Merge branch 'rcu/srcu' of git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu into core/rcu
Pull SRCU changes from Paul E. McKenney.
" These include debugging aids, updates that move towards the goal
of permitting srcu_read_lock() and srcu_read_unlock() to be used
from idle and offline CPUs, and a few small fixes. "
Signed-off-by: Ingo Molnar <mingo@kernel.org>
diff --git a/include/linux/srcu.h b/include/linux/srcu.h
index 6eb691b..04f4121 100644
--- a/include/linux/srcu.h
+++ b/include/linux/srcu.h
@@ -151,30 +151,14 @@
* Checks debug_lockdep_rcu_enabled() to prevent false positives during boot
* and while lockdep is disabled.
*
- * Note that if the CPU is in the idle loop from an RCU point of view
- * (ie: that we are in the section between rcu_idle_enter() and
- * rcu_idle_exit()) then srcu_read_lock_held() returns false even if
- * the CPU did an srcu_read_lock(). The reason for this is that RCU
- * ignores CPUs that are in such a section, considering these as in
- * extended quiescent state, so such a CPU is effectively never in an
- * RCU read-side critical section regardless of what RCU primitives it
- * invokes. This state of affairs is required --- we need to keep an
- * RCU-free window in idle where the CPU may possibly enter into low
- * power mode. This way we can notice an extended quiescent state to
- * other CPUs that started a grace period. Otherwise we would delay any
- * grace period as long as we run in the idle task.
- *
- * Similarly, we avoid claiming an SRCU read lock held if the current
- * CPU is offline.
+ * Note that SRCU is based on its own statemachine and it doesn't
+ * relies on normal RCU, it can be called from the CPU which
+ * is in the idle loop from an RCU point of view or offline.
*/
static inline int srcu_read_lock_held(struct srcu_struct *sp)
{
if (!debug_lockdep_rcu_enabled())
return 1;
- if (rcu_is_cpu_idle())
- return 0;
- if (!rcu_lockdep_current_cpu_online())
- return 0;
return lock_is_held(&sp->dep_map);
}
@@ -236,8 +220,6 @@
int retval = __srcu_read_lock(sp);
rcu_lock_acquire(&(sp)->dep_map);
- rcu_lockdep_assert(!rcu_is_cpu_idle(),
- "srcu_read_lock() used illegally while idle");
return retval;
}
@@ -251,8 +233,6 @@
static inline void srcu_read_unlock(struct srcu_struct *sp, int idx)
__releases(sp)
{
- rcu_lockdep_assert(!rcu_is_cpu_idle(),
- "srcu_read_unlock() used illegally while idle");
rcu_lock_release(&(sp)->dep_map);
__srcu_read_unlock(sp, idx);
}
diff --git a/kernel/srcu.c b/kernel/srcu.c
index 2b85982..01d5ccb 100644
--- a/kernel/srcu.c
+++ b/kernel/srcu.c
@@ -282,12 +282,8 @@
*/
void cleanup_srcu_struct(struct srcu_struct *sp)
{
- int sum;
-
- sum = srcu_readers_active(sp);
- WARN_ON(sum); /* Leakage unless caller handles error. */
- if (sum != 0)
- return;
+ if (WARN_ON(srcu_readers_active(sp)))
+ return; /* Leakage unless caller handles error. */
free_percpu(sp->per_cpu_ref);
sp->per_cpu_ref = NULL;
}
@@ -302,9 +298,8 @@
{
int idx;
+ idx = ACCESS_ONCE(sp->completed) & 0x1;
preempt_disable();
- idx = rcu_dereference_index_check(sp->completed,
- rcu_read_lock_sched_held()) & 0x1;
ACCESS_ONCE(this_cpu_ptr(sp->per_cpu_ref)->c[idx]) += 1;
smp_mb(); /* B */ /* Avoid leaking the critical section. */
ACCESS_ONCE(this_cpu_ptr(sp->per_cpu_ref)->seq[idx]) += 1;
@@ -321,10 +316,8 @@
*/
void __srcu_read_unlock(struct srcu_struct *sp, int idx)
{
- preempt_disable();
smp_mb(); /* C */ /* Avoid leaking the critical section. */
- ACCESS_ONCE(this_cpu_ptr(sp->per_cpu_ref)->c[idx]) -= 1;
- preempt_enable();
+ this_cpu_dec(sp->per_cpu_ref->c[idx]);
}
EXPORT_SYMBOL_GPL(__srcu_read_unlock);
@@ -423,6 +416,7 @@
!lock_is_held(&rcu_sched_lock_map),
"Illegal synchronize_srcu() in same-type SRCU (or RCU) read-side critical section");
+ might_sleep();
init_completion(&rcu.completion);
head->next = NULL;
@@ -455,10 +449,12 @@
* synchronize_srcu - wait for prior SRCU read-side critical-section completion
* @sp: srcu_struct with which to synchronize.
*
- * Flip the completed counter, and wait for the old count to drain to zero.
- * As with classic RCU, the updater must use some separate means of
- * synchronizing concurrent updates. Can block; must be called from
- * process context.
+ * Wait for the count to drain to zero of both indexes. To avoid the
+ * possible starvation of synchronize_srcu(), it waits for the count of
+ * the index=((->completed & 1) ^ 1) to drain to zero at first,
+ * and then flip the completed and wait for the count of the other index.
+ *
+ * Can block; must be called from process context.
*
* Note that it is illegal to call synchronize_srcu() from the corresponding
* SRCU read-side critical section; doing so will result in deadlock.
@@ -480,12 +476,11 @@
* Wait for an SRCU grace period to elapse, but be more aggressive about
* spinning rather than blocking when waiting.
*
- * Note that it is illegal to call this function while holding any lock
- * that is acquired by a CPU-hotplug notifier. It is also illegal to call
- * synchronize_srcu_expedited() from the corresponding SRCU read-side
- * critical section; doing so will result in deadlock. However, it is
- * perfectly legal to call synchronize_srcu_expedited() on one srcu_struct
- * from some other srcu_struct's read-side critical section, as long as
+ * Note that it is also illegal to call synchronize_srcu_expedited()
+ * from the corresponding SRCU read-side critical section;
+ * doing so will result in deadlock. However, it is perfectly legal
+ * to call synchronize_srcu_expedited() on one srcu_struct from some
+ * other srcu_struct's read-side critical section, as long as
* the resulting graph of srcu_structs is acyclic.
*/
void synchronize_srcu_expedited(struct srcu_struct *sp)