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rcu: Consolidate PREEMPT and !PREEMPT synchronize_rcu_expedited() 

The CONFIG_PREEMPT=n and CONFIG_PREEMPT=y implementations of
synchronize_rcu_expedited() are quite similar, and with small
modifications to rcu_blocking_is_gp() can be made identical.  This commit
therefore makes this change in order to save a few lines of code and to
reduce the amount of duplicate code.

Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
hifive-unleashed-5.1
Paul E. McKenney 2018-11-29 10:01:52 -08:00
parent 142d106d5e
commit 3cd4ca47aa
1 changed files with 49 additions and 56 deletions
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105
kernel/rcu/tree_exp.h
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@ -643,6 +643,33 @@ static void _synchronize_rcu_expedited(void)
mutex_unlock(&rcu_state.exp_mutex);
}
/*
* During early boot, any blocking grace-period wait automatically
* implies a grace period. Later on, this is never the case for PREEMPT.
*
* Howevr, because a context switch is a grace period for !PREEMPT, any
* blocking grace-period wait automatically implies a grace period if
* there is only one CPU online at any point time during execution of
* either synchronize_rcu() or synchronize_rcu_expedited(). It is OK to
* occasionally incorrectly indicate that there are multiple CPUs online
* when there was in fact only one the whole time, as this just adds some
* overhead: RCU still operates correctly.
*/
static int rcu_blocking_is_gp(void)
{
int ret;
if (rcu_scheduler_active == RCU_SCHEDULER_INACTIVE)
return true;
if (IS_ENABLED(CONFIG_PREEMPT))
return false;
might_sleep(); /* Check for RCU read-side critical section. */
preempt_disable();
ret = num_online_cpus() <= 1;
preempt_enable();
return ret;
}
#ifdef CONFIG_PREEMPT_RCU
/*
@ -729,39 +756,6 @@ static void sync_sched_exp_online_cleanup(int cpu)
{
}
/**
* synchronize_rcu_expedited - Brute-force RCU grace period
*
* Wait for an RCU-preempt grace period, but expedite it. The basic
* idea is to IPI all non-idle non-nohz online CPUs. The IPI handler
* checks whether the CPU is in an RCU-preempt critical section, and
* if so, it sets a flag that causes the outermost rcu_read_unlock()
* to report the quiescent state. On the other hand, if the CPU is
* not in an RCU read-side critical section, the IPI handler reports
* the quiescent state immediately.
*
* Although this is a greate improvement over previous expedited
* implementations, it is still unfriendly to real-time workloads, so is
* thus not recommended for any sort of common-case code. In fact, if
* you are using synchronize_rcu_expedited() in a loop, please restructure
* your code to batch your updates, and then Use a single synchronize_rcu()
* instead.
*
* This has the same semantics as (but is more brutal than) synchronize_rcu().
*/
void synchronize_rcu_expedited(void)
{
RCU_LOCKDEP_WARN(lock_is_held(&rcu_bh_lock_map) ||
lock_is_held(&rcu_lock_map) ||
lock_is_held(&rcu_sched_lock_map),
"Illegal synchronize_rcu_expedited() in RCU read-side critical section");
if (rcu_scheduler_active == RCU_SCHEDULER_INACTIVE)
return;
_synchronize_rcu_expedited();
}
EXPORT_SYMBOL_GPL(synchronize_rcu_expedited);
#else /* #ifdef CONFIG_PREEMPT_RCU */
/* Invoked on each online non-idle CPU for expedited quiescent state. */
@ -801,27 +795,28 @@ static void sync_sched_exp_online_cleanup(int cpu)
WARN_ON_ONCE(ret);
}
/*
* Because a context switch is a grace period for !PREEMPT, any
* blocking grace-period wait automatically implies a grace period if
* there is only one CPU online at any point time during execution of
* either synchronize_rcu() or synchronize_rcu_expedited(). It is OK to
* occasionally incorrectly indicate that there are multiple CPUs online
* when there was in fact only one the whole time, as this just adds some
* overhead: RCU still operates correctly.
#endif /* #else #ifdef CONFIG_PREEMPT_RCU */
/**
* synchronize_rcu_expedited - Brute-force RCU grace period
*
* Wait for an RCU grace period, but expedite it. The basic idea is to
* IPI all non-idle non-nohz online CPUs. The IPI handler checks whether
* the CPU is in an RCU critical section, and if so, it sets a flag that
* causes the outermost rcu_read_unlock() to report the quiescent state
* for RCU-preempt or asks the scheduler for help for RCU-sched. On the
* other hand, if the CPU is not in an RCU read-side critical section,
* the IPI handler reports the quiescent state immediately.
*
* Although this is a greate improvement over previous expedited
* implementations, it is still unfriendly to real-time workloads, so is
* thus not recommended for any sort of common-case code. In fact, if
* you are using synchronize_rcu_expedited() in a loop, please restructure
* your code to batch your updates, and then Use a single synchronize_rcu()
* instead.
*
* This has the same semantics as (but is more brutal than) synchronize_rcu().
*/
static int rcu_blocking_is_gp(void)
{
int ret;
might_sleep(); /* Check for RCU read-side critical section. */
preempt_disable();
ret = num_online_cpus() <= 1;
preempt_enable();
return ret;
}
/* PREEMPT=n implementation of synchronize_rcu_expedited(). */
void synchronize_rcu_expedited(void)
{
RCU_LOCKDEP_WARN(lock_is_held(&rcu_bh_lock_map) ||
@ -829,12 +824,10 @@ void synchronize_rcu_expedited(void)
lock_is_held(&rcu_sched_lock_map),
"Illegal synchronize_rcu_expedited() in RCU read-side critical section");
/* If only one CPU, this is automatically a grace period. */
/* Is the state is such that the call is a grace period? */
if (rcu_blocking_is_gp())
return;
_synchronize_rcu_expedited();
}
EXPORT_SYMBOL_GPL(synchronize_rcu_expedited);
#endif /* #else #ifdef CONFIG_PREEMPT_RCU */