Re: [-mm PATCH] RCU: debug sleep check

[Dipankar Sarma <dipankar@in.ibm.com>]

This adds an overhead of a function call and local_irq_save/restore
in the rcu reader path when CONFIG_DEBUG_SPINLOCK_SLEEP is set.
Hopefully that is not much of a concern. Comments are welcome.
Applies on top of my earlier patches. The full patchset is
at http://www.hill9.org/linux/kernel/patches/2.6.18-mm1/.

Add a debug check for rcu read-side critical section code calling
a function that might sleep which is illegal. The check is enabled only
if CONFIG_DEBUG_SPINLOCK_SLEEP is set.

Signed-off-by: Dipankar Sarma <dipankar@in.ibm.com>
---


 include/linux/rcupdate.h |   34 ++++++++++++++++++++++++++++++----
 kernel/rcupdate.c        |   33 +++++++++++++++++++++++++++++++++
 kernel/sched.c           |    2 +-
 3 files changed, 64 insertions(+), 5 deletions(-)

diff -puN include/linux/rcupdate.h~rcu-reader-sleep-check include/linux/rcupdate.h
--- linux-2.6.18-mm1-rcu/include/linux/rcupdate.h~rcu-reader-sleep-check	2006-09-24 23:18:35.000000000 +0530
+++ linux-2.6.18-mm1-rcu-dipankar/include/linux/rcupdate.h	2006-09-24 23:18:35.000000000 +0530
@@ -64,6 +64,16 @@ struct rcu_head {
        (ptr)->next = NULL; (ptr)->func = NULL; \
 } while (0)
 
+#ifdef CONFIG_DEBUG_SPINLOCK_SLEEP
+extern int rcu_read_in_atomic(void);
+extern void rcu_add_read_count(void);
+extern void rcu_sub_read_count(void);
+#else
+static inline int rcu_read_in_atomic(void) { return 0;}
+static inline void rcu_add_read_count(void) {}
+static inline void rcu_sub_read_count(void) {}
+#endif
+
 /**
  * rcu_read_lock - mark the beginning of an RCU read-side critical section.
  *
@@ -93,14 +103,22 @@ struct rcu_head {
  *
  * It is illegal to block while in an RCU read-side critical section.
  */
-#define rcu_read_lock() __rcu_read_lock()
+#define rcu_read_lock()	\
+	do {	\
+		rcu_add_read_count();	\
+		__rcu_read_lock();	\
+	} while (0)
 
 /**
  * rcu_read_unlock - marks the end of an RCU read-side critical section.
  *
  * See rcu_read_lock() for more information.
  */
-#define rcu_read_unlock() __rcu_read_unlock()
+#define rcu_read_unlock()	\
+	do {	\
+		__rcu_read_unlock();	\
+		rcu_sub_read_count();	\
+	} while (0)
 
 /*
  * So where is rcu_write_lock()?  It does not exist, as there is no
@@ -123,14 +141,22 @@ struct rcu_head {
  * can use just rcu_read_lock().
  *
  */
-#define rcu_read_lock_bh()	__rcu_read_lock_bh()
+#define rcu_read_lock_bh()	\
+	do {	\
+		rcu_add_read_count();	\
+		__rcu_read_lock_bh();	\
+	} while (0)
   
 /**
  * rcu_read_unlock_bh - marks the end of a softirq-only RCU critical section
  *
  * See rcu_read_lock_bh() for more information.
  */
-#define rcu_read_unlock_bh()	__rcu_read_unlock_bh()
+#define rcu_read_unlock_bh()	\
+	do {	\
+		__rcu_read_unlock_bh();	\
+		rcu_sub_read_count();	\
+	} while (0)
   
 /**
  * rcu_dereference - fetch an RCU-protected pointer in an
diff -puN kernel/rcupdate.c~rcu-reader-sleep-check kernel/rcupdate.c
--- linux-2.6.18-mm1-rcu/kernel/rcupdate.c~rcu-reader-sleep-check	2006-09-24 23:18:35.000000000 +0530
+++ linux-2.6.18-mm1-rcu-dipankar/kernel/rcupdate.c	2006-09-24 23:18:35.000000000 +0530
@@ -127,5 +127,38 @@ void __init rcu_init(void)
 	__rcu_init();
 }
 
+#ifdef CONFIG_DEBUG_SPINLOCK_SLEEP
+DEFINE_PER_CPU(int, rcu_read_count);
+int rcu_read_in_atomic(void)
+{
+	int val;
+	int cpu = get_cpu();
+	val = per_cpu(rcu_read_count, cpu);
+	put_cpu();
+	return val;
+}
+
+void rcu_add_read_count(void)
+{
+	int cpu, flags;
+	local_irq_save(flags);
+	cpu = smp_processor_id();
+	per_cpu(rcu_read_count, cpu)++;
+	local_irq_restore(flags);
+}
+
+void rcu_sub_read_count(void)
+{
+	int cpu, flags;
+	local_irq_save(flags);
+	cpu = smp_processor_id();
+	per_cpu(rcu_read_count, cpu)--;
+	local_irq_restore(flags);
+}
+EXPORT_SYMBOL_GPL(rcu_read_in_atomic);
+EXPORT_SYMBOL_GPL(rcu_add_read_count);
+EXPORT_SYMBOL_GPL(rcu_sub_read_count);
+#endif
+
 EXPORT_SYMBOL_GPL(rcu_barrier);
 EXPORT_SYMBOL_GPL(synchronize_rcu);
diff -puN kernel/sched.c~rcu-reader-sleep-check kernel/sched.c
--- linux-2.6.18-mm1-rcu/kernel/sched.c~rcu-reader-sleep-check	2006-09-24 23:18:35.000000000 +0530
+++ linux-2.6.18-mm1-rcu-dipankar/kernel/sched.c	2006-09-24 23:18:35.000000000 +0530
@@ -6974,7 +6974,7 @@ void __might_sleep(char *file, int line)
 #ifdef in_atomic
 	static unsigned long prev_jiffy;	/* ratelimiting */
 
-	if ((in_atomic() || irqs_disabled()) &&
+	if ((in_atomic() || irqs_disabled() || rcu_read_in_atomic()) &&
 	    system_state == SYSTEM_RUNNING && !oops_in_progress) {
 		if (time_before(jiffies, prev_jiffy + HZ) && prev_jiffy)
 			return;

_
-
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