Re: [1/4] rcu: Detect uses of rcu read side in extended quiescent states

Paul E. McKenney
2011-06-06T18:10:28Z

On Sun, Jun 05, 2011 at 10:44:33PM -0500, Milton Miller wrote:
> On Mon, 06 Jun 2011 about 03:10:55 -0000, Frederic Weisbecker wrote:
> > @@ -3994,8 +3995,13 @@ void lockdep_rcu_dereference(const char *file, const int line)
> > printk("\n===================================================\n");
> > printk( "[ INFO: suspicious rcu_dereference_check() usage. ]\n");
> > printk( "---------------------------------------------------\n");
> > - printk("%s:%d invoked rcu_dereference_check() without protection!\n",
> > - file, line);
> > + printk("%s:%d invoked rcu_dereference_check() ", file, line);
> > +
> > + if (type == RCU_WARN_UNPROTECTED)
> > + printk("without protection!\n");
> > + else if (type == RCU_WARN_EXT_QS)
> > + printk("while in RCU extended quiescent state!\n");
> > +
> > printk("\nother info that might help us debug this:\n\n");
> > printk("\nrcu_scheduler_active = %d, debug_locks = %d\n", rcu_scheduler_active, debug_locks);
> > lockdep_print_held_locks(curr);
>
> Can we keep the above in one printk? That way the printing is
> guaranteed to come out on one line. Probably the easiest way would
> be add char *why = "" then assign a string based on the current
> conditions. Do all of that before the first printk which gets the
> a %s added.

I have the following queued the -rcu tree which does add the string.

Frederic, would it be possible to base on this patch?

Thanx, Paul

------------------------------------------------------------------------

commit c15d76f26712bd5228aa0c6af7a7e7c492a812c9
Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Date: Tue May 24 08:31:09 2011 -0700

rcu: Restore checks for blocking in RCU read-side critical sections

Long ago, using TREE_RCU with PREEMPT would result in "scheduling
while atomic" diagnostics if you blocked in an RCU read-side critical
section. However, PREEMPT now implies TREE_PREEMPT_RCU, which defeats
this diagnostic. This commit therefore adds a replacement diagnostic
based on PROVE_RCU.

Because rcu_lockdep_assert() and lockdep_rcu_dereference() are now being
used for things that have nothing to do with rcu_dereference(), rename
lockdep_rcu_dereference() to lockdep_rcu_suspicious() and add a third
argument that is a string indicating what is suspicious. This third
argument is passed in from a new third argument to rcu_lockdep_assert().
Update all calls to rcu_lockdep_assert() to add an informative third
argument.

Finally, add a pair of rcu_lockdep_assert() calls from within
rcu_note_context_switch(), one complaining if a context switch occurs
in an RCU-bh read-side critical section and another complaining if a
context switch occurs in an RCU-sched read-side critical section.
These are present only if the PROVE_RCU kernel parameter is enabled.

Again, you must enable PROVE_RCU to see these new diagnostics. But you
are enabling PROVE_RCU to check out new RCU uses in any case, aren't you?

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
index 4aef1dd..bfa66e7 100644
--- a/include/linux/lockdep.h
+++ b/include/linux/lockdep.h
@@ -545,7 +545,7 @@ do { \
#endif

#ifdef CONFIG_PROVE_RCU
-extern void lockdep_rcu_dereference(const char *file, const int line);
+void lockdep_rcu_suspicious(const char *file, const int line, const char *s);
#endif

#endif /* __LINUX_LOCKDEP_H */
diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 58b13f1..fb2933d 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -297,19 +297,20 @@ extern int rcu_my_thread_group_empty(void);
/**
* rcu_lockdep_assert - emit lockdep splat if specified condition not met
* @c: condition to check
+ * @s: informative message
*/
-#define rcu_lockdep_assert(c) \
+#define rcu_lockdep_assert(c, s) \
do { \
static bool __warned; \
if (debug_lockdep_rcu_enabled() && !__warned && !(c)) { \
__warned = true; \
- lockdep_rcu_dereference(__FILE__, __LINE__); \
+ lockdep_rcu_suspicious(__FILE__, __LINE__, s); \
} \
} while (0)

#else /* #ifdef CONFIG_PROVE_RCU */

-#define rcu_lockdep_assert(c) do { } while (0)
+#define rcu_lockdep_assert(c, s) do { } while (0)

#endif /* #else #ifdef CONFIG_PROVE_RCU */

@@ -338,14 +339,16 @@ extern int rcu_my_thread_group_empty(void);
#define __rcu_dereference_check(p, c, space) \
({ \
typeof(*p) *_________p1 = (typeof(*p)*__force )ACCESS_ONCE(p); \
- rcu_lockdep_assert(c); \
+ rcu_lockdep_assert(c, "suspicious rcu_dereference_check()" \
+ " usage"); \
rcu_dereference_sparse(p, space); \
smp_read_barrier_depends(); \
((typeof(*p) __force __kernel *)(_________p1)); \
})
#define __rcu_dereference_protected(p, c, space) \
({ \
- rcu_lockdep_assert(c); \
+ rcu_lockdep_assert(c, "suspicious rcu_dereference_protected()" \
+ " usage"); \
rcu_dereference_sparse(p, space); \
((typeof(*p) __force __kernel *)(p)); \
})
@@ -359,7 +362,9 @@ extern int rcu_my_thread_group_empty(void);
#define __rcu_dereference_index_check(p, c) \
({ \
typeof(p) _________p1 = ACCESS_ONCE(p); \
- rcu_lockdep_assert(c); \
+ rcu_lockdep_assert(c, \
+ "suspicious rcu_dereference_index_check()" \
+ " usage"); \
smp_read_barrier_depends(); \
(_________p1); \
})
diff --git a/kernel/lockdep.c b/kernel/lockdep.c
index 53a6895..9789028 100644
--- a/kernel/lockdep.c
+++ b/kernel/lockdep.c
@@ -3798,7 +3798,7 @@ void lockdep_sys_exit(void)
}
}

-void lockdep_rcu_dereference(const char *file, const int line)
+void lockdep_rcu_suspicious(const char *file, const int line, const char *s)
{
struct task_struct *curr = current;

@@ -3807,9 +3807,10 @@ void lockdep_rcu_dereference(const char *file, const int line)
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");
+ printk("\n===============================\n");
+ printk( "[ INFO: suspicious RCU usage. ]\n");
+ printk( "-------------------------------\n");
+ printk( "%s\n", s);
printk("%s:%d invoked rcu_dereference_check() without protection!\n",
file, line);
printk("\nother info that might help us debug this:\n\n");
@@ -3818,4 +3819,4 @@ void lockdep_rcu_dereference(const char *file, const int line)
printk("\nstack backtrace:\n");
dump_stack();
}
-EXPORT_SYMBOL_GPL(lockdep_rcu_dereference);
+EXPORT_SYMBOL_GPL(lockdep_rcu_suspicious);
diff --git a/kernel/pid.c b/kernel/pid.c
index 57a8346..a7577b3 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -419,7 +419,9 @@ EXPORT_SYMBOL(pid_task);
*/
struct task_struct *find_task_by_pid_ns(pid_t nr, struct pid_namespace *ns)
{
- rcu_lockdep_assert(rcu_read_lock_held());
+ rcu_lockdep_assert(rcu_read_lock_held(),
+ "find_task_by_pid_ns() needs rcu_read_lock()"
+ " protection");
return pid_task(find_pid_ns(nr, ns), PIDTYPE_PID);
}

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 88547c8..8b4b3da 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -153,6 +153,12 @@ void rcu_bh_qs(int cpu)
*/
void rcu_note_context_switch(int cpu)
{
+ rcu_lockdep_assert(!lock_is_held(&rcu_bh_lock_map),
+ "Illegal context switch in RCU-bh"
+ " read-side critical section");
+ rcu_lockdep_assert(!lock_is_held(&rcu_sched_lock_map),
+ "Illegal context switch in RCU-sched"
+ " read-side critical section");
rcu_sched_qs(cpu);
rcu_preempt_note_context_switch(cpu);
}