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Merge tag 'rcu.2024.07.12a' of git://git.kernel.org/pub/scm/linux/ker…
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Pull RCU updates from Paul McKenney:

 - Update Tasks RCU and Tasks Rude RCU description in Requirements.rst
   and clarify rcu_assign_pointer() and rcu_dereference() ordering
   properties

 - Add lockdep assertions for RCU readers, limit inline wakeups for
   callback-bypass synchronize_rcu(), add an
   rcutree.nohz_full_patience_delay to reduce nohz_full OS jitter, add
   Uladzislau Rezki as RCU maintainer, and fix a subtle
   callback-migration memory-ordering issue

 - Remove a number of redundant memory barriers

 - Remove unnecessary bypass-list lock-contention mitigation, use
   parking API instead of open-coded ad-hoc equivalent, and upgrade
   obsolete comments

 - Revert avoidance of a deadlock that can no longer occur and properly
   synchronize Tasks Trace RCU checking of runqueues

 - Add tests for handling of double-call_rcu() bug, add missing
   MODULE_DESCRIPTION, and add a script that histograms the number of
   calls to RCU updaters

 - Fill out SRCU polled-grace-period API

* tag 'rcu.2024.07.12a' of git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu: (29 commits)
  rcu: Fix rcu_barrier() VS post CPUHP_TEARDOWN_CPU invocation
  rcu: Eliminate lockless accesses to rcu_sync->gp_count
  MAINTAINERS: Add Uladzislau Rezki as RCU maintainer
  rcu: Add rcutree.nohz_full_patience_delay to reduce nohz_full OS jitter
  rcu/exp: Remove redundant full memory barrier at the end of GP
  rcu: Remove full memory barrier on RCU stall printout
  rcu: Remove full memory barrier on boot time eqs sanity check
  rcu/exp: Remove superfluous full memory barrier upon first EQS snapshot
  rcu: Remove superfluous full memory barrier upon first EQS snapshot
  rcu: Remove full ordering on second EQS snapshot
  srcu: Fill out polled grace-period APIs
  srcu: Update cleanup_srcu_struct() comment
  srcu: Add NUM_ACTIVE_SRCU_POLL_OLDSTATE
  srcu: Disable interrupts directly in srcu_gp_end()
  rcu: Disable interrupts directly in rcu_gp_init()
  rcu/tree: Reduce wake up for synchronize_rcu() common case
  rcu/tasks: Fix stale task snaphot for Tasks Trace
  tools/rcu: Add rcu-updaters.sh script
  rcutorture: Add missing MODULE_DESCRIPTION() macros
  rcutorture: Fix rcu_torture_fwd_cb_cr() data race
  ...
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torvalds committed Jul 15, 2024
2 parents 253e1e9 + 02219ca commit 9855e87
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6 changes: 3 additions & 3 deletions Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.rst
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Expand Up @@ -149,9 +149,9 @@ This case is handled by calls to the strongly ordered
``atomic_add_return()`` read-modify-write atomic operation that
is invoked within ``rcu_dynticks_eqs_enter()`` at idle-entry
time and within ``rcu_dynticks_eqs_exit()`` at idle-exit time.
The grace-period kthread invokes ``rcu_dynticks_snap()`` and
``rcu_dynticks_in_eqs_since()`` (both of which invoke
an ``atomic_add_return()`` of zero) to detect idle CPUs.
The grace-period kthread invokes first ``ct_dynticks_cpu_acquire()``
(preceded by a full memory barrier) and ``rcu_dynticks_in_eqs_since()``
(both of which rely on acquire semantics) to detect idle CPUs.

+-----------------------------------------------------------------------+
| **Quick Quiz**: |
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16 changes: 16 additions & 0 deletions Documentation/RCU/Design/Requirements/Requirements.rst
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Expand Up @@ -2357,6 +2357,7 @@ section.
#. `Sched Flavor (Historical)`_
#. `Sleepable RCU`_
#. `Tasks RCU`_
#. `Tasks Trace RCU`_

Bottom-Half Flavor (Historical)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Expand Down Expand Up @@ -2610,6 +2611,16 @@ critical sections that are delimited by voluntary context switches, that
is, calls to schedule(), cond_resched(), and
synchronize_rcu_tasks(). In addition, transitions to and from
userspace execution also delimit tasks-RCU read-side critical sections.
Idle tasks are ignored by Tasks RCU, and Tasks Rude RCU may be used to
interact with them.

Note well that involuntary context switches are *not* Tasks-RCU quiescent
states. After all, in preemptible kernels, a task executing code in a
trampoline might be preempted. In this case, the Tasks-RCU grace period
clearly cannot end until that task resumes and its execution leaves that
trampoline. This means, among other things, that cond_resched() does
not provide a Tasks RCU quiescent state. (Instead, use rcu_softirq_qs()
from softirq or rcu_tasks_classic_qs() otherwise.)

The tasks-RCU API is quite compact, consisting only of
call_rcu_tasks(), synchronize_rcu_tasks(), and
Expand All @@ -2632,6 +2643,11 @@ moniker. And this operation is considered to be quite rude by real-time
workloads that don't want their ``nohz_full`` CPUs receiving IPIs and
by battery-powered systems that don't want their idle CPUs to be awakened.

Once kernel entry/exit and deep-idle functions have been properly tagged
``noinstr``, Tasks RCU can start paying attention to idle tasks (except
those that are idle from RCU's perspective) and then Tasks Rude RCU can
be removed from the kernel.

The tasks-rude-RCU API is also reader-marking-free and thus quite compact,
consisting of call_rcu_tasks_rude(), synchronize_rcu_tasks_rude(),
and rcu_barrier_tasks_rude().
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30 changes: 19 additions & 11 deletions Documentation/RCU/whatisRCU.rst
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Expand Up @@ -250,21 +250,25 @@ rcu_assign_pointer()
^^^^^^^^^^^^^^^^^^^^
void rcu_assign_pointer(p, typeof(p) v);

Yes, rcu_assign_pointer() **is** implemented as a macro, though it
would be cool to be able to declare a function in this manner.
(Compiler experts will no doubt disagree.)
Yes, rcu_assign_pointer() **is** implemented as a macro, though
it would be cool to be able to declare a function in this manner.
(And there has been some discussion of adding overloaded functions
to the C language, so who knows?)

The updater uses this spatial macro to assign a new value to an
RCU-protected pointer, in order to safely communicate the change
in value from the updater to the reader. This is a spatial (as
opposed to temporal) macro. It does not evaluate to an rvalue,
but it does execute any memory-barrier instructions required
for a given CPU architecture. Its ordering properties are that
of a store-release operation.

Perhaps just as important, it serves to document (1) which
pointers are protected by RCU and (2) the point at which a
given structure becomes accessible to other CPUs. That said,
but it does provide any compiler directives and memory-barrier
instructions required for a given compile or CPU architecture.
Its ordering properties are that of a store-release operation,
that is, any prior loads and stores required to initialize the
structure are ordered before the store that publishes the pointer
to that structure.

Perhaps just as important, rcu_assign_pointer() serves to document
(1) which pointers are protected by RCU and (2) the point at which
a given structure becomes accessible to other CPUs. That said,
rcu_assign_pointer() is most frequently used indirectly, via
the _rcu list-manipulation primitives such as list_add_rcu().

Expand All @@ -283,7 +287,11 @@ rcu_dereference()
executes any needed memory-barrier instructions for a given
CPU architecture. Currently, only Alpha needs memory barriers
within rcu_dereference() -- on other CPUs, it compiles to a
volatile load.
volatile load. However, no mainstream C compilers respect
address dependencies, so rcu_dereference() uses volatile casts,
which, in combination with the coding guidelines listed in
rcu_dereference.rst, prevent current compilers from breaking
these dependencies.

Common coding practice uses rcu_dereference() to copy an
RCU-protected pointer to a local variable, then dereferences
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8 changes: 8 additions & 0 deletions Documentation/admin-guide/kernel-parameters.txt
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Expand Up @@ -5015,6 +5015,14 @@
the ->nocb_bypass queue. The definition of "too
many" is supplied by this kernel boot parameter.

rcutree.nohz_full_patience_delay= [KNL]
On callback-offloaded (rcu_nocbs) CPUs, avoid
disturbing RCU unless the grace period has
reached the specified age in milliseconds.
Defaults to zero. Large values will be capped
at five seconds. All values will be rounded down
to the nearest value representable by jiffies.

rcutree.qhimark= [KNL]
Set threshold of queued RCU callbacks beyond which
batch limiting is disabled.
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1 change: 1 addition & 0 deletions MAINTAINERS
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Expand Up @@ -18863,6 +18863,7 @@ M: Neeraj Upadhyay <[email protected]> (kernel/rcu/tasks.h)
M: Joel Fernandes <[email protected]>
M: Josh Triplett <[email protected]>
M: Boqun Feng <[email protected]>
M: Uladzislau Rezki <[email protected]>
R: Steven Rostedt <[email protected]>
R: Mathieu Desnoyers <[email protected]>
R: Lai Jiangshan <[email protected]>
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88 changes: 38 additions & 50 deletions include/linux/rcu_segcblist.h
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Original file line number Diff line number Diff line change
Expand Up @@ -80,36 +80,35 @@ struct rcu_cblist {
* | SEGCBLIST_RCU_CORE | SEGCBLIST_LOCKING | SEGCBLIST_OFFLOADED |
* | |
* | Callbacks processed by rcu_core() from softirqs or local |
* | rcuc kthread, while holding nocb_lock. Waking up CB and GP kthreads, |
* | allowing nocb_timer to be armed. |
* | rcuc kthread, while holding nocb_lock. Waking up CB and GP kthreads. |
* ----------------------------------------------------------------------------
* |
* v
* -----------------------------------
* | |
* v v
* --------------------------------------- ----------------------------------|
* | SEGCBLIST_RCU_CORE | | | SEGCBLIST_RCU_CORE | |
* | SEGCBLIST_LOCKING | | | SEGCBLIST_LOCKING | |
* | SEGCBLIST_OFFLOADED | | | SEGCBLIST_OFFLOADED | |
* | SEGCBLIST_KTHREAD_CB | | SEGCBLIST_KTHREAD_GP |
* | | | |
* | | | |
* | CB kthread woke up and | | GP kthread woke up and |
* | acknowledged SEGCBLIST_OFFLOADED. | | acknowledged SEGCBLIST_OFFLOADED|
* | Processes callbacks concurrently | | |
* | with rcu_core(), holding | | |
* | nocb_lock. | | |
* --------------------------------------- -----------------------------------
* | |
* -----------------------------------
* ----------------------------------------------------------------------------
* | SEGCBLIST_RCU_CORE | SEGCBLIST_LOCKING | SEGCBLIST_OFFLOADED |
* | + unparked CB kthread |
* | |
* | CB kthread got unparked and processes callbacks concurrently with |
* | rcu_core(), holding nocb_lock. |
* ---------------------------------------------------------------------------
* |
* v
* ---------------------------------------------------------------------------|
* | SEGCBLIST_RCU_CORE | |
* | SEGCBLIST_LOCKING | |
* | SEGCBLIST_OFFLOADED | |
* | SEGCBLIST_KTHREAD_GP |
* | + unparked CB kthread |
* | |
* | GP kthread woke up and acknowledged nocb_lock. |
* ---------------------------------------- -----------------------------------
* |
* v
* |--------------------------------------------------------------------------|
* | SEGCBLIST_LOCKING | |
* | SEGCBLIST_OFFLOADED | |
* | SEGCBLIST_LOCKING | |
* | SEGCBLIST_OFFLOADED | |
* | SEGCBLIST_KTHREAD_GP | |
* | SEGCBLIST_KTHREAD_CB |
* | + unparked CB kthread |
* | |
* | Kthreads handle callbacks holding nocb_lock, local rcu_core() stops |
* | handling callbacks. Enable bypass queueing. |
Expand All @@ -125,8 +124,8 @@ struct rcu_cblist {
* |--------------------------------------------------------------------------|
* | SEGCBLIST_LOCKING | |
* | SEGCBLIST_OFFLOADED | |
* | SEGCBLIST_KTHREAD_CB | |
* | SEGCBLIST_KTHREAD_GP |
* | + unparked CB kthread |
* | |
* | CB/GP kthreads handle callbacks holding nocb_lock, local rcu_core() |
* | ignores callbacks. Bypass enqueue is enabled. |
Expand All @@ -137,11 +136,11 @@ struct rcu_cblist {
* | SEGCBLIST_RCU_CORE | |
* | SEGCBLIST_LOCKING | |
* | SEGCBLIST_OFFLOADED | |
* | SEGCBLIST_KTHREAD_CB | |
* | SEGCBLIST_KTHREAD_GP |
* | + unparked CB kthread |
* | |
* | CB/GP kthreads handle callbacks holding nocb_lock, local rcu_core() |
* | handles callbacks concurrently. Bypass enqueue is enabled. |
* | handles callbacks concurrently. Bypass enqueue is disabled. |
* | Invoke RCU core so we make sure not to preempt it in the middle with |
* | leaving some urgent work unattended within a jiffy. |
* ----------------------------------------------------------------------------
Expand All @@ -150,42 +149,31 @@ struct rcu_cblist {
* |--------------------------------------------------------------------------|
* | SEGCBLIST_RCU_CORE | |
* | SEGCBLIST_LOCKING | |
* | SEGCBLIST_KTHREAD_CB | |
* | SEGCBLIST_KTHREAD_GP |
* | + unparked CB kthread |
* | |
* | CB/GP kthreads and local rcu_core() handle callbacks concurrently |
* | holding nocb_lock. Wake up CB and GP kthreads if necessary. Disable |
* | bypass enqueue. |
* | holding nocb_lock. Wake up GP kthread if necessary. |
* ----------------------------------------------------------------------------
* |
* v
* -----------------------------------
* | |
* v v
* ---------------------------------------------------------------------------|
* | | |
* | SEGCBLIST_RCU_CORE | | SEGCBLIST_RCU_CORE | |
* | SEGCBLIST_LOCKING | | SEGCBLIST_LOCKING | |
* | SEGCBLIST_KTHREAD_CB | SEGCBLIST_KTHREAD_GP |
* | | |
* | GP kthread woke up and | CB kthread woke up and |
* | acknowledged the fact that | acknowledged the fact that |
* | SEGCBLIST_OFFLOADED got cleared. | SEGCBLIST_OFFLOADED got cleared. |
* | | The CB kthread goes to sleep |
* | The callbacks from the target CPU | until it ever gets re-offloaded. |
* | will be ignored from the GP kthread | |
* | loop. | |
* |--------------------------------------------------------------------------|
* | SEGCBLIST_RCU_CORE | |
* | SEGCBLIST_LOCKING | |
* | + unparked CB kthread |
* | |
* | GP kthread woke up and acknowledged the fact that SEGCBLIST_OFFLOADED |
* | got cleared. The callbacks from the target CPU will be ignored from the|
* | GP kthread loop. |
* ----------------------------------------------------------------------------
* | |
* -----------------------------------
* |
* v
* ----------------------------------------------------------------------------
* | SEGCBLIST_RCU_CORE | SEGCBLIST_LOCKING |
* | + parked CB kthread |
* | |
* | Callbacks processed by rcu_core() from softirqs or local |
* | rcuc kthread, while holding nocb_lock. Forbid nocb_timer to be armed. |
* | Flush pending nocb_timer. Flush nocb bypass callbacks. |
* | CB kthread is parked. Callbacks processed by rcu_core() from softirqs or |
* | local rcuc kthread, while holding nocb_lock. |
* ----------------------------------------------------------------------------
* |
* v
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62 changes: 60 additions & 2 deletions include/linux/rcupdate.h
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Original file line number Diff line number Diff line change
Expand Up @@ -209,7 +209,6 @@ void synchronize_rcu_tasks_rude(void);

#define rcu_note_voluntary_context_switch(t) rcu_tasks_qs(t, false)
void exit_tasks_rcu_start(void);
void exit_tasks_rcu_stop(void);
void exit_tasks_rcu_finish(void);
#else /* #ifdef CONFIG_TASKS_RCU_GENERIC */
#define rcu_tasks_classic_qs(t, preempt) do { } while (0)
Expand All @@ -218,7 +217,6 @@ void exit_tasks_rcu_finish(void);
#define call_rcu_tasks call_rcu
#define synchronize_rcu_tasks synchronize_rcu
static inline void exit_tasks_rcu_start(void) { }
static inline void exit_tasks_rcu_stop(void) { }
static inline void exit_tasks_rcu_finish(void) { }
#endif /* #else #ifdef CONFIG_TASKS_RCU_GENERIC */

Expand Down Expand Up @@ -421,11 +419,71 @@ static inline void rcu_preempt_sleep_check(void) { }
"Illegal context switch in RCU-sched read-side critical section"); \
} while (0)

// See RCU_LOCKDEP_WARN() for an explanation of the double call to
// debug_lockdep_rcu_enabled().
static inline bool lockdep_assert_rcu_helper(bool c)
{
return debug_lockdep_rcu_enabled() &&
(c || !rcu_is_watching() || !rcu_lockdep_current_cpu_online()) &&
debug_lockdep_rcu_enabled();
}

/**
* lockdep_assert_in_rcu_read_lock - WARN if not protected by rcu_read_lock()
*
* Splats if lockdep is enabled and there is no rcu_read_lock() in effect.
*/
#define lockdep_assert_in_rcu_read_lock() \
WARN_ON_ONCE(lockdep_assert_rcu_helper(!lock_is_held(&rcu_lock_map)))

/**
* lockdep_assert_in_rcu_read_lock_bh - WARN if not protected by rcu_read_lock_bh()
*
* Splats if lockdep is enabled and there is no rcu_read_lock_bh() in effect.
* Note that local_bh_disable() and friends do not suffice here, instead an
* actual rcu_read_lock_bh() is required.
*/
#define lockdep_assert_in_rcu_read_lock_bh() \
WARN_ON_ONCE(lockdep_assert_rcu_helper(!lock_is_held(&rcu_bh_lock_map)))

/**
* lockdep_assert_in_rcu_read_lock_sched - WARN if not protected by rcu_read_lock_sched()
*
* Splats if lockdep is enabled and there is no rcu_read_lock_sched()
* in effect. Note that preempt_disable() and friends do not suffice here,
* instead an actual rcu_read_lock_sched() is required.
*/
#define lockdep_assert_in_rcu_read_lock_sched() \
WARN_ON_ONCE(lockdep_assert_rcu_helper(!lock_is_held(&rcu_sched_lock_map)))

/**
* lockdep_assert_in_rcu_reader - WARN if not within some type of RCU reader
*
* Splats if lockdep is enabled and there is no RCU reader of any
* type in effect. Note that regions of code protected by things like
* preempt_disable, local_bh_disable(), and local_irq_disable() all qualify
* as RCU readers.
*
* Note that this will never trigger in PREEMPT_NONE or PREEMPT_VOLUNTARY
* kernels that are not also built with PREEMPT_COUNT. But if you have
* lockdep enabled, you might as well also enable PREEMPT_COUNT.
*/
#define lockdep_assert_in_rcu_reader() \
WARN_ON_ONCE(lockdep_assert_rcu_helper(!lock_is_held(&rcu_lock_map) && \
!lock_is_held(&rcu_bh_lock_map) && \
!lock_is_held(&rcu_sched_lock_map) && \
preemptible()))

#else /* #ifdef CONFIG_PROVE_RCU */

#define RCU_LOCKDEP_WARN(c, s) do { } while (0 && (c))
#define rcu_sleep_check() do { } while (0)

#define lockdep_assert_in_rcu_read_lock() do { } while (0)
#define lockdep_assert_in_rcu_read_lock_bh() do { } while (0)
#define lockdep_assert_in_rcu_read_lock_sched() do { } while (0)
#define lockdep_assert_in_rcu_reader() do { } while (0)

#endif /* #else #ifdef CONFIG_PROVE_RCU */

/*
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