It is now legal to invoke synchronize_sched() at early boot, which causes
Tiny RCU's synchronize_sched() to emit spurious splats.  This commit
therefore removes the cond_resched() from Tiny RCU's synchronize_sched().

Fixes: 8b355e3b ("rcu: Drive expedited grace periods from workqueue")
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: <stable@vger.kernel.org> # 4.9.0-

The __latent_entropy gcc attribute can be used only on functions and
variables.  If it is on a function then the plugin will instrument it for
gathering control-flow entropy. If the attribute is on a variable then
the plugin will initialize it with random contents.  The variable must
be an integer, an integer array type or a structure with integer fields.

These specific functions have been selected because they are init
functions (to help gather boot-time entropy), are called at unpredictable
times, or they have variable loops, each of which provide some level of
latent entropy.
Signed-off-by: NEmese Revfy <re.emese@gmail.com>
[kees: expanded commit message]
Signed-off-by: NKees Cook <keescook@chromium.org>

As we now have rcu_callback_t typedefs as the type of rcu callbacks, we
should use it in call_rcu*() and friends as the type of parameters. This
could save us a few lines of code and make it clear which function
requires an rcu callbacks rather than other callbacks as its argument.

Besides, this can also help cscope to generate a better database for
code reading.
Signed-off-by: NBoqun Feng <boqun.feng@gmail.com>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

This commit renames rcu_lockdep_assert() to RCU_LOCKDEP_WARN() for
consistency with the WARN() series of macros.  This also requires
inverting the sense of the conditional, which this commit also does.
Reported-by: NIngo Molnar <mingo@kernel.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NIngo Molnar <mingo@kernel.org>

If, at the time __rcu_process_callbacks() is invoked,  there are callbacks
in Tiny RCU's callback list, but none of them are ready to be invoked,
the current list-management code will knit the non-ready callbacks out
of the list.  This can result in hangs and possibly worse.  This commit
therefore inserts a check for there being no callbacks that can be
invoked immediately.

This bug is unlikely to occur -- you have to get a new callback between
the time rcu_sched_qs() or rcu_bh_qs() was called, but before we get to
__rcu_process_callbacks().  It was detected by the addition of RCU-bh
testing to rcutorture, which in turn was instigated by Iftekhar Ahmed's
mutation testing.  Although this bug was made much more likely by
915e8a4f (rcu: Remove fastpath from __rcu_process_callbacks()), this
did not cause the bug, but rather made it much more probable.   That
said, it takes more than 40 hours of rcutorture testing, on average,
for this bug to appear, so this fix cannot be considered an emergency.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: <stable@vger.kernel.org>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

The Tiny RCU counterparts to rcu_idle_enter(), rcu_idle_exit(),
rcu_irq_enter(), and rcu_irq_exit() are empty functions, but each has
EXPORT_SYMBOL_GPL(), which needlessly consumes extra memory, especially
in kernels built with module support.  This commit therefore moves these
functions to static inlines in rcutiny.h, removing the need for exports.

This won't affect the size of the tiniest kernels, which are likely
built without module support, but might help semi-tiny kernels that
might include module support.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

The term "ftrace" is really the infrastructure of the function hooks,
and not the trace events. Rename ftrace_event.h to trace_events.h to
represent the trace_event infrastructure and decouple the term ftrace
from it.
Signed-off-by: NSteven Rostedt <rostedt@goodmis.org>

The standard code path accommodates a condition when no
RCU callbacks are ready to invoke. Since size of the code
is a priority for tiny RCU, remove the fast path.

Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Signed-off-by: NAlexander Gordeev <agordeev@redhat.com>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

When the ->curtail and ->donetail pointers differ, ->rcucblist
always points to the beginning of the current list and thus
cannot be NULL. Therefore, the check ->rcucblist != NULL is
redundant and this commit removes it.

Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Signed-off-by: NAlexander Gordeev <agordeev@redhat.com>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

The current tiny RCU stall-warning code assumes that the jiffies counter
starts at zero, however, it is sometimes initialized to other values,
for example, -30,000.  This commit therefore changes rcu_init() to
invoke reset_cpu_stall_ticks() for both flavors of RCU to initialize
the stall-warning times properly at boot.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

For RCU in UP, context-switch = QS = GP, thus we can force a
context-switch when any call_rcu_[bh|sched]() is happened on idle_task.
After doing so, rcu_idle/irq_enter/exit() are useless, so we can simply
make these functions empty.

More important, this change does not change the functionality logically.
Note: raise_softirq(RCU_SOFTIRQ)/rcu_sched_qs() in rcu_idle_enter() and
outmost rcu_irq_exit() will have to wake up the ksoftirqd
(due to in_interrupt() == 0).

Before this patch		After this patch:
call_rcu_sched() in idle;	call_rcu_sched() in idle
				  set resched
do other stuffs;		do other stuffs
outmost rcu_irq_exit()		outmost rcu_irq_exit() (empty function)
  (or rcu_idle_enter())		  (or rcu_idle_enter(), also empty function)
				start to resched. (see above)
  rcu_sched_qs()		rcu_sched_qs()
    QS,and GP and advance cb	  QS,and GP and advance cb
    wake up the ksoftirqd	    wake up the ksoftirqd
      set resched
resched to ksoftirqd (or other)	resched to ksoftirqd (or other)

These two code patches are almost the same.

Size changed after patched:

size kernel/rcu/tiny-old.o kernel/rcu/tiny-patched.o
   text	   data	    bss	    dec	    hex	filename
   3449	    206	      8	   3663	    e4f	kernel/rcu/tiny-old.o
   2406	    144	      8	   2558	    9fe	kernel/rcu/tiny-patched.o
Signed-off-by: NLai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

Let's start assuming that something in the idle loop posts a callback,
and scheduling-clock interrupt occurs:

1. The system is idle and stays that way, no runnable tasks.

2. Scheduling-clock interrupt occurs, rcu_check_callbacks() is called
   as result, which in turn calls rcu_is_cpu_rrupt_from_idle().

3. rcu_is_cpu_rrupt_from_idle() reports the CPU was interrupted from
   idle, which results in rcu_sched_qs() call, which does a
   raise_softirq(RCU_SOFTIRQ).

4. Upon return from interrupt, rcu_irq_exit() is invoked, which calls
   rcu_idle_enter_common(), which in turn calls rcu_sched_qs() again,
   which does another raise_softirq(RCU_SOFTIRQ).

5. The softirq happens shortly and invokes rcu_process_callbacks(),
   which invokes __rcu_process_callbacks().

6. So now callbacks can be invoked. At least they can be if
   ->donetail has been updated. Which it will have been because
   rcu_sched_qs() invokes rcu_qsctr_help().

In the described scenario rcu_sched_qs() and raise_softirq(RCU_SOFTIRQ)
get called twice in steps 3 and 4. This redundancy could be eliminated
by removing rcu_is_cpu_rrupt_from_idle() function.
Signed-off-by: NAlexander Gordeev <agordeev@redhat.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

Add early boot self tests for RCU under CONFIG_PROVE_RCU.

Currently the only test is adding a dummy callback which increments a counter
which we then later verify after calling rcu_barrier*().
Signed-off-by: NPranith Kumar <bobby.prani@gmail.com>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

The "cpu" argument was kept around on the off-chance that RCU might
offload scheduler-clock interrupts.  However, this offload approach
has been replaced by NO_HZ_FULL, which offloads -all- RCU processing
from qualifying CPUs.  It is therefore time to remove the "cpu" argument
to rcu_check_callbacks(), which this commit does.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NPranith Kumar <bobby.prani@gmail.com>

The rcu_bh_qs(), rcu_preempt_qs(), and rcu_sched_qs() functions use
old-style per-CPU variable access and write to ->passed_quiesce even
if it is already set.  This commit therefore updates to use the new-style
per-CPU variable access functions and avoids the spurious writes.
This commit also eliminates the "cpu" argument to these functions because
they are always invoked on the indicated CPU.
Reported-by: NPeter Zijlstra <peterz@infradead.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

This commit adds a new RCU-tasks flavor of RCU, which provides
call_rcu_tasks().  This RCU flavor's quiescent states are voluntary
context switch (not preemption!) and userspace execution (not the idle
loop -- use some sort of schedule_on_each_cpu() if you need to handle the
idle tasks.  Note that unlike other RCU flavors, these quiescent states
occur in tasks, not necessarily CPUs.  Includes fixes from Steven Rostedt.

This RCU flavor is assumed to have very infrequent latency-tolerant
updaters.  This assumption permits significant simplifications, including
a single global callback list protected by a single global lock, along
with a single task-private linked list containing all tasks that have not
yet passed through a quiescent state.  If experience shows this assumption
to be incorrect, the required additional complexity will be added.
Suggested-by: NSteven Rostedt <rostedt@goodmis.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

The rcu_idle_enter_common() and rcu_idle_exit_common() functions contain
error checks that have to the best of my knowledge have never triggered
over the past several years.  These are nevertheless valuable when
creating new architectures or doing other low-level changes, so the
checks should not be deleted.  This commit instead places these checks
under #ifdef CONFIG_RCU_TRACE so that they are executed only when
specifically requested.

The savings are significant:

	Before:

	   text    data     bss     dec     hex filename
	   1749      39       0    1788     6fc /tmp/b/kernel/rcu/tiny.o
	    632     152       0     784     310 /tmp/b/kernel/rcu/update.o
				   ----
				   2572

	After:

	   text    data     bss     dec     hex filename
	   1281      37       0    1318     526 /tmp/b/kernel/rcu/tiny.o
	    632     152       0     784     310 /tmp/b/kernel/rcu/update.o
				   ----
				   2102

This amounts to 470 bytes, or 18% of the original.

Switched from #ifdef to IS_ENABLED() on Josh Triplett's advice.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

This commit updates the references to rcutree.c which is now rcu/tree.c
Signed-off-by: NPranith Kumar <bobby.prani@gmail.com>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

The kbuild test bot uncovered an implicit dependence on the
trace header being present before rcu.h in ia64 allmodconfig
that looks like this:

In file included from kernel/ksysfs.c:22:0:
kernel/rcu/rcu.h: In function '__rcu_reclaim':
kernel/rcu/rcu.h:107:3: error: implicit declaration of function 'trace_rcu_invoke_kfree_callback' [-Werror=implicit-function-declaration]
kernel/rcu/rcu.h:112:3: error: implicit declaration of function 'trace_rcu_invoke_callback' [-Werror=implicit-function-declaration]
cc1: some warnings being treated as errors

Looking at other rcu.h users, we can find that they all
were sourcing the trace header in advance of rcu.h itself,
as seen in the context of this diff.  There were also some
inconsistencies as to whether it was or wasn't sourced based
on the parent tracing Kconfig.

Rather than "fix" it at each use site, and have inconsistent
use based on whether "#ifdef CONFIG_RCU_TRACE" was used or not,
lets just source the trace header just once, in the actual consumer
of it, which is rcu.h itself.  We include it unconditionally, as
build testing shows us that is a hard requirement for some files.
Reported-by: Nkbuild test robot <fengguang.wu@intel.com>
Signed-off-by: NPaul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

All of the RCU source files have the usual GPL header, which contains a
long-obsolete postal address for FSF.  To avoid the need to track the
FSF office's movements, this commit substitutes the URL where GPL may
be found.
Reported-by: NGreg KH <gregkh@linuxfoundation.org>
Reported-by: NSteven Rostedt <rostedt@goodmis.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NIngo Molnar <mingo@kernel.org>

The old rcu_is_cpu_idle() function is just __rcu_is_watching() with
preemption disabled.  This commit therefore renames rcu_is_cpu_idle()
to rcu_is_watching.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

There is currently no way for kernel code to determine whether it
is safe to enter an RCU read-side critical section, in other words,
whether or not RCU is paying attention to the currently running CPU.
Given the large and increasing quantity of code shared by the idle loop
and non-idle code, the this shortcoming is becoming increasingly painful.

This commit therefore adds __rcu_is_watching(), which returns true if
it is safe to enter an RCU read-side critical section on the currently
running CPU.  This function is quite fast, using only a __this_cpu_read().
However, the caller must disable preemption.
Reported-by: NSteven Rostedt <rostedt@goodmis.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

The event-tracing macros do not like bool tracing arguments, so this
commit makes them be of type char.  This change has the knock-on effect
of making it illegal to pass a pointer into one of these arguments, so
also change rcutiny's first call to trace_rcu_batch_end() to convert
from pointer to boolean, prefixing with "!!".
Reported-by: Nkbuild test robot <fengguang.wu@intel.com>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

This commit extends the work done in f7f7bac9 (rcu: Have the RCU
tracepoints use the tracepoint_string infrastructure) to cover rcutiny.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>

All the RCU tracepoints and functions that reference char pointers do
so with just 'char *' even though they do not modify the contents of
the string itself. This will cause warnings if a const char * is used
in one of these functions.

The RCU tracepoints store the pointer to the string to refer back to them
when the trace output is displayed. As this can be minutes, hours or
even days later, those strings had better be constant.

This change also opens the door to allow the RCU tracepoint strings and
their addresses to be exported so that userspace tracing tools can
translate the contents of the pointers of the RCU tracepoints.
Signed-off-by: NSteven Rostedt <rostedt@goodmis.org>

TINY_RCU's reset_cpu_stall_ticks() and check_cpu_stalls() functions
are defined unconditionally, and are empty functions if CONFIG_RCU_TRACE
is disabled (which in turns disables detection of RCU CPU stalls).
This commit saves a few lines of source code by defining these functions
only if CONFIG_RCU_TRACE=y.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

TINY_PREEMPT_RCU could use a kthread to handle RCU callback invocation,
which required an API to abstract kthread vs. softirq invocation.
Now that TINY_PREEMPT_RCU is no longer with us, this commit retires
this API in favor of direct use of the relevant softirq primitives.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

With the removal of CONFIG_TINY_PREEMPT_RCU, rcu_preempt_process_callbacks()
is now an empty function.  This commit therefore eliminates it by
inlining it.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

With the removal of CONFIG_TINY_PREEMPT_RCU, rcu_preempt_remove_callbacks()
is now an empty function.  This commit therefore eliminates it by
inlining it.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

With the removal of CONFIG_TINY_PREEMPT_RCU, rcu_preempt_check_callbacks()
is now an empty function.  This commit therefore eliminates it by
inlining it.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

Tiny RCU has historically omitted RCU CPU stall warnings in order to
reduce memory requirements, however, lack of these warnings caused
Thomas Gleixner some debugging pain recently.  Therefore, this commit
adds RCU CPU stall warnings to tiny RCU if RCU_TRACE=y.  This keeps
the memory footprint small, while still enabling CPU stall warnings
in kernels built to enable them.

Updated to include Josh Triplett's suggested use of RCU_STALL_COMMON
config variable to simplify #if expressions.
Reported-by: NThomas Gleixner <tglx@linutronix.de>
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

Both rcutiny and rcutree define a helper function named
rcu_is_cpu_rrupt_from_idle(), each used exactly once, later in the
same file.  This commit therefore declares these helper functions static.
Signed-off-by: NJosh Triplett <josh@joshtriplett.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

The rcu_is_cpu_rrupt_from_idle() needs to allow for one interrupt level
from the idle loop, but TINY_RCU checks for a call from the idle loop
itself.  This commit fixes this issue.
Reported-by: NJosh Triplett <josh@joshtriplett.org>
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

TINY_RCU's rcu_idle_enter_common() invokes rcu_sched_qs() in order
to inform the RCU core of the quiescent state implied by idle entry.
Of course, idle is also an extended quiescent state, so that the call
to rcu_sched_qs() speeds up RCU's invoking of any callbacks that might
be queued.  This speed-up is important when entering into dyntick-idle
mode -- if there are no further scheduling-clock interrupts, the callbacks
might never be invoked, which could result in a system hang.

However, processing callbacks does event tracing, which in turn
implies RCU read-side critical sections, which are illegal in extended
quiescent states.  This patch therefore moves the call to rcu_sched_qs()
so that it precedes the point at which we inform lockdep that RCU has
entered an extended quiescent state.
Signed-off-by: NLi Zhong <zhong@linux.vnet.ibm.com>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

There is a need to use RCU from interrupt context, but either before
rcu_irq_enter() is called or after rcu_irq_exit() is called.  If the
interrupt occurs from idle, then lockdep-RCU will complain about such
uses, as they appear to be illegal uses of RCU from the idle loop.
In other environments, RCU_NONIDLE() could be used to properly protect
the use of RCU, but RCU_NONIDLE() currently cannot be invoked except
from process context.

This commit therefore modifies RCU_NONIDLE() to permit its use more
globally.
Reported-by: NSteven Rostedt <rostedt@goodmis.org>
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

Because TINY_RCU's idle detection keys directly off of the nesting
level, rather than from a separate variable as in TREE_RCU, the
TINY_RCU dyntick-idle tracing on transition to idle must happen
before the change to the nesting level.  This commit therefore makes
this change by passing the desired new value (rather than the old value)
of the nesting level in to rcu_idle_enter_common().

[ paulmck: Add fix for wrong-variable bug spotted by
  Michael Wang <wangyun@linux.vnet.ibm.com>. ]
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

The rcu_is_cpu_idle() function is used if CONFIG_DEBUG_LOCK_ALLOC,
but TINY_RCU defines it only when CONFIG_PROVE_RCU.  This causes
build failures when CONFIG_DEBUG_LOCK_ALLOC=y but CONFIG_PROVE_RCU=n.
This commit therefore adjusts the #ifdefs for rcu_is_cpu_idle() so
that it is defined when CONFIG_DEBUG_LOCK_ALLOC=y.
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

RCU, RCU-bh, and RCU-sched read-side critical sections are forbidden
in the inner idle loop, that is, between the rcu_idle_enter() and the
rcu_idle_exit() -- RCU will happily ignore any such read-side critical
sections.  However, things like powertop need tracepoints in the inner
idle loop.

This commit therefore provides an RCU_NONIDLE() macro that can be used to
wrap code in the idle loop that requires RCU read-side critical sections.
Suggested-by: NSteven Rostedt <rostedt@goodmis.org>
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>
Acked-by: NDeepthi Dharwar <deepthi@linux.vnet.ibm.com>

Use of RCU in the idle loop is incorrect, quite a few instances of
just that have made their way into mainline, primarily event tracing.
The problem with RCU read-side critical sections on CPUs that RCU believes
to be idle is that RCU is completely ignoring the CPU, along with any
attempts and RCU read-side critical sections.

The approaches of eliminating the offending uses and of pushing the
definition of idle down beyond the offending uses have both proved
impractical.  The new approach is to encapsulate offending uses of RCU
with rcu_idle_exit() and rcu_idle_enter(), but this requires nesting
for code that is invoked both during idle and and during normal execution.
Therefore, this commit modifies rcu_idle_enter() and rcu_idle_exit() to
permit nesting.
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>
Acked-by: NDeepthi Dharwar <deepthi@linux.vnet.ibm.com>