The scheduler is currently required to hold rq/pi locks across the entire
RCU read-side critical section or not at all.  This is inconvenient and
leaves traps for the unwary, including the author of this commit.

But now that excessively long grace periods enable scheduling-clock
interrupts for holdout nohz_full CPUs, the nohz_full rescue logic in
rcu_read_unlock_special() can be dispensed with.  In other words, the
rcu_read_unlock_special() function can refrain from doing wakeups unless
such wakeups are guaranteed safe.

This commit therefore avoids unsafe wakeups, freeing the scheduler to
hold rq/pi locks across rcu_read_unlock() even if the corresponding RCU
read-side critical section might have been preempted.  This commit also
updates RCU's requirements documentation.

This commit is inspired by a patch from Lai Jiangshan:
https://lore.kernel.org/lkml/20191102124559.1135-2-laijs@linux.alibaba.com
This commit is further intended to be a step towards his goal of permitting
the inlining of RCU-preempt's rcu_read_lock() and rcu_read_unlock().

Cc: Lai Jiangshan <laijs@linux.alibaba.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

For testing mainline, the kvm.sh rcutorture script is the preferred
approach to testing.  This commit therefore adds it to the torture.txt
documentation.

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

Signed-off-by: SeongJae Park <sjpark@amazon.de>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Signed-off-by: SeongJae Park <sjpark@amazon.de>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Signed-off-by: SeongJae Park <sjpark@amazon.de>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

listRCU.rst document gives an example with 'ipc_lock()', but the
function has dropped off by commit 82061c57ce93 ("ipc: drop
ipc_lock()").  Because the main logic of 'ipc_lock()' has melded in
'shm_lock()' by the commit, this commit updates the document to use
'shm_lock()' instead.

Reviewed-by: Madhuparna Bhowmik <madhuparnabhowmik04@gmail.com>
Signed-off-by: SeongJae Park <sjpark@amazon.de>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Signed-off-by: SeongJae Park <sjpark@amazon.de>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Commit ccc9971e2147 ("docs: rcu: convert some articles from html to
ReST") has converted a few of html RCU docs into ReST files, but a few
of html tags which not supported on rst is remaining.  This commit
converts those to ReST appropriate alternatives.

Reviewed-by: Madhuparna Bhowmik <madhuparnabhowmik04@gmail.com>
Signed-off-by: SeongJae Park <sjpark@amazon.de>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

- Add more information about RCU list patterns taking examples
from audit subsystem in the linux kernel.

- Keep the current audit examples, even though the kernel has changed.

- Modify inline text for better passage quality.

- Fix typo in code-blocks and improve code comments.

- Add text formatting (italics, bold and code) for better emphasis.

Patch originally submitted at
https://lore.kernel.org/patchwork/patch/1082804/

Co-developed-by: Amol Grover <frextrite@gmail.com>
Signed-off-by: Amol Grover <frextrite@gmail.com>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

doc.2019.12.10a: Documentations updates
exp.2019.12.09a: Expedited grace-period updates
fixes.2020.01.24a: Miscellaneous fixes
kfree_rcu.2020.01.24a: Batch kfree_rcu() work
list.2020.01.10a: RCU-protected-list updates
preempt.2020.01.24a: Preemptible RCU updates
torture.2019.12.09a: Torture-test updates