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rcu: Don't disable preemption for Tiny and Tree RCU readers

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Because preempt_disable() maps to barrier() for non-debug builds,
it forces the compiler to spill and reload registers.  Because Tree
RCU and Tiny RCU now only appear in CONFIG_PREEMPT=n builds, these
barrier() instances generate needless extra code for each instance of
rcu_read_lock() and rcu_read_unlock().  This extra code slows down Tree
RCU and bloats Tiny RCU.

This commit therefore removes the preempt_disable() and preempt_enable()
from the non-preemptible implementations of __rcu_read_lock() and
__rcu_read_unlock(), respectively.  However, for debug purposes,
preempt_disable() and preempt_enable() are still invoked if
CONFIG_PREEMPT_COUNT=y, because this allows detection of sleeping inside
atomic sections in non-preemptible kernels.

However, Tiny and Tree RCU operates by coalescing all RCU read-side
critical sections on a given CPU that lie between successive quiescent
states.  It is therefore necessary to compensate for removing barriers
from __rcu_read_lock() and __rcu_read_unlock() by adding them to a
couple of the RCU functions invoked during quiescent states, namely to
rcu_all_qs() and rcu_note_context_switch().  However, note that the latter
is more paranoia than necessity, at least until link-time optimizations
become more aggressive.

This is based on an earlier patch by Paul E. McKenney, fixing
a bug encountered in kernels built with CONFIG_PREEMPT=n and
CONFIG_PREEMPT_COUNT=y.

Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit is contained in:
Boqun Feng 2015-07-30 16:55:38 -07:00 committed by Paul E. McKenney
parent db3e8db45e
commit bb73c52bad
3 changed files with 14 additions and 2 deletions
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6
include/linux/rcupdate.h
View File

@ -297,12 +297,14 @@ void synchronize_rcu(void);
static inline void __rcu_read_lock(void)
{
preempt_disable();
if (IS_ENABLED(CONFIG_PREEMPT_COUNT))
preempt_disable();
}
static inline void __rcu_read_unlock(void)
{
preempt_enable();
if (IS_ENABLED(CONFIG_PREEMPT_COUNT))
preempt_enable();
}
static inline void synchronize_rcu(void)

1
include/linux/rcutiny.h
View File

@ -216,6 +216,7 @@ static inline bool rcu_is_watching(void)
static inline void rcu_all_qs(void)
{
barrier(); /* Avoid RCU read-side critical sections leaking across. */
}
#endif /* __LINUX_RCUTINY_H */

9
kernel/rcu/tree.c
View File

@ -337,12 +337,14 @@ static void rcu_momentary_dyntick_idle(void)
*/
void rcu_note_context_switch(void)
{
barrier(); /* Avoid RCU read-side critical sections leaking down. */
trace_rcu_utilization(TPS("Start context switch"));
rcu_sched_qs();
rcu_preempt_note_context_switch();
if (unlikely(raw_cpu_read(rcu_sched_qs_mask)))
rcu_momentary_dyntick_idle();
trace_rcu_utilization(TPS("End context switch"));
barrier(); /* Avoid RCU read-side critical sections leaking up. */
}
EXPORT_SYMBOL_GPL(rcu_note_context_switch);
@ -353,12 +355,19 @@ EXPORT_SYMBOL_GPL(rcu_note_context_switch);
* RCU flavors in desperate need of a quiescent state, which will normally
* be none of them). Either way, do a lightweight quiescent state for
* all RCU flavors.
*
* The barrier() calls are redundant in the common case when this is
* called externally, but just in case this is called from within this
* file.
*
*/
void rcu_all_qs(void)
{
barrier(); /* Avoid RCU read-side critical sections leaking down. */
if (unlikely(raw_cpu_read(rcu_sched_qs_mask)))
rcu_momentary_dyntick_idle();
this_cpu_inc(rcu_qs_ctr);
barrier(); /* Avoid RCU read-side critical sections leaking up. */
}
EXPORT_SYMBOL_GPL(rcu_all_qs);