and overflow checking.  This commit therefore
makes ->dynticks_nesting be a simple counter.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

RCU currently uses two different mechanisms for tracking irqs and NMIs.
This is unnecessary complexity: Given that NMIs can nest and given that
RCU's tracking handles such nesting, the NMI tracking mechanism can also
be used to track irqs.  This commit therefore defines rcu_irq_enter()
in terms of rcu_nmi_enter() and rcu_irq_exit() in terms of rcu_nmi_exit().

Unfortunately, callers must still distinguish between the irq and NMI
functions because additional actions are taken when an irq interrupts
idle or nohz_full usermode execution, and these actions cannot always
be taken from NMI handlers.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

In preparation for merging dyntick-idle irq handling into the NMI
algorithm, clamp ->dynticks_nmi_nesting value to allow for interrupts
that enter but never leave and vice versa.

It is important that the clamping happen outside of the extended quiescent
state.  Otherwise, there will be short windows where irqs and NMIs fail
to convince RCU to start watching.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

This is a code-motion-only commit that prepares to define rcu_irq_enter()
in terms of rcu_nmi_enter() and rcu_irq_exit() in terms of rcu_irq_exit().
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

Both extended-quiescent-state entry and exit first update the nesting
counter and then adjust the dyntick-idle state.  This means that there
are four states: (1) Both nesting and dyntick idle indicate idle,
(2) Nesting indicates idle but dyntick idle does not, (3) Nesting indicates
non-idle and dyntick idle does not, and (4) Both nesting and dyntick
idle indicate non-idle.  This commit simplifies the state space by
eliminating #3, reversing the order of updates on exit from extended
quiescent state.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

NMIs can nest, and store tearing could in theory happen on carries
from one byte to the next.  This commit therefore adds the WRITE_ONCE()
macros preventing this.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

Lockdep now has an integrated IRQs disabled/enabled sanity check. Just
use it instead of the ad-hoc RCU version.
Signed-off-by: NFrederic Weisbecker <frederic@kernel.org>
Acked-by: NThomas Gleixner <tglx@linutronix.de>
Acked-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: David S . Miller <davem@davemloft.net>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Tejun Heo <tj@kernel.org>
Link: http://lkml.kernel.org/r/1509980490-4285-15-git-send-email-frederic@kernel.orgSigned-off-by: NIngo Molnar <mingo@kernel.org>

Because many of RCU's files have not been included into docbook, a
number of errors have accumulated.  This commit fixes them.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

If you add or remove calls to rcu_idle_enter(), rcu_user_enter(),
rcu_irq_exit(), rcu_irq_exit_irqson(), rcu_idle_exit(), rcu_user_exit(),
rcu_irq_enter(), rcu_irq_enter_irqson(), rcu_nmi_enter(), or
rcu_nmi_exit(), you should run a full set of tests on a kernel built
with CONFIG_RCU_EQS_DEBUG=y.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

One common question upon seeing an RCU CPU stall warning is "did
the stalled CPUs have interrupts disabled?"  However, the current
stall warnings are silent on this point.  This commit therefore
uses irq_work to check whether stalled CPUs still respond to IPIs,
and flags this state in the RCU CPU stall warning console messages.
Reported-by: NSteven Rostedt <rostedt@goodmis.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

There is some confusion as to which of cond_resched() or
cond_resched_rcu_qs() should be added to long in-kernel loops.
This commit therefore eliminates the decision by adding RCU quiescent
states to cond_resched().  This commit also simplifies the code that
used to interact with cond_resched_rcu_qs(), and that now interacts with
cond_resched(), to reduce its overhead.  This reduction is necessary to
allow the heavier-weight cond_resched_rcu_qs() mechanism to be invoked
everywhere that cond_resched() is invoked.

Part of that reduction in overhead converts the jiffies_till_sched_qs
kernel parameter to read-only at runtime, thus eliminating the need for
bounds checking.
Reported-by: NMichal Hocko <mhocko@kernel.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
[ paulmck: Keep PREEMPT=n cond_resched a no-op, per Peter Zijlstra. ]

The read of ->dynticks_nmi_nesting in rcu_irq_enter() and rcu_irq_exit()
is currently protected with READ_ONCE().  However, this protection is
unnecessary because (1) ->dynticks_nmi_nesting is updated only by the
current CPU, (2) Although NMI handlers can update this field, they reset
it back to its old value before return, and (3) Interrupts are disabled,
so nothing else can modify it.  The value of ->dynticks_nmi_nesting is
thus effectively constant, and so no protection is required.

This commit therefore removes the READ_ONCE() protection from these
two accesses.

Link: http://lkml.kernel.org/r/20170926031902.GA2074@linux.vnet.ibm.comReported-by: NLinus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: NSteven Rostedt (VMware) <rostedt@goodmis.org>

A number of architecture invoke rcu_irq_enter() on exception entry in
order to allow RCU read-side critical sections in the exception handler
when the exception is from an idle or nohz_full CPU.  This works, at
least unless the exception happens in an NMI handler.  In that case,
rcu_nmi_enter() would already have exited the extended quiescent state,
which would mean that rcu_irq_enter() would (incorrectly) cause RCU
to think that it is again in an extended quiescent state.  This will
in turn result in lockdep splats in response to later RCU read-side
critical sections.

This commit therefore causes rcu_irq_enter() and rcu_irq_exit() to
take no action if there is an rcu_nmi_enter() in effect, thus avoiding
the unscheduled return to RCU quiescent state.  This in turn should
make the kernel safe for on-demand RCU voyeurism.

Link: http://lkml.kernel.org/r/20170922211022.GA18084@linux.vnet.ibm.com

Cc: stable@vger.kernel.org
Fixes: 0be964be ("module: Sanitize RCU usage and locking")
Reported-by: NSteven Rostedt <rostedt@goodmis.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: NSteven Rostedt (VMware) <rostedt@goodmis.org>

First, number of CPUs can't be negative number.

Second, different signnnedness leads to suboptimal code in the following
cases:

1)
	kmalloc(nr_cpu_ids * sizeof(X));

"int" has to be sign extended to size_t.

2)
	while (loff_t *pos < nr_cpu_ids)

MOVSXD is 1 byte longed than the same MOV.

Other cases exist as well. Basically compiler is told that nr_cpu_ids
can't be negative which can't be deduced if it is "int".

Code savings on allyesconfig kernel: -3KB

	add/remove: 0/0 grow/shrink: 25/264 up/down: 261/-3631 (-3370)
	function                                     old     new   delta
	coretemp_cpu_online                          450     512     +62
	rcu_init_one                                1234    1272     +38
	pci_device_probe                             374     399     +25

				...

	pgdat_reclaimable_pages                      628     556     -72
	select_fallback_rq                           446     369     -77
	task_numa_find_cpu                          1923    1807    -116

Link: http://lkml.kernel.org/r/20170819114959.GA30580@avx2Signed-off-by: NAlexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

The rcu_idle_exit() and rcu_idle_enter() functions are exported because
they were originally used by RCU_NONIDLE(), which was intended to
be usable from modules.  However, RCU_NONIDLE() now instead uses
rcu_irq_enter_irqson() and rcu_irq_exit_irqson(), which are not
exported, and there have been no complaints.

This commit therefore removes the exports from rcu_idle_exit() and
rcu_idle_enter().
Reported-by: NPeter Zijlstra <peterz@infradead.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

All current callers of rcu_idle_enter() have irqs disabled, and
rcu_idle_enter() relies on this, but doesn't check.  This commit
therefore adds a RCU_LOCKDEP_WARN() to add some verification to the trust.
While we are there, pass "true" rather than "1" to rcu_eqs_enter().
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

All callers to rcu_idle_enter() have irqs disabled, so there is no
point in rcu_idle_enter disabling them again.  This commit therefore
replaces the irq disabling with a RCU_LOCKDEP_WARN().
Signed-off-by: NPeter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

This commit adds assertions verifying the consistency of the rcu_node
structure's ->blkd_tasks list and its ->gp_tasks, ->exp_tasks, and
->boost_tasks pointers.  In particular, the ->blkd_tasks lists must be
empty except for leaf rcu_node structures.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

Set disable_rcu_irq_enter on not only rcu_eqs_enter_common() but also
rcu_eqs_exit(), since rcu_eqs_exit() suffers from the same issue as was
fixed for rcu_eqs_enter_common() by commit 03ecd3f4 ("rcu/tracing:
Add rcu_disabled to denote when rcu_irq_enter() will not work").
Signed-off-by: NMasami Hiramatsu <mhiramat@kernel.org>
Acked-by: NSteven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

The _rcu_barrier_trace() function is a wrapper for trace_rcu_barrier(),
which needs TPS() protection for strings passed through the second
argument.  However, it has escaped prior TPS()-ification efforts because
it _rcu_barrier_trace() does not start with "trace_".  This commit
therefore adds the needed TPS() protection
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: NSteven Rostedt (VMware) <rostedt@goodmis.org>

These RCU waits were set to use interruptible waits to avoid the kthreads
contributing to system load average, even though they are not interruptible
as they are spawned from a kthread. Use the new TASK_IDLE swaits which makes
our goal clear, and removes confusion about these paths possibly being
interruptible -- they are not.

When the system is idle the RCU grace-period kthread will spend all its time
blocked inside the swait_event_interruptible(). If the interruptible() was
not used, then this kthread would contribute to the load average. This means
that an idle system would have a load average of 2 (or 3 if PREEMPT=y),
rather than the load average of 0 that almost fifty years of UNIX has
conditioned sysadmins to expect.

The same argument applies to swait_event_interruptible_timeout() use. The
RCU grace-period kthread spends its time blocked inside this call while
waiting for grace periods to complete. In particular, if there was only one
busy CPU, but that CPU was frequently invoking call_rcu(), then the RCU
grace-period kthread would spend almost all its time blocked inside the
swait_event_interruptible_timeout(). This would mean that the load average
would be 2 rather than the expected 1 for the single busy CPU.
Acked-by: N"Eric W. Biederman" <ebiederm@xmission.com>
Tested-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: NLuis R. Rodriguez <mcgrof@kernel.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

After adopting callbacks from a newly offlined CPU, the adopting CPU
checks to make sure that its callback list's count is zero only if the
list has no callbacks and vice versa.  Unfortunately, it does so after
enabling interrupts, which means that false positives are possible due to
interrupt handlers invoking call_rcu().  Although these false positives
are improbable, rcutorture did make it happen once.

This commit therefore moves this check to an irq-disabled region of code,
thus suppressing the false positive.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>