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Commit 6f56f714 authored by Paul E. McKenney's avatar Paul E. McKenney
Browse files

rcu: Improve RCU-tasks naming and comments 



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: default avatarByungchul Park <byungchul.park@lge.com>
Reported-by: default avatarJoel Fernandes <joel@joelfernandes.org>
Reported-by: default avatarSteven Rostedt <rostedt@goodmis.org>
Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
parent a7538352

include/linux/rcupdate.h

+6 −6
Original line number Diff line number Diff line
@@ -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)

include/linux/rcutiny.h

+1 −1
Original line number Diff line number Diff line
@@ -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)

kernel/rcu/tree.c

+1 −1
Original line number Diff line number Diff line
@@ -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. */

kernel/rcu/update.c

+14 −13
Original line number Diff line number Diff line
@@ -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.