There is a rather obtuse string that can be printed as part of an
expedited RCU CPU stall-warning message that starts with "blocking
rcu_node structures".  Under normal conditions, most of this message
is just repeating the list of CPUs blocking the current expedited grace
period, but in a manner that is rather difficult to read.  This commit
therefore marks this message as "(internal RCU debug)" in an effort to
give people the option of avoiding wasting time attempting to extract
nonexistent additional meaning from this portion of the message.

Reported-by: Jonathan Lemon <bsd@fb.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

This commit moves the initialization of the CONFIG_PREEMPT=n version of
the rcu_exp_handler() function's rdp and rnp local variables into their
respective declarations to save a couple lines of code.

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

This commit converts the schedule_timeout_uninterruptible() call used
by RCU's expedited grace-period processing to schedule_timeout_idle().
This conversion avoids polluting the load-average with RCU-related
sleeping.

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

fixes.2020.04.27a:  Miscellaneous fixes.
kfree_rcu.2020.04.27a:  Changes related to kfree_rcu().
rcu-tasks.2020.04.27a:  Addition of new RCU-tasks flavors.
stall.2020.04.27a:  RCU CPU stall-warning updates.
torture.2020.05.07a:  Torture-test updates.

Although the accesses used to determine whether or not an expedited
stall should be printed are an integral part of the concurrency algorithm
governing use of the corresponding variables, the values that are simply
printed are ancillary.  As such, it is best to use data_race() for these
accesses in order to provide the greatest latitude in the use of KCSAN
for the other accesses that are an integral part of the algorithm.  This
commit therefore changes the relevant uses of READ_ONCE() to data_race().

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

Now that RCU flavors have been consolidated, an RCU-preempt
rcu_read_unlock() in an interrupt or softirq handler cannot possibly
end the RCU read-side critical section.  Consider the old vulnerability
involving rcu_read_unlock() being invoked within such a handler that
interrupted an __rcu_read_unlock_special(), in which a wakeup might be
invoked with a scheduler lock held.  Because rcu_read_unlock_special()
no longer does wakeups in such situations, it is no longer necessary
for __rcu_read_unlock() to set the nesting level negative.

This commit therefore removes this recursion-protection code from
__rcu_read_unlock().

[ paulmck: Let rcu_exp_handler() continue to call rcu_report_exp_rdp(). ]
[ paulmck: Adjust other checks given no more negative nesting. ]
Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

There are lockless loads from the rcu_node structure's ->exp_tasks field,
so this commit causes all stores to use WRITE_ONCE() and all lockless
loads to use READ_ONCE() or data_race(), with the latter for debug
prints.  This code also did a unprotected traversal of the linked list
pointed into by ->exp_tasks, so this commit also acquires the rcu_node
structure's ->lock to properly protect this traversal.  This list was
traversed unprotected only when printing an RCU CPU stall warning for
an expedited grace period, so the odds of seeing this in production are
not all that high.

This data race was reported by KCSAN.

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

doc.2020.02.27a: Documentation updates.
fixes.2020.03.21a: Miscellaneous fixes.
kfree_rcu.2020.02.20a: Updates to kfree_rcu().
locktorture.2020.02.20a: Lock torture-test updates.
ovld.2020.02.20a: Updates to callback-overload handling.
rcu-tasks.2020.02.20a: RCU-tasks updates.
srcu.2020.02.20a: SRCU updates.
torture.2020.02.20a: Torture-test updates.

In normal production, an RCU CPU stall warning at boottime is often
just as bad as at any other time.  In fact, given the desire for fast
boot, any sort of long-term stall at boot is a bad idea.  However,
heavy rcutorture testing on large hyperthreaded systems can generate
boottime RCU CPU stalls as a matter of course.  This commit therefore
provides a kernel boot parameter that suppresses reporting of boottime
RCU CPU stall warnings and similarly of rcutorture writer stalls.

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

Some larger systems can take in excess of 50 seconds to complete their
early boot initcalls prior to spawing init.  This does not in any way
help the forward-progress judgments of built-in rcutorture (when
rcutorture is built as a module, the insmod or modprobe command normally
cannot happen until some time after boot completes).  This commit
therefore suppresses such complaints until about the time that init
is spawned.

This also includes a fix to a stupid error located by kbuild test robot.

[ paulmck: Apply kbuild test robot feedback. ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
[ paulmck: Fix to nohz_full slow-expediting recovery logic, per bpetkov. ]
[ paulmck: Restrict splat to CONFIG_PREEMPT_RT=y kernels and simplify. ]
Tested-by: Borislav Petkov <bp@alien8.de>