[PATCH tip/core/rcu 05/27] rcu: Improve RCU-tasks naming and comments

Paul E. McKenney
2018-06-26T00:37:38Z

The naming and comments associated with some RCU-tasks code make
the faulty assumption that context switches due to cond_resched()
are voluntary. As several people pointed out, this is not the case.
This commit therefore updates function names and comments to better
reflect current reality.

Reported-by: Byungchul Park <byungchul.park@lge.com>
Reported-by: Joel Fernandes <joel@joelfernandes.org>
Reported-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
include/linux/rcupdate.h | 12 ++++++------
include/linux/rcutiny.h | 2 +-
kernel/rcu/tree.c | 2 +-
kernel/rcu/update.c | 27 ++++++++++++++-------------
4 files changed, 22 insertions(+), 21 deletions(-)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index dacc90358b33..75e5b393cf44 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -158,11 +158,11 @@ static inline void rcu_init_nohz(void) { }
} while (0)

/*
- * Note a voluntary context switch for RCU-tasks benefit. This is a
- * macro rather than an inline function to avoid #include hell.
+ * Note a quasi-voluntary context switch for RCU-tasks's benefit.
+ * This is a macro rather than an inline function to avoid #include hell.
*/
#ifdef CONFIG_TASKS_RCU
-#define rcu_note_voluntary_context_switch_lite(t) \
+#define rcu_tasks_qs(t) \
do { \
if (READ_ONCE((t)->rcu_tasks_holdout)) \
WRITE_ONCE((t)->rcu_tasks_holdout, false); \
@@ -170,14 +170,14 @@ static inline void rcu_init_nohz(void) { }
#define rcu_note_voluntary_context_switch(t) \
do { \
rcu_all_qs(); \
- rcu_note_voluntary_context_switch_lite(t); \
+ rcu_tasks_qs(t); \
} while (0)
void call_rcu_tasks(struct rcu_head *head, rcu_callback_t func);
void synchronize_rcu_tasks(void);
void exit_tasks_rcu_start(void);
void exit_tasks_rcu_finish(void);
#else /* #ifdef CONFIG_TASKS_RCU */
-#define rcu_note_voluntary_context_switch_lite(t) do { } while (0)
+#define rcu_tasks_qs(t) do { } while (0)
#define rcu_note_voluntary_context_switch(t) rcu_all_qs()
#define call_rcu_tasks call_rcu_sched
#define synchronize_rcu_tasks synchronize_sched
@@ -194,7 +194,7 @@ static inline void exit_tasks_rcu_finish(void) { }
*/
#define cond_resched_tasks_rcu_qs() \
do { \
- rcu_note_voluntary_context_switch_lite(current); \
+ rcu_tasks_qs(current); \
cond_resched(); \
} while (0)

diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
index 7b3c82e8a625..8d9a0ea8f0b5 100644
--- a/include/linux/rcutiny.h
+++ b/include/linux/rcutiny.h
@@ -93,7 +93,7 @@ static inline void kfree_call_rcu(struct rcu_head *head,
#define rcu_note_context_switch(preempt) \
do { \
rcu_sched_qs(); \
- rcu_note_voluntary_context_switch_lite(current); \
+ rcu_tasks_qs(current); \
} while (0)

static inline int rcu_needs_cpu(u64 basemono, u64 *nextevt)
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index fcb0355a8084..1ea971244512 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -457,7 +457,7 @@ void rcu_note_context_switch(bool preempt)
rcu_momentary_dyntick_idle();
this_cpu_inc(rcu_dynticks.rcu_qs_ctr);
if (!preempt)
- rcu_note_voluntary_context_switch_lite(current);
+ rcu_tasks_qs(current);
out:
trace_rcu_utilization(TPS("End context switch"));
barrier(); /* Avoid RCU read-side critical sections leaking up. */
diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index 4c230a60ece4..5783bdf86e5a 100644
--- a/kernel/rcu/update.c
+++ b/kernel/rcu/update.c
@@ -507,14 +507,15 @@ early_initcall(check_cpu_stall_init);
#ifdef CONFIG_TASKS_RCU

/*
- * Simple variant of RCU whose quiescent states are voluntary context switch,
- * user-space execution, and idle. As such, grace periods can take one good
- * long time. There are no read-side primitives similar to rcu_read_lock()
- * and rcu_read_unlock() because this implementation is intended to get
- * the system into a safe state for some of the manipulations involved in
- * tracing and the like. Finally, this implementation does not support
- * high call_rcu_tasks() rates from multiple CPUs. If this is required,
- * per-CPU callback lists will be needed.
+ * Simple variant of RCU whose quiescent states are voluntary context
+ * switch, cond_resched_rcu_qs(), user-space execution, and idle.
+ * As such, grace periods can take one good long time. There are no
+ * read-side primitives similar to rcu_read_lock() and rcu_read_unlock()
+ * because this implementation is intended to get the system into a safe
+ * state for some of the manipulations involved in tracing and the like.
+ * Finally, this implementation does not support high call_rcu_tasks()
+ * rates from multiple CPUs. If this is required, per-CPU callback lists
+ * will be needed.
*/

/* Global list of callbacks and associated lock. */
@@ -542,11 +543,11 @@ static struct task_struct *rcu_tasks_kthread_ptr;
* period elapses, in other words after all currently executing RCU
* read-side critical sections have completed. call_rcu_tasks() assumes
* that the read-side critical sections end at a voluntary context
- * switch (not a preemption!), entry into idle, or transition to usermode
- * execution. As such, there are no read-side primitives analogous to
- * rcu_read_lock() and rcu_read_unlock() because this primitive is intended
- * to determine that all tasks have passed through a safe state, not so
- * much for data-strcuture synchronization.
+ * switch (not a preemption!), cond_resched_rcu_qs(), entry into idle,
+ * or transition to usermode execution. As such, there are no read-side
+ * primitives analogous to rcu_read_lock() and rcu_read_unlock() because
+ * this primitive is intended to determine that all tasks have passed
+ * through a safe state, not so much for data-strcuture synchronization.
*
* See the description of call_rcu() for more detailed information on
* memory ordering guarantees.
--
2.17.1