rcu: Add event-trace markers to TREE_RCU kthreads

Add event-trace markers to TREE_RCU kthreads to allow including these
kthread's CPU time in the utilization calculations.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
rcu: Make TINY_RCU also use softirq for RCU_BOOST=n

This patch #ifdefs TINY_RCU kthreads out of the kernel unless RCU_BOOST=y,
thus eliminating context-switch overhead if RCU priority boosting has
not been configured.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
rcu: Add grace-period, quiescent-state, and call_rcu trace events

Add trace events to record grace-period start and end, quiescent states,
CPUs noticing grace-period start and end, grace-period initialization,
call_rcu() invocation, tasks blocking in RCU read-side critical sections,
tasks exiting those same critical sections, force_quiescent_state()
detection of dyntick-idle and offline CPUs, CPUs entering and leaving
dyntick-idle mode (except from NMIs), CPUs coming online and going
offline, and CPUs being kicked for staying in dyntick-idle mode for too
long (as in many weeks, even on 32-bit systems).

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

rcu: Add the rcu flavor to callback trace events

The earlier trace events for registering RCU callbacks and for invoking
them did not include the RCU flavor (rcu_bh, rcu_preempt, or rcu_sched).
This commit adds the RCU flavor to those trace events.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
rcu: Simplify quiescent-state accounting

There is often a delay between the time that a CPU passes through a
quiescent state and the time that this quiescent state is reported to the
RCU core.  It is quite possible that the grace period ended before the
quiescent state could be reported, for example, some other CPU might have
deduced that this CPU passed through dyntick-idle mode.  It is critically
important that quiescent state be counted only against the grace period
that was in effect at the time that the quiescent state was detected.

Previously, this was handled by recording the number of the last grace
period to complete when passing through a quiescent state.  The RCU
core then checks this number against the current value, and rejects
the quiescent state if there is a mismatch.  However, one additional
possibility must be accounted for, namely that the quiescent state was
recorded after the prior grace period completed but before the current
grace period started.  In this case, the RCU core must reject the
quiescent state, but the recorded number will match.  This is handled
when the CPU becomes aware of a new grace period -- at that point,
it invalidates any prior quiescent state.

This works, but is a bit indirect.  The new approach records the current
grace period, and the RCU core checks to see (1) that this is still the
current grace period and (2) that this grace period has not yet ended.
This approach simplifies reasoning about correctness, and this commit
changes over to this new approach.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
rcu: Not necessary to pass rcu_read_lock_held() to rcu_dereference_protected()

Since ca5ecddf (rcu: define __rcu address space modifier for sparse)
rcu_dereference_check() use rcu_read_lock_held() as a part of condition
automatically.  Therefore, callers of rcu_dereference_check() no longer
need to pass rcu_read_lock_held() to rcu_dereference_check().

Signed-off-by: Michal Hocko <mhocko@suse.cz>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
rcu: Update documentation for additional RCU lockdep functions

Add documentation for rcu_dereference_bh_check(),
rcu_dereference_sched_check(), srcu_dereference_check(), and
rcu_dereference_index_check().

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
rcu: Remove unused and redundant interfaces

The rcu_dereference_bh_protected() and rcu_dereference_sched_protected()
macros are synonyms for rcu_dereference_protected() and are not used
anywhere in mainline.  This commit therefore removes them.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
rcu: Allow rcutorture's stat_interval parameter to be changed at runtime

When rcutorture is compiled directly into the kernel
(instead of separately as a module), it is necessary to specify
rcutorture.stat_interval as a kernel command-line parameter, otherwise,
the rcu_torture_stats kthread is never started.  However, when working
with the system after it has booted, it is convenient to be able to
change the time between statistic printing, particularly when logged
into the console.

This commit therefore allows the stat_interval parameter to be changed
at runtime.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
rcu: Document interpretation of RCU-lockdep splats

There has been quite a bit of confusion about what RCU-lockdep splats
mean, so this commit adds some documentation describing how to
interpret them.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
rcu: Eliminate in_irq() checks in rcu_enter_nohz()

The in_irq() check in rcu_enter_nohz() is redundant because if we really
are in an interrupt, the attempt to re-enter dyntick-idle mode will invoke
rcu_needs_cpu() in any case, which will force the check for RCU callbacks.
So this commit removes the check along with the set_need_resched().

Suggested-by: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
rcu: Make rcu_implicit_dynticks_qs() locals be correct size

When the ->dynticks field in the rcu_dynticks structure changed to an
atomic_t, its size on 64-bit systems changed from 64 bits to 32 bits.
The local variables in rcu_implicit_dynticks_qs() need to change as
well, hence this commit.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
rcu: Make rcu_assign_pointer() unconditionally insert a memory barrier

Recent changes to gcc give warning messages on rcu_assign_pointers()'s
checks that allow it to determine when it is OK to omit the memory
barrier.  Stephen Hemminger tried a number of gcc tricks to silence
this warning, but #pragmas and CPP macros do not work together in the
way that would be required to make this work.

However, we now have RCU_INIT_POINTER(), which already omits this
memory barrier, and which therefore may be used when assigning NULL to
an RCU-protected pointer that is accessible to readers.  This commit
therefore makes rcu_assign_pointer() unconditionally emit the memory
barrier.

Reported-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
rcu: Improve rcu_assign_pointer() and RCU_INIT_POINTER() documentation

The differences between rcu_assign_pointer() and RCU_INIT_POINTER() are
subtle, and it is easy to use the the cheaper RCU_INIT_POINTER() when
the more-expensive rcu_assign_pointer() should have been used instead.
The consequences of this mistake are quite severe.

This commit therefore carefully lays out the situations in which it it
permissible to use RCU_INIT_POINTER().

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
rcu: Move __rcu_read_unlock()'s barrier() within if-statement

We only need to constrain the compiler if we are actually exiting
the top-level RCU read-side critical section.  This commit therefore
moves the first barrier() cal in __rcu_read_unlock() to inside the
"if" statement, thus avoiding needless register flushes for inner
rcu_read_unlock() calls.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
rcu: Dump local stack if cannot dump all CPUs' stacks

The trigger_all_cpu_backtrace() function is a no-op in architectures that
do not define arch_trigger_all_cpu_backtrace.  On such architectures, RCU
CPU stall warning messages contain no stack trace information, which makes
debugging quite difficult.  This commit therefore substitutes dump_stack()
for architectures that do not define arch_trigger_all_cpu_backtrace,
so that at least the local CPU's stack is dumped as part of the RCU CPU
stall warning message.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
rcu: Prevent early boot set_need_resched() from __rcu_pending()

There isn't a whole lot of point in poking the scheduler before there
are other tasks to switch to.  This commit therefore adds a check
for rcu_scheduler_fully_active in __rcu_pending() to suppress any
pre-scheduler calls to set_need_resched().  The downside of this approach
is additional runtime overhead in a reasonably hot code path.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
rcu: Simplify unboosting checks

Commit 7765be (Fix RCU_BOOST race handling current->rcu_read_unlock_special)
introduced a new ->rcu_boosted field in the task structure.  This is
redundant because the existing ->rcu_boost_mutex will be non-NULL at
any time that ->rcu_boosted is nonzero.  Therefore, this commit removes
->rcu_boosted and tests ->rcu_boost_mutex instead.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
rcu: Prohibit grace periods during early boot

Greater use of RCU during early boot (before the scheduler is operating)
is causing RCU to attempt to start grace periods during that time, which
in turn is resulting in both RCU and the callback functions attempting
to use the scheduler before it is ready.

This commit prevents these problems by prohibiting RCU grace periods
until after the scheduler has spawned the first non-idle task.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>