rcu-tasks: Initialize callback lists at rcu_init() time

In order for RCU Tasks to reliably maintain per-CPU lists of exiting
tasks, those lists must be initialized before it is possible for tasks
to exit, especially given that the boot CPU is not necessarily CPU 0
(an example being, powerpc kexec() kernels). And at the time that
rcu_init_tasks_generic() is called, a task could potentially exit,
unconventional though that sort of thing might be.

This commit therefore moves the calls to cblist_init_generic() from
functions called from rcu_init_tasks_generic() to a new function named
tasks_cblist_init_generic() that is invoked from rcu_init().

This constituted a bug in a commit that never went to mainline, so
there is no need for any backporting to -stable.

Reported-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>

rcu: Dump memory object info if callback function is invalid

When a structure containing an RCU callback rhp is (incorrectly) freed
and reallocated after rhp is passed to call_rcu(), it is not unusual for
rhp->func to be set to NULL. This defeats the debugging prints used by
__call_rcu_common() in kernels built with CONFIG_DEBUG_OBJECTS_RCU_HEAD=y,
which expect to identify the offending code using the identity of this
function.

And in kernels build without CONFIG_DEBUG_OBJECTS_RCU_HEAD=y, things
are even worse, as can be seen from this splat:

Unable to handle kernel NULL pointer dereference at virtual address 0
... ...
PC is at 0x0
LR is at rcu_do_batch+0x1c0/0x3b8
... ...
(rcu_do_batch) from (rcu_core+0x1d4/0x284)
(rcu_core) from (__do_softirq+0x24c/0x344)
(__do_softirq) from (__irq_exit_rcu+0x64/0x108)
(__irq_exit_rcu) from (irq_exit+0x8/0x10)
(irq_exit) from (__handle_domain_irq+0x74/0x9c)
(__handle_domain_irq) from (gic_handle_irq+0x8c/0x98)
(gic_handle_irq) from (__irq_svc+0x5c/0x94)
(__irq_svc) from (arch_cpu_idle+0x20/0x3c)
(arch_cpu_idle) from (default_idle_call+0x4c/0x78)
(default_idle_call) from (do_idle+0xf8/0x150)
(do_idle) from (cpu_startup_entry+0x18/0x20)
(cpu_startup_entry) from (0xc01530)

This commit therefore adds calls to mem_dump_obj(rhp) to output some
information, for example:

slab kmalloc-256 start ffff410c45019900 pointer offset 0 size 256

This provides the rough size of the memory block and the offset of the
rcu_head structure, which as least provides at least a few clues to help
locate the problem. If the problem is reproducible, additional slab
debugging can be enabled, for example, CONFIG_DEBUG_SLAB=y, which can
provide significantly more information.

Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>

rcu: Refactor kvfree_call_rcu() and high-level helpers

Currently a kvfree_call_rcu() takes an offset within a structure as
a second parameter, so a helper such as a kvfree_rcu_arg_2() has to
convert rcu_head and a freed ptr to an offset in order to pass it. That
leads to an extra conversion on macro entry.

Instead of converting, refactor the code in way that a pointer that has
to be freed is passed directly to the kvfree_call_rcu().

This patch does not make any functional change and is transparent to
all kvfree_rcu() users.

Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

rcu: Make call_rcu() lazy to save power

Implement timer-based RCU callback batching (also known as lazy
callbacks). With this we save about 5-10% of power consumed due
to RCU requests that happen when system is lightly loaded or idle.

By default, all async callbacks (queued via call_rcu) are marked
lazy. An alternate API call_rcu_hurry() is provided for the few users,
for example synchronize_rcu(), that need the old behavior.

The batch is flushed whenever a certain amount of time has passed, or
the batch on a particular CPU grows too big. Also memory pressure will
flush it in a future patch.

To handle several corner cases automagically (such as rcu_barrier() and
hotplug), we re-use bypass lists which were originally introduced to
address lock contention, to handle lazy CBs as well. The bypass list
length has the lazy CB length included in it. A separate lazy CB length
counter is also introduced to keep track of the number of lazy CBs.

[ paulmck: Fix formatting of inline call_rcu_lazy() definition. ]
[ paulmck: Apply Zqiang feedback. ]
[ paulmck: Apply s/call_rcu_flush/call_rcu_hurry/ feedback from Tejun Heo. ]

Suggested-by: Paul McKenney <paulmck@kernel.org>
Acked-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

rcu: Add full-sized polling for get_completed*() and poll_state*()

The get_completed_synchronize_rcu() and poll_state_synchronize_rcu()
APIs compress the combined expedited and normal grace-period states into a
single unsigned long, which conserves storage, but can miss grace periods
in certain cases involving overlapping normal and expedited grace periods.
Missing the occasional grace period is usually not a problem, but there
are use cases that care about each and every grace period.

This commit therefore adds the first members of the full-state RCU
grace-period polling API, namely the get_completed_synchronize_rcu_full()
and poll_state_synchronize_rcu_full() functions. These use up to three
times the storage (rcu_gp_oldstate structure instead of unsigned long),
but which are guaranteed not to miss grace periods, at least in situations
where the single-CPU grace-period optimization does not apply.

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

rcu: Make tiny RCU support leak callbacks for debug-object errors

Currently, only Tree RCU leaks callbacks setting when it detects a
duplicate call_rcu(). This commit causes Tiny RCU to also leak
callbacks in this situation.

Because this is Tiny RCU, kernel size is important:

1. CONFIG_TINY_RCU=y and CONFIG_DEBUG_OBJECTS_RCU_HEAD=n
(Production kernel)

Original:
text data bss dec hex filename
26290663 20159823 15212544 61663030 3ace736 vmlinux

With this commit:
text data bss dec hex filename
26290663 20159823 15212544 61663030 3ace736 vmlinux

2. CONFIG_TINY_RCU=y and CONFIG_DEBUG_OBJECTS_RCU_HEAD=y
(Debugging kernel)

Original:
text data bss dec hex filename
26291319 20160143 15212544 61664006 3aceb06 vmlinux

With this commit:
text data bss dec hex filename
26291319 20160431 15212544 61664294 3acec26 vmlinux

These results show that the kernel size is unchanged for production
kernels, as desired.

Signed-off-by: Zqiang <qiang1.zhang@intel.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

rcu: Make Tiny RCU grace periods visible to polled APIs

This commit makes the Tiny RCU implementation of synchronize_rcu()
increment the rcu_ctrlblk.gp_seq counter, thus making both
synchronize_rcu() and synchronize_rcu_expedited() visible to
get_state_synchronize_rcu() and friends.

Link: https://lore.kernel.org/all/20220121142454.1994916-1-bfoster@redhat.com/
Link: https://docs.google.com/document/d/1RNKWW9jQyfjxw2E8dsXVTdvZYh0HnYeSHDKog9jhdN8/edit?usp=sharing
Cc: Brian Foster <bfoster@redhat.com>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Ian Kent <raven@themaw.net>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

rcu: tiny: Record kvfree_call_rcu() call stack for KASAN

When running KASAN with Tiny RCU (e.g. under ARCH=um, where
a working KASAN patch is now available), we don't get any
information on the original kfree_rcu() (or similar) caller
when a problem is reported, as Tiny RCU doesn't record this.

Add the recording, which required pulling kvfree_call_rcu()
out of line for the KASAN case since the recording function
(kasan_record_aux_stack_noalloc) is neither exported, nor
can we include kasan.h into rcutiny.h.

without KASAN, the patch has no size impact (ARCH=um kernel):
text data bss dec hex filename
6151515 4423154 33148520 43723189 29b29b5 linux
6151515 4423154 33148520 43723189 29b29b5 linux + patch

with KASAN, the impact on my build was minimal:
text data bss dec hex filename
13915539 7388050 33282304 54585893 340ea25 linux
13911266 7392114 33282304 54585684 340e954 linux + patch
-4273 +4064 +-0 -209

Acked-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

rcu: Provide a get_completed_synchronize_rcu() function

It is currently up to the caller to handle stale return values from
get_state_synchronize_rcu(). If poll_state_synchronize_rcu() returned
true once, a grace period has elapsed, regardless of the fact that counter
wrap might cause some future poll_state_synchronize_rcu() invocation to
return false. For example, the caller might store a separate flag that
indicates whether some previous call to poll_state_synchronize_rcu()
determined that the relevant grace period had already ended.

This approach works, but it requires extra storage and is easy to get
wrong. This commit therefore introduces a get_completed_synchronize_rcu()
that returns a cookie that causes poll_state_synchronize_rcu() to always
return true. This already-completed cookie can be stored in place of the
cookie that previously caused poll_state_synchronize_rcu() to return true.
It can also be used to flag a given structure as not having been exposed
to readers, and thus not requiring a grace period to elapse.

This commit is in preparation for polled expedited grace periods.

Link: https://lore.kernel.org/all/20220121142454.1994916-1-bfoster@redhat.com/
Link: https://docs.google.com/document/d/1RNKWW9jQyfjxw2E8dsXVTdvZYh0HnYeSHDKog9jhdN8/edit?usp=sharing
Cc: Brian Foster <bfoster@redhat.com>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Ian Kent <raven@themaw.net>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

srcu: Initialize SRCU after timers

Once srcu_init() is called, the SRCU core will make use of delayed
workqueues, which rely on timers. However init_timers() is called
several steps after rcu_init(). This means that a call_srcu() after
rcu_init() but before init_timers() would find itself within a dangerously
uninitialized timer core.

This commit therefore creates a separate call to srcu_init() after
init_timer() completes, which ensures that we stay in early SRCU mode
until timers are safe(r).

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Uladzislau Rezki <urezki@gmail.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Neeraj Upadhyay <neeraju@codeaurora.org>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Joel Fernandes <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

rcu: Provide polling interfaces for Tiny RCU grace periods

There is a need for a non-blocking polling interface for RCU grace
periods, so this commit supplies start_poll_synchronize_rcu() and
poll_state_synchronize_rcu() for this purpose. Note that the existing
get_state_synchronize_rcu() may be used if future grace periods are
inevitable (perhaps due to a later call_rcu() invocation). The new
start_poll_synchronize_rcu() is to be used if future grace periods
might not otherwise happen. Finally, poll_state_synchronize_rcu()
provides a lockless check for a grace period having elapsed since
the corresponding call to either of the get_state_synchronize_rcu()
or start_poll_synchronize_rcu().

As with get_state_synchronize_rcu(), the return value from either
get_state_synchronize_rcu() or start_poll_synchronize_rcu() is passed in
to a later call to either poll_state_synchronize_rcu() or the existing
(might_sleep) cond_synchronize_rcu().

[ paulmck: Revert cond_synchronize_rcu() to might_sleep() per Frederic Weisbecker feedback. ]
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

rcu: Rename *_kfree_callback/*_kfree_rcu_offset/kfree_call_*

The following changes are introduced:

1. Rename rcu_invoke_kfree_callback() to rcu_invoke_kvfree_callback(),
as well as the associated trace events, so the rcu_kfree_callback(),
becomes rcu_kvfree_callback(). The reason is to be aligned with kvfree()
notation.

2. Rename __is_kfree_rcu_offset to __is_kvfree_rcu_offset. All RCU
paths use kvfree() now instead of kfree(), thus rename it.

3. Rename kfree_call_rcu() to the kvfree_call_rcu(). The reason is,
it is capable of freeing vmalloc() memory now. Do the same with
__kfree_rcu() macro, it becomes __kvfree_rcu(), the goal is the
same.

Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Co-developed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

rcu/tiny: support vmalloc in tiny-RCU

Replace kfree() with kvfree() in rcu_reclaim_tiny().
This makes it possible to release either SLAB or vmalloc
objects after a GP.

Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

rcu: Remove kfree_rcu() special casing and lazy-callback handling

This commit removes kfree_rcu() special-casing and the lazy-callback
handling from Tree RCU. It moves some of this special casing to Tiny RCU,
the removal of which will be the subject of later commits.

This results in a nice negative delta.

Suggested-by: Paul E. McKenney <paulmck@linux.ibm.com>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
[ paulmck: Add slab.h #include, thanks to kbuild test robot <lkp@intel.com>. ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

rcu: rcu_qs -- Use raise_softirq_irqoff to not save irqs twice

The rcu_qs is disabling IRQs by self so no need to do the same in raise_softirq
but instead we can save some cycles using raise_softirq_irqoff directly.

CC: Paul E. McKenney <paulmck@linux.ibm.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>

rcu/tiny: Convert to SPDX license identifier

Replace the license boiler plate with a SPDX license identifier.
While in the area, update an email address.

Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>

rcu: Rename rcu_check_callbacks() to rcu_sched_clock_irq()

The name rcu_check_callbacks() arguably made sense back in the early
2000s when RCU was quite a bit simpler than it is today, but it has
become quite misleading, especially with the advent of dyntick-idle
and NO_HZ_FULL. The rcu_check_callbacks() function is RCU's hook into
the scheduling-clock interrupt, and is now but one of many ways that
callbacks get promoted to invocable state.

This commit therefore changes the name to rcu_sched_clock_irq(),
which is the same number of characters and clearly indicates this
function's relation to the rest of the Linux kernel. In addition, for
the sake of consistency, rcu_flavor_check_callbacks() is also renamed
to rcu_flavor_sched_clock_irq().

While in the area, the header comments for both functions are reworked.

Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>

srcu: Make call_srcu() available during very early boot

Event tracing is moving to SRCU in order to take advantage of the fact
that SRCU may be safely used from idle and even offline CPUs. However,
event tracing can invoke call_srcu() very early in the boot process,
even before workqueue_init_early() is invoked (let alone rcu_init()).
Therefore, call_srcu()'s attempts to queue work fail miserably.

This commit therefore detects this situation, and refrains from attempting
to queue work before rcu_init() time, but does everything else that it
would have done, and in addition, adds the srcu_struct to a global list.
The rcu_init() function now invokes a new srcu_init() function, which
is empty if CONFIG_SRCU=n. Otherwise, srcu_init() queues work for
each srcu_struct on the list. This all happens early enough in boot
that there is but a single CPU with interrupts disabled, which allows
synchronization to be dispensed with.

Of course, the queued work won't actually be invoked until after
workqueue_init() is invoked, which happens shortly after the scheduler
is up and running. This means that although call_srcu() may be invoked
any time after per-CPU variables have been set up, there is still a very
narrow window when synchronize_srcu() won't work, and this window
extends from the time that the scheduler starts until the time that
workqueue_init() returns. This can be fixed in a manner similar to
the fix for synchronize_rcu_expedited() and friends, but until someone
actually needs to use synchronize_srcu() during this window, this fix
is added churn for no benefit.

Finally, note that Tree SRCU's new srcu_init() function invokes
queue_work() rather than the queue_delayed_work() function that is
invoked post-boot. The reason is that queue_delayed_work() will (as you
would expect) post a timer, and timers have not yet been initialized.
So use of queue_work() avoids the complaints about use of uninitialized
spinlocks that would otherwise result. Besides, some delay is already
provide by the aforementioned fact that the queued work won't actually
be invoked until after the scheduler is up and running.

Requested-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

rcu: Motivate Tiny RCU forward progress

If a long-running CPU-bound in-kernel task invokes call_rcu(), the
callback won't be invoked until the next context switch. If there are
no other runnable tasks (which is not an uncommon situation on deep
embedded systems), the callback might never be invoked.

This commit therefore causes rcu_check_callbacks() to ask the scheduler
for a context switch if there are callbacks posted that are still waiting
for a grace period.

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

rcu: Clean up flavor-related definitions and comments in tiny.c

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

rcu: Express Tiny RCU updates in terms of RCU rather than RCU-sched

This commit renames Tiny RCU functions so that the lowest level of
functionality is RCU (e.g., synchronize_rcu()) rather than RCU-sched
(e.g., synchronize_sched()). This provides greater naming compatibility
with Tree RCU, which will in turn permit more LoC removal once
the RCU-sched and RCU-bh update-side API is removed.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
[ paulmck: Fix Tiny call_rcu()'s EXPORT_SYMBOL() in response to a bug
report from kbuild test robot. ]

rcu: Define RCU-bh update API in terms of RCU

Now that the main RCU API knows about softirq disabling and softirq's
quiescent states, the RCU-bh update code can be dispensed with.
This commit therefore removes the RCU-bh update-side implementation and
defines RCU-bh's update-side API in terms of that of either RCU-preempt or
RCU-sched, depending on the setting of the CONFIG_PREEMPT Kconfig option.

In kernels built with CONFIG_RCU_NOCB_CPU=y this has the knock-on effect
of reducing by one the number of rcuo kthreads per CPU.

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

rcu: Improve rcu_note_voluntary_context_switch() reporting

We expect a quiescent state of TASKS_RCU when cond_resched_tasks_rcu_qs()
is called, no matter whether it actually be scheduled or not. However,
it currently doesn't report the quiescent state when the task enters
into __schedule() as it's called with preempt = true. So make it report
the quiescent state unconditionally when cond_resched_tasks_rcu_qs() is
called.

And in TINY_RCU, even though the quiescent state of rcu_bh also should
be reported when the tick interrupt comes from user, it doesn't. So make
it reported.

Lastly in TREE_RCU, rcu_note_voluntary_context_switch() should be
reported when the tick interrupt comes from not only user but also idle,
as an extended quiescent state.

Signed-off-by: Byungchul Park <byungchul.park@lge.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
[ paulmck: Simplify rcutiny portion given no RCU-tasks for !PREEMPT. ]

srcu: Move rcu_scheduler_starting() from Tiny RCU to Tiny SRCU

Other than lockdep support, Tiny RCU has no need for the
scheduler status. However, Tiny SRCU will need this to control
boot-time behavior independent of lockdep. Therefore, this commit
moves rcu_scheduler_starting() from kernel/rcu/tiny_plugin.h to
kernel/rcu/srcutiny.c. This in turn allows the complete removal of
kernel/rcu/tiny_plugin.h.

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

rcu: Remove RCU CPU stall warnings from Tiny RCU

Tiny RCU's job is to be tiny, so this commit removes its RCU CPU
stall warning code. After this, there is no longer any need for
rcu_sched_ctrlblk and rcu_bh_ctrlblk to be in tiny_plugin.h, so this
commit also moves them to tiny.c.

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

rcu: Remove event tracing from Tiny RCU

This commit saves a few lines by getting rid of Tiny RCU's event tracing.

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

rcu: Eliminate the unused __rcu_is_watching() function

The __rcu_is_watching() function is currently not used, aside from
to implement the rcu_is_watching() function. This commit therefore
eliminates __rcu_is_watching(), which has the beneficial side-effect
of shrinking include/linux/rcupdate.h a bit.

Reported-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

rcu: Semicolon inside RCU_TRACE() for Tiny RCU

The current use of "RCU_TRACE(statement);" can cause odd bugs, especially
where "statement" is a local-variable declaration, as it can leave a
misplaced ";" in the source code. This commit therefore converts these
to "RCU_TRACE(statement;)", which avoids the misplaced ";".

Reported-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

rcu: Separate the RCU synchronization types and APIs into <linux/rcupdate_wait.h>

So rcupdate.h is a pretty complex header, in particular it includes
<linux/completion.h> which includes <linux/wait.h> - creating a
dependency that includes <linux/wait.h> in <linux/sched.h>,
which prevents the isolation of <linux/sched.h> from the derived
<linux/wait.h> header.

Solve part of the problem by decoupling rcupdate.h from completions:
this can be done by separating out the rcu_synchronize types and APIs,
and updating their usage sites.

Since this is a mostly RCU-internal types this will not just simplify
<linux/sched.h>'s dependencies, but will make all the hundreds of
.c files that include rcupdate.h but not completions or wait.h build
faster.

( For rcutiny this means that two dependent APIs have to be uninlined,
but that shouldn't be much of a problem as they are rare variants. )

Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

rcu: Remove unneeded rcu_process_callbacks() declarations

The declarations of __rcu_process_callbacks() and rcu_process_callbacks()
are not needed, as the definition of both of these functions appear before
any uses. This commit therefore removes both declarations.

Reported-by: "Ahmed, Iftekhar" <ahmedi@oregonstate.edu>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

rcu: Remove cond_resched() from Tiny synchronize_sched()

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: 8b355e3bc140 ("rcu: Drive expedited grace periods from workqueue")
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: <stable@vger.kernel.org> # 4.9.0-

latent_entropy: Mark functions with __latent_entropy

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: Emese Revfy <re.emese@gmail.com>
[kees: expanded commit message]
Signed-off-by: Kees Cook <keescook@chromium.org>

rcu: Use rcu_callback_t in call_rcu*() and friends

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: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

rcu: Rename rcu_lockdep_assert() to RCU_LOCKDEP_WARN()

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

rcu: Correctly handle non-empty Tiny RCU callback list with none ready

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
915e8a4fe45e (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: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: <stable@vger.kernel.org>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

rcu: Further shrink Tiny RCU by making empty functions static inlines

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: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

tracing: Rename ftrace_event.h to trace_events.h

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: Steven Rostedt <rostedt@goodmis.org>

rcu: Remove fastpath from __rcu_process_callbacks()

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

rcu: Remove unnecessary condition check in rcu_qsctr_help()

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

rcu: Initialize tiny RCU stall-warning timeouts at boot

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: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

tiny_rcu: Directly force QS when call_rcu_[bh|sched]() on idle_task

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: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

rcu: Remove redundant rcu_is_cpu_rrupt_from_idle() from tiny RCU

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

rcutorture: Add early boot self tests

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: Pranith Kumar <bobby.prani@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

rcu: Remove "cpu" argument to rcu_check_callbacks()

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: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>

rcu: Per-CPU operation cleanups to rcu_*_qs() functions

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: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

rcu: Add call_rcu_tasks()

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: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

rcu: Make TINY_RCU tinier by putting error checks under #ifdef

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: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

rcu: Update tiny.c references to tree.c

This commit updates the references to rcutree.c which is now rcu/tree.c

Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

rcu: Ensure kernel/rcu/rcu.h can be sourced/used stand-alone

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: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

rcu: Stop tracking FSF's postal address

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: Greg KH <gregkh@linuxfoundation.org>
Reported-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

rcu: Move RCU-related source code to kernel/rcu directory

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