rcu: optionally leave lockdep enabled after RCU lockdep splat

There is no need to disable lockdep after an RCU lockdep splat,
so remove the debug_lockdeps_off() from lockdep_rcu_dereference().
To avoid repeated lockdep splats, use a static variable in the inlined
rcu_dereference_check() and rcu_dereference_protected() macros so that
a given instance splats only once, but so that multiple instances can
be detected per boot.

This is controlled by a new config variable CONFIG_PROVE_RCU_REPEATEDLY,
which is disabled by default.  This provides the normal lockdep behavior
by default, but permits people who want to find multiple RCU-lockdep
splats per boot to easily do so.

Requested-by: Eric Paris <eparis@redhat.com>
Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Tested-by: Eric Paris <eparis@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

3 files changed, 26 insertions, 4 deletions

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index db266bbed23f..4dca2752cfde 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -192,6 +192,15 @@ static inline int rcu_read_lock_sched_held(void)
 
 extern int rcu_my_thread_group_empty(void);
 
+#define __do_rcu_dereference_check(c)                                   \
+        do {                                                            \
+                static bool __warned;                                   \
+                if (debug_lockdep_rcu_enabled() && !__warned && !(c)) { \
+                        __warned = true;                                \
+                        lockdep_rcu_dereference(__FILE__, __LINE__);    \
+                }                                                       \
+        } while (0)
+
 /**
  * rcu_dereference_check - rcu_dereference with debug checking
  * @p: The pointer to read, prior to dereferencing
@@ -221,8 +230,7 @@ extern int rcu_my_thread_group_empty(void);
  */
 #define rcu_dereference_check(p, c) \
         ({ \
-                if (debug_lockdep_rcu_enabled() && !(c)) \
-                        lockdep_rcu_dereference(__FILE__, __LINE__); \
+                __do_rcu_dereference_check(c); \
                 rcu_dereference_raw(p); \
         })
 
@@ -239,8 +247,7 @@ extern int rcu_my_thread_group_empty(void);
  */
 #define rcu_dereference_protected(p, c) \
         ({ \
-                if (debug_lockdep_rcu_enabled() && !(c)) \
-                        lockdep_rcu_dereference(__FILE__, __LINE__); \
+                __do_rcu_dereference_check(c); \
                 (p); \
         })
 
diff --git a/kernel/lockdep.c b/kernel/lockdep.c
index 2594e1ce41cb..3a756ba8d5d8 100644
--- a/kernel/lockdep.c
+++ b/kernel/lockdep.c
@@ -3801,8 +3801,11 @@ void lockdep_rcu_dereference(const char *file, const int line)
 {
         struct task_struct *curr = current;
 
+#ifndef CONFIG_PROVE_RCU_REPEATEDLY
         if (!debug_locks_off())
                 return;
+#endif /* #ifdef CONFIG_PROVE_RCU_REPEATEDLY */
+        /* Note: the following can be executed concurrently, so be careful. */
         printk("\n===================================================\n");
         printk(  "[ INFO: suspicious rcu_dereference_check() usage. ]\n");
         printk(  "---------------------------------------------------\n");
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 935248bdbc47..94090b4bb7d2 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -512,6 +512,18 @@ config PROVE_RCU
 
          Say N if you are unsure.
 
+config PROVE_RCU_REPEATEDLY
+        bool "RCU debugging: don't disable PROVE_RCU on first splat"
+        depends on PROVE_RCU
+        default n
+        help
+         By itself, PROVE_RCU will disable checking upon issuing the
+         first warning (or "splat").  This feature prevents such
+         disabling, allowing multiple RCU-lockdep warnings to be printed
+         on a single reboot.
+
+         Say N if you are unsure.
+
 config LOCKDEP
         bool
         depends on DEBUG_KERNEL && TRACE_IRQFLAGS_SUPPORT && STACKTRACE_SUPPORT && LOCKDEP_SUPPORT