Commit bb73c52b authored by Boqun Feng's avatar Boqun Feng Committed by Paul E. McKenney

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

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: default avatarBoqun Feng <boqun.feng@gmail.com>
Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
parent db3e8db4
...@@ -297,11 +297,13 @@ void synchronize_rcu(void); ...@@ -297,11 +297,13 @@ void synchronize_rcu(void);
static inline void __rcu_read_lock(void) static inline void __rcu_read_lock(void)
{ {
if (IS_ENABLED(CONFIG_PREEMPT_COUNT))
preempt_disable(); preempt_disable();
} }
static inline void __rcu_read_unlock(void) static inline void __rcu_read_unlock(void)
{ {
if (IS_ENABLED(CONFIG_PREEMPT_COUNT))
preempt_enable(); preempt_enable();
} }
......
...@@ -216,6 +216,7 @@ static inline bool rcu_is_watching(void) ...@@ -216,6 +216,7 @@ static inline bool rcu_is_watching(void)
static inline void rcu_all_qs(void) static inline void rcu_all_qs(void)
{ {
barrier(); /* Avoid RCU read-side critical sections leaking across. */
} }
#endif /* __LINUX_RCUTINY_H */ #endif /* __LINUX_RCUTINY_H */
...@@ -337,12 +337,14 @@ static void rcu_momentary_dyntick_idle(void) ...@@ -337,12 +337,14 @@ static void rcu_momentary_dyntick_idle(void)
*/ */
void rcu_note_context_switch(void) void rcu_note_context_switch(void)
{ {
barrier(); /* Avoid RCU read-side critical sections leaking down. */
trace_rcu_utilization(TPS("Start context switch")); trace_rcu_utilization(TPS("Start context switch"));
rcu_sched_qs(); rcu_sched_qs();
rcu_preempt_note_context_switch(); rcu_preempt_note_context_switch();
if (unlikely(raw_cpu_read(rcu_sched_qs_mask))) if (unlikely(raw_cpu_read(rcu_sched_qs_mask)))
rcu_momentary_dyntick_idle(); rcu_momentary_dyntick_idle();
trace_rcu_utilization(TPS("End context switch")); trace_rcu_utilization(TPS("End context switch"));
barrier(); /* Avoid RCU read-side critical sections leaking up. */
} }
EXPORT_SYMBOL_GPL(rcu_note_context_switch); EXPORT_SYMBOL_GPL(rcu_note_context_switch);
...@@ -353,12 +355,19 @@ 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 * RCU flavors in desperate need of a quiescent state, which will normally
* be none of them). Either way, do a lightweight quiescent state for * be none of them). Either way, do a lightweight quiescent state for
* all RCU flavors. * 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) void rcu_all_qs(void)
{ {
barrier(); /* Avoid RCU read-side critical sections leaking down. */
if (unlikely(raw_cpu_read(rcu_sched_qs_mask))) if (unlikely(raw_cpu_read(rcu_sched_qs_mask)))
rcu_momentary_dyntick_idle(); rcu_momentary_dyntick_idle();
this_cpu_inc(rcu_qs_ctr); this_cpu_inc(rcu_qs_ctr);
barrier(); /* Avoid RCU read-side critical sections leaking up. */
} }
EXPORT_SYMBOL_GPL(rcu_all_qs); EXPORT_SYMBOL_GPL(rcu_all_qs);
......
Markdown is supported
0% or
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment