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Subject[PATCH 05/11] rcu: Warn when srcu_read_lock() is used in an extended quiescent state
Date
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>

Catch SRCU up to the other variants of RCU by making PROVE_RCU
complain if either srcu_read_lock() or srcu_read_lock_held() are
used from within RCU-idle mode.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
---
include/linux/srcu.h | 35 ++++++++++++++++++++++-------------
1 files changed, 22 insertions(+), 13 deletions(-)

diff --git a/include/linux/srcu.h b/include/linux/srcu.h
index 58971e8..553c003 100644
--- a/include/linux/srcu.h
+++ b/include/linux/srcu.h
@@ -28,6 +28,7 @@
#define _LINUX_SRCU_H

#include <linux/mutex.h>
+#include <linux/rcupdate.h>

struct srcu_struct_array {
int c[2];
@@ -60,18 +61,10 @@ int __init_srcu_struct(struct srcu_struct *sp, const char *name,
__init_srcu_struct((sp), #sp, &__srcu_key); \
})

-# define srcu_read_acquire(sp) \
- lock_acquire(&(sp)->dep_map, 0, 0, 2, 1, NULL, _THIS_IP_)
-# define srcu_read_release(sp) \
- lock_release(&(sp)->dep_map, 1, _THIS_IP_)
-
#else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */

int init_srcu_struct(struct srcu_struct *sp);

-# define srcu_read_acquire(sp) do { } while (0)
-# define srcu_read_release(sp) do { } while (0)
-
#endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */

void cleanup_srcu_struct(struct srcu_struct *sp);
@@ -90,12 +83,28 @@ long srcu_batches_completed(struct srcu_struct *sp);
* read-side critical section. In absence of CONFIG_DEBUG_LOCK_ALLOC,
* this assumes we are in an SRCU read-side critical section unless it can
* prove otherwise.
+ *
+ * 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.
*/
static inline int srcu_read_lock_held(struct srcu_struct *sp)
{
- if (debug_locks)
- return lock_is_held(&sp->dep_map);
- return 1;
+ if (rcu_is_cpu_idle())
+ return 0;
+
+ if (!debug_locks)
+ return 1;
+
+ return lock_is_held(&sp->dep_map);
}

#else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
@@ -150,7 +159,7 @@ static inline int srcu_read_lock(struct srcu_struct *sp) __acquires(sp)
{
int retval = __srcu_read_lock(sp);

- srcu_read_acquire(sp);
+ rcu_lock_acquire(&(sp)->dep_map);
return retval;
}

@@ -164,7 +173,7 @@ static inline int srcu_read_lock(struct srcu_struct *sp) __acquires(sp)
static inline void srcu_read_unlock(struct srcu_struct *sp, int idx)
__releases(sp)
{
- srcu_read_release(sp);
+ rcu_lock_release(&(sp)->dep_map);
__srcu_read_unlock(sp, idx);
}

--
1.7.5.4


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