[ Upstream commit 8e4b1d2bc198e34b48fc7cc3a3c5a2fcb269e271 ]

Currently, rcu_spawn_core_kthreads() is invoked via an early_initcall(),
which works, except that rcu_spawn_gp_kthread() is also invoked via an
early_initcall() and rcu_spawn_core_kthreads() relies on adjustments to
kthread_prio that are carried out by rcu_spawn_gp_kthread().  There is
no guaranttee of ordering among early_initcall() handlers, and thus no
guarantee that kthread_prio will be properly checked and range-limited
at the time that rcu_spawn_core_kthreads() needs it.

In most cases, this bug is harmless.  After all, the only reason that
rcu_spawn_gp_kthread() adjusts the value of kthread_prio is if the user
specified a nonsensical value for this boot parameter, which experience
indicates is rare.

Nevertheless, a bug is a bug.  This commit therefore causes the
rcu_spawn_core_kthreads() function to be invoked directly from
rcu_spawn_gp_kthread() after any needed adjustments to kthread_prio have
been carried out.

Fixes: 48d07c04b4cc ("rcu: Enable elimination of Tree-RCU softirq processing")
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

bitmaprange.2021.03.08a:  Allow 3-N for bitmap ranges.
fixes.2021.03.15a:  Miscellaneous fixes.
kvfree_rcu.2021.03.08a:  kvfree_rcu() updates.
mmdumpobj.2021.03.08a:  mem_dump_obj() updates.
nocb.2021.03.15a:  RCU NOCB CPU updates, including limited deoffloading.
poll.2021.03.24a:  Polling grace-period interfaces for RCU.
rt.2021.03.08a:  Realtime-related RCU changes.
tasks.2021.03.08a:  Tasks-RCU updates.
torture.2021.03.08a:  Torture-test updates.
torturescript.2021.03.22a:  Torture-test scripting updates.

This commit causes rcutorture to test the new start_poll_synchronize_rcu()
and poll_state_synchronize_rcu() functions.  Because of the difficulty of
determining the nature of a synchronous RCU grace (expedited or not),
the test that insisted that poll_state_synchronize_rcu() detect an
intervening synchronize_rcu() had to be dropped.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

There is a need for a non-blocking polling interface for RCU grace
periods, so this commit supplies start_poll_synchronize_rcu() and
poll_state_synchronize_rcu() for this purpose.  Note that the existing
get_state_synchronize_rcu() may be used if future grace periods are
inevitable (perhaps due to a later call_rcu() invocation).  The new
start_poll_synchronize_rcu() is to be used if future grace periods
might not otherwise happen.  Finally, poll_state_synchronize_rcu()
provides a lockless check for a grace period having elapsed since
the corresponding call to either of the get_state_synchronize_rcu()
or start_poll_synchronize_rcu().

As with get_state_synchronize_rcu(), the return value from either
get_state_synchronize_rcu() or start_poll_synchronize_rcu() is passed in
to a later call to either poll_state_synchronize_rcu() or the existing
(might_sleep) cond_synchronize_rcu().

[ paulmck: Revert cond_synchronize_rcu() to might_sleep() per Frederic Weisbecker feedback. ]
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

There is a need for a non-blocking polling interface for RCU grace
periods, so this commit supplies start_poll_synchronize_rcu() and
poll_state_synchronize_rcu() for this purpose.  Note that the existing
get_state_synchronize_rcu() may be used if future grace periods are
inevitable (perhaps due to a later call_rcu() invocation).  The new
start_poll_synchronize_rcu() is to be used if future grace periods
might not otherwise happen.  Finally, poll_state_synchronize_rcu()
provides a lockless check for a grace period having elapsed since
the corresponding call to either of the get_state_synchronize_rcu()
or start_poll_synchronize_rcu().

As with get_state_synchronize_rcu(), the return value from either
get_state_synchronize_rcu() or start_poll_synchronize_rcu() is passed in
to a later call to either poll_state_synchronize_rcu() or the existing
(might_sleep) cond_synchronize_rcu().

[ paulmck: Remove redundant smp_mb() per Frederic Weisbecker feedback. ]
[ Update poll_state_synchronize_rcu() docbook per Frederic Weisbecker feedback. ]
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Those tracing calls don't need to be under ->nocb_lock.  This commit
therefore moves them outside of that lock.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Neeraj Upadhyay <neeraju@codeaurora.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

This commit removes a stale comment claiming that the cblist must be
empty before changing the offloading state.  This claim was correct back
when the offloaded state was defined exclusively at boot.

Reported-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Neeraj Upadhyay <neeraju@codeaurora.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Currently, the bypass is flushed at the very last moment in the
deoffloading procedure.  However, this approach leads to a larger state
space than would be preferred.  This commit therefore disables the
bypass at soon as the deoffloading procedure begins, then flushes it.
This guarantees that the bypass remains empty and thus out of the way
of the deoffloading procedure.

Symmetrically, this commit waits to enable the bypass until the offloading
procedure has completed.

Reported-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Neeraj Upadhyay <neeraju@codeaurora.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

This sequence of events can lead to a failure to requeue a CPU's
->nocb_timer:

1.	There are no callbacks queued for any CPU covered by CPU 0-2's
	->nocb_gp_kthread.  Note that ->nocb_gp_kthread is associated
	with CPU 0.

2.	CPU 1 enqueues its first callback with interrupts disabled, and
	thus must defer awakening its ->nocb_gp_kthread.  It therefore
	queues its rcu_data structure's ->nocb_timer.  At this point,
	CPU 1's rdp->nocb_defer_wakeup is RCU_NOCB_WAKE.

3.	CPU 2, which shares the same ->nocb_gp_kthread, also enqueues a
	callback, but with interrupts enabled, allowing it to directly
	awaken the ->nocb_gp_kthread.

4.	The newly awakened ->nocb_gp_kthread associates both CPU 1's
	and CPU 2's callbacks with a future grace period and arranges
	for that grace period to be started.

5.	This ->nocb_gp_kthread goes to sleep waiting for the end of this
	future grace period.

6.	This grace period elapses before the CPU 1's timer fires.
	This is normally improbably given that the timer is set for only
	one jiffy, but timers can be delayed.  Besides, it is possible
	that kernel was built with CONFIG_RCU_STRICT_GRACE_PERIOD=y.

7.	The grace period ends, so rcu_gp_kthread awakens the
	->nocb_gp_kthread, which in turn awakens both CPU 1's and
	CPU 2's ->nocb_cb_kthread.  Then ->nocb_gb_kthread sleeps
	waiting for more newly queued callbacks.

8.	CPU 1's ->nocb_cb_kthread invokes its callback, then sleeps
	waiting for more invocable callbacks.

9.	Note that neither kthread updated any ->nocb_timer state,
	so CPU 1's ->nocb_defer_wakeup is still set to RCU_NOCB_WAKE.

10.	CPU 1 enqueues its second callback, this time with interrupts
 	enabled so it can wake directly	->nocb_gp_kthread.
	It does so with calling wake_nocb_gp() which also cancels the
	pending timer that got queued in step 2. But that doesn't reset
	CPU 1's ->nocb_defer_wakeup which is still set to RCU_NOCB_WAKE.
	So CPU 1's ->nocb_defer_wakeup and its ->nocb_timer are now
	desynchronized.

11.	->nocb_gp_kthread associates the callback queued in 10 with a new
	grace period, arranges for that grace period to start and sleeps
	waiting for it to complete.

12.	The grace period ends, rcu_gp_kthread awakens ->nocb_gp_kthread,
	which in turn wakes up CPU 1's ->nocb_cb_kthread which then
	invokes the callback queued in 10.

13.	CPU 1 enqueues its third callback, this time with interrupts
	disabled so it must queue a timer for a deferred wakeup. However
	the value of its ->nocb_defer_wakeup is RCU_NOCB_WAKE which
	incorrectly indicates that a timer is already queued.  Instead,
	CPU 1's ->nocb_timer was cancelled in 10.  CPU 1 therefore fails
	to queue the ->nocb_timer.

14.	CPU 1 has its pending callback and it may go unnoticed until
	some other CPU ever wakes up ->nocb_gp_kthread or CPU 1 ever
	calls an explicit deferred wakeup, for example, during idle entry.

This commit fixes this bug by resetting rdp->nocb_defer_wakeup everytime
we delete the ->nocb_timer.

It is quite possible that there is a similar scenario involving
->nocb_bypass_timer and ->nocb_defer_wakeup.  However, despite some
effort from several people, a failure scenario has not yet been located.
However, that by no means guarantees that no such scenario exists.
Finding a failure scenario is left as an exercise for the reader, and the
"Fixes:" tag below relates to ->nocb_bypass_timer instead of ->nocb_timer.

Fixes: d1b222c6be1f (rcu/nocb: Add bypass callback queueing)
Cc: <stable@vger.kernel.org>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Reviewed-by: Neeraj Upadhyay <neeraju@codeaurora.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

RCU triggerse the following sparse warning:

kernel/rcu/tree_plugin.h:1497:5: warning: symbol
'nocb_nobypass_lim_per_jiffy' was not declared. Should it be static?

This commit therefore makes this variable static.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Reported-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>