Add some missing critical pieces of explanation to understand the need
for full memory barriers throughout the whole grace period state machine,
thanks to Paul's explanations.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Neeraj Upadhyay <neeraju@codeaurora.org>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Uladzislau Rezki <urezki@gmail.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
[ paulmck: Adjust code block per Akira Yokosawa. ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

To avoid the ``foo`` markup inside the `bar`__ hyperlink marker,
use the "replace" directive [1].

This should restore the intended appearance of the link.

Tested with sphinx versions 1.7.9 and 2.4.4.

[1]: https://docutils.sourceforge.io/docs/ref/rst/directives.html#replace

Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

The three diagrams describing rcu_gp_init() all spuriously refer to
the same figure, probably due to a copy/paste issue.  This commit fixes
these references.

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

The sentence defining the relationship of accesses before a grace
period to read-side accesses following that same grace period was
missing a small word: "not".  This commit therefore adds it.

Reported-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Pull RCU updates from Paul E. McKenney:

- Documentation updates.

- Miscellaneous fixes.

- kfree_rcu() updates: Addition of mem_dump_obj() to provide allocator return
  addresses to more easily locate bugs.  This has a couple of RCU-related commits,
  but is mostly MM.  Was pulled in with akpm's agreement.

- Per-callback-batch tracking of numbers of callbacks,
  which enables better debugging information and smarter
  reactions to large numbers of callbacks.

- The first round of changes to allow CPUs to be runtime switched from and to
  callback-offloaded state.

- CONFIG_PREEMPT_RT-related changes.

- RCU CPU stall warning updates.
- Addition of polling grace-period APIs for SRCU.

- Torture-test and torture-test scripting updates, including a "torture everything"
  script that runs rcutorture, locktorture, scftorture, rcuscale, and refscale.
  Plus does an allmodconfig build.

Signed-off-by: Ingo Molnar <mingo@kernel.org>

doc.2021.01.06a: Documentation updates.
fixes.2021.01.04b: Miscellaneous fixes.
kfree_rcu.2021.01.04a: kfree_rcu() updates.
mmdumpobj.2021.01.22a: Dump allocation point for memory blocks.
nocb.2021.01.06a: RCU callback offload updates and cblist segment lengths.
rt.2021.01.04a: Real-time updates.
stall.2021.01.06a: RCU CPU stall warning updates.
torture.2021.01.12a: Torture-test updates and polling SRCU grace-period API.
tortureall.2021.01.06a: Torture-test script updates.

CONFIG_PREEMPTION is selected by CONFIG_PREEMPT and by CONFIG_PREEMPT_RT.
Both PREEMPT and PREEMPT_RT require the same functionality which today
depends on CONFIG_PREEMPT.

Update the documents and mention CONFIG_PREEMPTION. Spell out
CONFIG_PREEMPT_RT (instead PREEMPT_RT) since it is an option now.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

The PREEMPT_RT wiki moved from kernel.org to the Linux Foundation wiki.
The kernel.org wiki is read only.

This commit therefore updates the URL of the active PREEMPT_RT wiki.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

This commit adds requirements documentation for the
get_state_synchronize_srcu(), start_poll_synchronize_srcu(), and
poll_state_synchronize_srcu() functions.

Link: https://lore.kernel.org/rcu/20201112201547.GF3365678@moria.home.lan/
Reported-by: Kent Overstreet <kent.overstreet@gmail.com>
Reviewed-by: Neeraj Upadhyay <neeraju@codeaurora.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Back in the day, RCU_INIT_POINTER() was the only way to avoid
memory-barrier instructions while storing NULL to an RCU-protected
pointer.  Fortunately, in 2016, rcu_assign_pointer() started checking for
compile-time NULL pointers and omitting the memory-barrier instructions in
that case.  Unfortunately, RCU's Requirements.rst document was not updated
accordingly.  This commit therefore at long last carries out that update.

Fixes: 3a37f7275cda ("rcu: No ordering for rcu_assign_pointer() of NULL")
Link: https://lore.kernel.org/lkml/20201209230755.GV7338@casper.infradead.org/
Reported-by: Matthew Wilcox <willy@infradead.org>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>