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rcu: Improve RCU-tasks naming and comments

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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>
This commit is contained in:
Paul E. McKenney 2018-05-14 13:52:27 -07:00
parent a7538352da
commit 6f56f714db
4 changed files with 22 additions and 21 deletions
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12
include/linux/rcupdate.h
View File

@ -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)

2
include/linux/rcutiny.h
View File

@ -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)

2
kernel/rcu/tree.c
View File

@ -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. */

27
kernel/rcu/update.c
View File

@ -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.