[PATCH 0/7] [GIT PULL] lockdep: Show description of lockdep problems

[PATCH 0/7] [GIT PULL] lockdep: Show description of lockdep problems

[Steven Rostedt]

Ingo,

Please pull the latest tip/lockdep/devel tree, which can be found at:

  git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-2.6-trace.git
tip/lockdep/devel


Steven Rostedt (7):
      lockdep: Print a nice description of an irq locking issue
      lockdep: Print a nice description of normal deadlocks
      lockdep: Print a nice description of simple deadlock
      lockdep: Printk nice description for irq inversion bug
      lockdep: Replace bad path error message with something sane
      lockdep: Print a nice description of simple irq inversion
      lockdep: Remove cmpxchg to update nr_chain_hlocks

----
 kernel/lockdep.c |  206 +++++++++++++++++++++++++++++++++++++++++++++++++++---
 1 files changed, 195 insertions(+), 11 deletions(-)

[PATCH 1/7] lockdep: Print a nice description of an irq locking issue

[Steven Rostedt]

[-- Attachment #1: 0001-lockdep-Print-a-nice-description-of-an-irq-locking-i.patch --]
[-- Type: text/plain, Size: 5045 bytes --]

From: Steven Rostedt <srostedt@redhat.com>

Locking order inversion due to interrupts is a subtle problem.
When a locking inversion due to interrupts is discovered by lockdep,
it currently reports something like this:

[ INFO: HARDIRQ-safe -> HARDIRQ-unsafe lock order detected ]

And then writes the locks that are involved as well as back traces.
But several developers are confused by what a HARDIRQ->safe to unsafe
issue is all about, and sometimes even blow it off as a bug in lockdep.
As it is not obvious when lockdep describes this about a lock that
is never taken in interrupt context.

After explaining the problems that lockdep is reporting, I decided
to add a description of the problem in visual form. Now the following
is shown:

 ---
other info that might help us debug this:

 Possible interrupt unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(lockA);
                               local_irq_disable();
                               lock(&rq->lock);
                               lock(lockA);
  <Interrupt>
    lock(&rq->lock);

 *** DEADLOCK ***

 ---

The above is the case when the unsafe lock is taken while holding
a lock taken in irq context. But when a lock is taken that also
grabs a unsafe lock, the call chain is shown:

 ---
other info that might help us debug this:

Chain exists of:
  &rq->lock --> lockA --> lockC

 Possible interrupt unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(lockC);
                               local_irq_disable();
                               lock(&rq->lock);
                               lock(lockA);
  <Interrupt>
    lock(&rq->lock);

 *** DEADLOCK ***

Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
---
 kernel/lockdep.c |   70 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 files changed, 70 insertions(+), 0 deletions(-)

diff --git a/kernel/lockdep.c b/kernel/lockdep.c
index 0d2058d..bb77c030 100644
--- a/kernel/lockdep.c
+++ b/kernel/lockdep.c
@@ -490,6 +490,18 @@ void get_usage_chars(struct lock_class *class, char usage[LOCK_USAGE_CHARS])
 	usage[i] = '\0';
 }
 
+static int __print_lock_name(struct lock_class *class)
+{
+	char str[KSYM_NAME_LEN];
+	const char *name;
+
+	name = class->name;
+	if (!name)
+		name = __get_key_name(class->key, str);
+
+	return printk("%s", name);
+}
+
 static void print_lock_name(struct lock_class *class)
 {
 	char str[KSYM_NAME_LEN], usage[LOCK_USAGE_CHARS];
@@ -1325,6 +1337,62 @@ print_shortest_lock_dependencies(struct lock_list *leaf,
 	return;
 }
 
+static void
+print_irq_lock_scenario(struct lock_list *safe_entry,
+			struct lock_list *unsafe_entry,
+			struct held_lock *prev,
+			struct held_lock *next)
+{
+	struct lock_class *safe_class = safe_entry->class;
+	struct lock_class *unsafe_class = unsafe_entry->class;
+	struct lock_class *middle_class = hlock_class(prev);
+
+	if (middle_class == safe_class)
+		middle_class = hlock_class(next);
+
+	/*
+	 * A direct locking problem where unsafe_class lock is taken
+	 * directly by safe_class lock, then all we need to show
+	 * is the deadlock scenario, as it is obvious that the
+	 * unsafe lock is taken under the safe lock.
+	 *
+	 * But if there is a chain instead, where the safe lock takes
+	 * an intermediate lock (middle_class) where this lock is
+	 * not the same as the safe lock, then the lock chain is
+	 * used to describe the problem. Otherwise we would need
+	 * to show a different CPU case for each link in the chain
+	 * from the safe_class lock to the unsafe_class lock.
+	 */
+	if (middle_class != unsafe_class) {
+		printk("Chain exists of:\n  ");
+		__print_lock_name(safe_class);
+		printk(" --> ");
+		__print_lock_name(middle_class);
+		printk(" --> ");
+		__print_lock_name(unsafe_class);
+		printk("\n\n");
+	}
+
+	printk(" Possible interrupt unsafe locking scenario:\n\n");
+	printk("       CPU0                    CPU1\n");
+	printk("       ----                    ----\n");
+	printk("  lock(");
+	__print_lock_name(unsafe_class);
+	printk(");\n");
+	printk("                               local_irq_disable();\n");
+	printk("                               lock(");
+	__print_lock_name(safe_class);
+	printk(");\n");
+	printk("                               lock(");
+	__print_lock_name(middle_class);
+	printk(");\n");
+	printk("  <Interrupt>\n");
+	printk("    lock(");
+	__print_lock_name(safe_class);
+	printk(");\n");
+	printk("\n *** DEADLOCK ***\n\n");
+}
+
 static int
 print_bad_irq_dependency(struct task_struct *curr,
 			 struct lock_list *prev_root,
@@ -1376,6 +1444,8 @@ print_bad_irq_dependency(struct task_struct *curr,
 	print_stack_trace(forwards_entry->class->usage_traces + bit2, 1);
 
 	printk("\nother info that might help us debug this:\n\n");
+	print_irq_lock_scenario(backwards_entry, forwards_entry, prev, next);
+
 	lockdep_print_held_locks(curr);
 
 	printk("\nthe dependencies between %s-irq-safe lock", irqclass);
-- 
1.7.2.3

[PATCH 2/7] lockdep: Print a nice description of normal deadlocks

[Steven Rostedt]

[-- Attachment #1: 0002-lockdep-Print-a-nice-description-of-normal-deadlocks.patch --]
[-- Type: text/plain, Size: 3987 bytes --]

From: Steven Rostedt <srostedt@redhat.com>

The lockdep output can be pretty cryptic, having a nice output
can save a lot of head scratching. When a normal deadlock scenario
is detected by lockdep (lock A -> lock B and there exists a place
where lock B -> lock A) we get the following output:

other info that might help us debug this:

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(lockB);
                               lock(lockA);
                               lock(lockB);
  lock(lockA);

 *** DEADLOCK ***

On cases where there's a deeper chair, it shows the partial chain
that can cause the issue:

Chain exists of:
  lockC --> lockA --> lockB

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(lockB);
                               lock(lockA);
                               lock(lockB);
  lock(lockC);

 *** DEADLOCK ***

Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
---
 kernel/lockdep.c |   55 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 files changed, 55 insertions(+), 0 deletions(-)

diff --git a/kernel/lockdep.c b/kernel/lockdep.c
index bb77c030..1039008 100644
--- a/kernel/lockdep.c
+++ b/kernel/lockdep.c
@@ -1065,6 +1065,56 @@ print_circular_bug_entry(struct lock_list *target, int depth)
 	return 0;
 }
 
+static void
+print_circular_lock_scenario(struct held_lock *src,
+			     struct held_lock *tgt,
+			     struct lock_list *prt)
+{
+	struct lock_class *source = hlock_class(src);
+	struct lock_class *target = hlock_class(tgt);
+	struct lock_class *parent = prt->class;
+
+	/*
+	 * A direct locking problem where unsafe_class lock is taken
+	 * directly by safe_class lock, then all we need to show
+	 * is the deadlock scenario, as it is obvious that the
+	 * unsafe lock is taken under the safe lock.
+	 *
+	 * But if there is a chain instead, where the safe lock takes
+	 * an intermediate lock (middle_class) where this lock is
+	 * not the same as the safe lock, then the lock chain is
+	 * used to describe the problem. Otherwise we would need
+	 * to show a different CPU case for each link in the chain
+	 * from the safe_class lock to the unsafe_class lock.
+	 */
+	if (parent != source) {
+		printk("Chain exists of:\n  ");
+		__print_lock_name(source);
+		printk(" --> ");
+		__print_lock_name(parent);
+		printk(" --> ");
+		__print_lock_name(target);
+		printk("\n\n");
+	}
+
+	printk(" Possible unsafe locking scenario:\n\n");
+	printk("       CPU0                    CPU1\n");
+	printk("       ----                    ----\n");
+	printk("  lock(");
+	__print_lock_name(target);
+	printk(");\n");
+	printk("                               lock(");
+	__print_lock_name(parent);
+	printk(");\n");
+	printk("                               lock(");
+	__print_lock_name(target);
+	printk(");\n");
+	printk("  lock(");
+	__print_lock_name(source);
+	printk(");\n");
+	printk("\n *** DEADLOCK ***\n\n");
+}
+
 /*
  * When a circular dependency is detected, print the
  * header first:
@@ -1108,6 +1158,7 @@ static noinline int print_circular_bug(struct lock_list *this,
 {
 	struct task_struct *curr = current;
 	struct lock_list *parent;
+	struct lock_list *first_parent;
 	int depth;
 
 	if (!debug_locks_off_graph_unlock() || debug_locks_silent)
@@ -1121,6 +1172,7 @@ static noinline int print_circular_bug(struct lock_list *this,
 	print_circular_bug_header(target, depth, check_src, check_tgt);
 
 	parent = get_lock_parent(target);
+	first_parent = parent;
 
 	while (parent) {
 		print_circular_bug_entry(parent, --depth);
@@ -1128,6 +1180,9 @@ static noinline int print_circular_bug(struct lock_list *this,
 	}
 
 	printk("\nother info that might help us debug this:\n\n");
+	print_circular_lock_scenario(check_src, check_tgt,
+				     first_parent);
+
 	lockdep_print_held_locks(curr);
 
 	printk("\nstack backtrace:\n");
-- 
1.7.2.3

[PATCH 3/7] lockdep: Print a nice description of simple deadlock

[Steven Rostedt]

[-- Attachment #1: 0003-lockdep-Print-a-nice-description-of-simple-deadlock.patch --]
[-- Type: text/plain, Size: 1756 bytes --]

From: Steven Rostedt <srostedt@redhat.com>

The lockdep output can be pretty cryptic, having a nice output
can save a lot of head scratching. When a simple deadlock scenario
is detected by lockdep (lock A -> lock A) we get the following output:

other info that might help us debug this:
 Possible unsafe locking scenario:

       CPU0
       ----
  lock(&(lock)->rlock);
  lock(&(lock)->rlock);

 *** DEADLOCK ***

Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
---
 kernel/lockdep.c |   21 +++++++++++++++++++++
 1 files changed, 21 insertions(+), 0 deletions(-)

diff --git a/kernel/lockdep.c b/kernel/lockdep.c
index 1039008..a0f253b 100644
--- a/kernel/lockdep.c
+++ b/kernel/lockdep.c
@@ -1664,6 +1664,26 @@ static inline void inc_chains(void)
 
 #endif
 
+static void
+print_deadlock_scenario(struct held_lock *nxt,
+			     struct held_lock *prv)
+{
+	struct lock_class *next = hlock_class(nxt);
+	struct lock_class *prev = hlock_class(prv);
+
+	printk(" Possible unsafe locking scenario:\n\n");
+	printk("       CPU0\n");
+	printk("       ----\n");
+	printk("  lock(");
+	__print_lock_name(prev);
+	printk(");\n");
+	printk("  lock(");
+	__print_lock_name(next);
+	printk(");\n");
+	printk("\n *** DEADLOCK ***\n\n");
+	printk(" May be due to missing lock nesting notation\n\n");
+}
+
 static int
 print_deadlock_bug(struct task_struct *curr, struct held_lock *prev,
 		   struct held_lock *next)
@@ -1682,6 +1702,7 @@ print_deadlock_bug(struct task_struct *curr, struct held_lock *prev,
 	print_lock(prev);
 
 	printk("\nother info that might help us debug this:\n");
+	print_deadlock_scenario(next, prev);
 	lockdep_print_held_locks(curr);
 
 	printk("\nstack backtrace:\n");
-- 
1.7.2.3

[PATCH 4/7] lockdep: Printk nice description for irq inversion bug

[Steven Rostedt]

[-- Attachment #1: 0004-lockdep-Printk-nice-description-for-irq-inversion-bu.patch --]
[-- Type: text/plain, Size: 3925 bytes --]

From: Steven Rostedt <srostedt@redhat.com>

The irq inversion and irq dependency bug are only subtly different.
The diffenerence lies where the interrupt occurred.

For irq dependency:

	irq_disable
	lock(A)
	lock(B)
	unlock(B)
	unlock(A)
	irq_enable

	lock(B)
	unlock(B)

 	<interrupt>
	  lock(A)

The interrupt comes in after it has been established that lock A
can be held when taking an irq unsafe lock. Lockdep detects the problem
when taking lock A in interrupt context.

 	<interrupt>
	  lock(A)

	irq_disable
	lock(A)
	lock(B)
	unlock(B)
	unlock(A)
	irq_enable

	lock(B)
	unlock(B)

With the irq_inversion the irq happens before it is established
and lockdep detects the problem with the taking of lock B.

Since the problem with the locking for both of these issues are
in actuality the same, they both should report the same scenario.

other info that might help us debug this:

Chain exists of:
  &rq->lock --> lockA --> lockC

 Possible interrupt unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(lockC);
                               local_irq_disable();
                               lock(&rq->lock);
                               lock(lockA);
  <Interrupt>
    lock(&rq->lock);

 *** DEADLOCK ***

Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
---
 kernel/lockdep.c |   34 +++++++++++++++++++++++++++++-----
 1 files changed, 29 insertions(+), 5 deletions(-)

diff --git a/kernel/lockdep.c b/kernel/lockdep.c
index a0f253b..8646b8c 100644
--- a/kernel/lockdep.c
+++ b/kernel/lockdep.c
@@ -1395,15 +1395,15 @@ print_shortest_lock_dependencies(struct lock_list *leaf,
 static void
 print_irq_lock_scenario(struct lock_list *safe_entry,
 			struct lock_list *unsafe_entry,
-			struct held_lock *prev,
-			struct held_lock *next)
+			struct lock_class *prev_class,
+			struct lock_class *next_class)
 {
 	struct lock_class *safe_class = safe_entry->class;
 	struct lock_class *unsafe_class = unsafe_entry->class;
-	struct lock_class *middle_class = hlock_class(prev);
+	struct lock_class *middle_class = prev_class;
 
 	if (middle_class == safe_class)
-		middle_class = hlock_class(next);
+		middle_class = next_class;
 
 	/*
 	 * A direct locking problem where unsafe_class lock is taken
@@ -1499,7 +1499,8 @@ print_bad_irq_dependency(struct task_struct *curr,
 	print_stack_trace(forwards_entry->class->usage_traces + bit2, 1);
 
 	printk("\nother info that might help us debug this:\n\n");
-	print_irq_lock_scenario(backwards_entry, forwards_entry, prev, next);
+	print_irq_lock_scenario(backwards_entry, forwards_entry,
+				hlock_class(prev), hlock_class(next));
 
 	lockdep_print_held_locks(curr);
 
@@ -2219,6 +2220,10 @@ print_irq_inversion_bug(struct task_struct *curr,
 			struct held_lock *this, int forwards,
 			const char *irqclass)
 {
+	struct lock_list *entry = other;
+	struct lock_list *middle = NULL;
+	int depth;
+
 	if (!debug_locks_off_graph_unlock() || debug_locks_silent)
 		return 0;
 
@@ -2237,6 +2242,25 @@ print_irq_inversion_bug(struct task_struct *curr,
 	printk("\n\nand interrupts could create inverse lock ordering between them.\n\n");
 
 	printk("\nother info that might help us debug this:\n");
+
+	/* Find a middle lock (if one exists) */
+	depth = get_lock_depth(other);
+	do {
+		if (depth == 0 && (entry != root)) {
+			printk("lockdep:%s bad path found in chain graph\n", __func__);
+			break;
+		}
+		middle = entry;
+		entry = get_lock_parent(entry);
+		depth--;
+	} while (entry && entry != root && (depth >= 0));
+	if (forwards)
+		print_irq_lock_scenario(root, other,
+			middle ? middle->class : root->class, other->class);
+	else
+		print_irq_lock_scenario(other, root,
+			middle ? middle->class : other->class, root->class);
+
 	lockdep_print_held_locks(curr);
 
 	printk("\nthe shortest dependencies between 2nd lock and 1st lock:\n");
-- 
1.7.2.3

[PATCH 5/7] lockdep: Replace bad path error message with something sane

[Steven Rostedt]

[-- Attachment #1: 0005-lockdep-Replace-bad-path-error-message-with-somethin.patch --]
[-- Type: text/plain, Size: 829 bytes --]

From: Steven Rostedt <srostedt@redhat.com>

The message of "Bad BFS generated tree" is a bit confusing.
Replace it with a more sane error message.

Thanks to Peter Zijlstra for helping me come up with a better
message.

Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
---
 kernel/lockdep.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/kernel/lockdep.c b/kernel/lockdep.c
index 8646b8c..b531c66 100644
--- a/kernel/lockdep.c
+++ b/kernel/lockdep.c
@@ -1381,7 +1381,7 @@ print_shortest_lock_dependencies(struct lock_list *leaf,
 		printk("\n");
 
 		if (depth == 0 && (entry != root)) {
-			printk("lockdep:%s bad BFS generated tree\n", __func__);
+			printk("lockdep:%s bad path found in chain graph\n", __func__);
 			break;
 		}
 
-- 
1.7.2.3

[PATCH 6/7] lockdep: Print a nice description of simple irq inversion

[Steven Rostedt]

[-- Attachment #1: 0006-lockdep-Print-a-nice-description-of-simple-irq-inver.patch --]
[-- Type: text/plain, Size: 1781 bytes --]

From: Steven Rostedt <srostedt@redhat.com>

The lockdep output can be pretty cryptic, having a nice output
can save a lot of head scratching. When a simple irq inversion scenario
is detected by lockdep (lock A taken in interrupt context but also in
thread context without disabling interrupts) we get the following output:

other info that might help us debug this:
 Possible unsafe locking scenario:

       CPU0
       ----
  lock(lockA);
  <Interrupt>
    lock(lockA);

 *** DEADLOCK ***

Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
---
 kernel/lockdep.c |   20 ++++++++++++++++++++
 1 files changed, 20 insertions(+), 0 deletions(-)

diff --git a/kernel/lockdep.c b/kernel/lockdep.c
index b531c66..3b1ac92 100644
--- a/kernel/lockdep.c
+++ b/kernel/lockdep.c
@@ -2158,6 +2158,24 @@ static void check_chain_key(struct task_struct *curr)
 #endif
 }
 
+static void
+print_usage_bug_scenario(struct held_lock *lock)
+{
+	struct lock_class *class = hlock_class(lock);
+
+	printk(" Possible unsafe locking scenario:\n\n");
+	printk("       CPU0\n");
+	printk("       ----\n");
+	printk("  lock(");
+	__print_lock_name(class);
+	printk(");\n");
+	printk("  <Interrupt>\n");
+	printk("    lock(");
+	__print_lock_name(class);
+	printk(");\n");
+	printk("\n *** DEADLOCK ***\n\n");
+}
+
 static int
 print_usage_bug(struct task_struct *curr, struct held_lock *this,
 		enum lock_usage_bit prev_bit, enum lock_usage_bit new_bit)
@@ -2186,6 +2204,8 @@ print_usage_bug(struct task_struct *curr, struct held_lock *this,
 
 	print_irqtrace_events(curr);
 	printk("\nother info that might help us debug this:\n");
+	print_usage_bug_scenario(this);
+
 	lockdep_print_held_locks(curr);
 
 	printk("\nstack backtrace:\n");
-- 
1.7.2.3

[PATCH 7/7] lockdep: Remove cmpxchg to update nr_chain_hlocks

[Steven Rostedt]

[-- Attachment #1: 0007-lockdep-Remove-cmpxchg-to-update-nr_chain_hlocks.patch --]
[-- Type: text/plain, Size: 1602 bytes --]

From: Steven Rostedt <srostedt@redhat.com>

For some reason nr_chain_hlocks is updated with cmpxchg, but this
is performed inside of the lockdep global "grab_lock()", which also
makes simple modification of this variable atomic.

Remove the cmpxchg logic for updating nr_chain_hlocks and simplify
the code.

Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
---
 kernel/lockdep.c |   14 ++++----------
 1 files changed, 4 insertions(+), 10 deletions(-)

diff --git a/kernel/lockdep.c b/kernel/lockdep.c
index 3b1ac92..60bce52 100644
--- a/kernel/lockdep.c
+++ b/kernel/lockdep.c
@@ -1973,7 +1973,7 @@ static inline int lookup_chain_cache(struct task_struct *curr,
 	struct list_head *hash_head = chainhashentry(chain_key);
 	struct lock_chain *chain;
 	struct held_lock *hlock_curr, *hlock_next;
-	int i, j, n, cn;
+	int i, j;
 
 	if (DEBUG_LOCKS_WARN_ON(!irqs_disabled()))
 		return 0;
@@ -2033,15 +2033,9 @@ cache_hit:
 	}
 	i++;
 	chain->depth = curr->lockdep_depth + 1 - i;
-	cn = nr_chain_hlocks;
-	while (cn + chain->depth <= MAX_LOCKDEP_CHAIN_HLOCKS) {
-		n = cmpxchg(&nr_chain_hlocks, cn, cn + chain->depth);
-		if (n == cn)
-			break;
-		cn = n;
-	}
-	if (likely(cn + chain->depth <= MAX_LOCKDEP_CHAIN_HLOCKS)) {
-		chain->base = cn;
+	if (likely(nr_chain_hlocks + chain->depth <= MAX_LOCKDEP_CHAIN_HLOCKS)) {
+		chain->base = nr_chain_hlocks;
+		nr_chain_hlocks += chain->depth;
 		for (j = 0; j < chain->depth - 1; j++, i++) {
 			int lock_id = curr->held_locks[i].class_idx - 1;
 			chain_hlocks[chain->base + j] = lock_id;
-- 
1.7.2.3

Re: [PATCH 0/7] [GIT PULL] lockdep: Show description of lockdep problems

[Yong Zhang]

On Thu, Apr 21, 2011 at 9:41 AM, Steven Rostedt <rostedt@goodmis.org> wrote:
> Steven Rostedt (7):
>      lockdep: Print a nice description of an irq locking issue
>      lockdep: Print a nice description of normal deadlocks
>      lockdep: Print a nice description of simple deadlock
>      lockdep: Printk nice description for irq inversion bug
>      lockdep: Replace bad path error message with something sane
>      lockdep: Print a nice description of simple irq inversion
>      lockdep: Remove cmpxchg to update nr_chain_hlocks
>
> ----
>  kernel/lockdep.c |  206 +++++++++++++++++++++++++++++++++++++++++++++++++++---
>  1 files changed, 195 insertions(+), 11 deletions(-)

This looks very nice :)

Thanks,
Yong


-- 
Only stand for myself

Re: [PATCH 1/7] lockdep: Print a nice description of an irq locking issue

[Yong Zhang]

On Thu, Apr 21, 2011 at 9:41 AM, Steven Rostedt <rostedt@goodmis.org> wrote:
>  ---
>
> The above is the case when the unsafe lock is taken while holding
> a lock taken in irq context. But when a lock is taken that also
> grabs a unsafe lock, the call chain is shown:
>
>  ---
> other info that might help us debug this:
>
> Chain exists of:
>  &rq->lock --> lockA --> lockC
>
>  Possible interrupt unsafe locking scenario:
>
>       CPU0                    CPU1
>       ----                    ----
>  lock(lockC);
>                               local_irq_disable();
>                               lock(&rq->lock);
>                               lock(lockA);
>  <Interrupt>
>    lock(&rq->lock);
>
>  *** DEADLOCK ***

But this is not a real deadlock, right? Or maybe you can teach me :)

I just assume your scenario should show a more real one, But for
a possible irq-dependence deadlock, it seems not easy to find out.

Thanks,
Yong

>
> Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
> ---
>  kernel/lockdep.c |   70 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 files changed, 70 insertions(+), 0 deletions(-)
>
> diff --git a/kernel/lockdep.c b/kernel/lockdep.c
> index 0d2058d..bb77c030 100644
> --- a/kernel/lockdep.c
> +++ b/kernel/lockdep.c
> @@ -490,6 +490,18 @@ void get_usage_chars(struct lock_class *class, char usage[LOCK_USAGE_CHARS])
>        usage[i] = '\0';
>  }
>
> +static int __print_lock_name(struct lock_class *class)
> +{
> +       char str[KSYM_NAME_LEN];
> +       const char *name;
> +
> +       name = class->name;
> +       if (!name)
> +               name = __get_key_name(class->key, str);
> +
> +       return printk("%s", name);
> +}
> +
>  static void print_lock_name(struct lock_class *class)
>  {
>        char str[KSYM_NAME_LEN], usage[LOCK_USAGE_CHARS];
> @@ -1325,6 +1337,62 @@ print_shortest_lock_dependencies(struct lock_list *leaf,
>        return;
>  }
>
> +static void
> +print_irq_lock_scenario(struct lock_list *safe_entry,
> +                       struct lock_list *unsafe_entry,
> +                       struct held_lock *prev,
> +                       struct held_lock *next)
> +{
> +       struct lock_class *safe_class = safe_entry->class;
> +       struct lock_class *unsafe_class = unsafe_entry->class;
> +       struct lock_class *middle_class = hlock_class(prev);
> +
> +       if (middle_class == safe_class)
> +               middle_class = hlock_class(next);
> +
> +       /*
> +        * A direct locking problem where unsafe_class lock is taken
> +        * directly by safe_class lock, then all we need to show
> +        * is the deadlock scenario, as it is obvious that the
> +        * unsafe lock is taken under the safe lock.
> +        *
> +        * But if there is a chain instead, where the safe lock takes
> +        * an intermediate lock (middle_class) where this lock is
> +        * not the same as the safe lock, then the lock chain is
> +        * used to describe the problem. Otherwise we would need
> +        * to show a different CPU case for each link in the chain
> +        * from the safe_class lock to the unsafe_class lock.
> +        */
> +       if (middle_class != unsafe_class) {
> +               printk("Chain exists of:\n  ");
> +               __print_lock_name(safe_class);
> +               printk(" --> ");
> +               __print_lock_name(middle_class);
> +               printk(" --> ");
> +               __print_lock_name(unsafe_class);
> +               printk("\n\n");
> +       }
> +
> +       printk(" Possible interrupt unsafe locking scenario:\n\n");
> +       printk("       CPU0                    CPU1\n");
> +       printk("       ----                    ----\n");
> +       printk("  lock(");
> +       __print_lock_name(unsafe_class);
> +       printk(");\n");
> +       printk("                               local_irq_disable();\n");
> +       printk("                               lock(");
> +       __print_lock_name(safe_class);
> +       printk(");\n");
> +       printk("                               lock(");
> +       __print_lock_name(middle_class);
> +       printk(");\n");
> +       printk("  <Interrupt>\n");
> +       printk("    lock(");
> +       __print_lock_name(safe_class);
> +       printk(");\n");
> +       printk("\n *** DEADLOCK ***\n\n");
> +}
> +
>  static int
>  print_bad_irq_dependency(struct task_struct *curr,
>                         struct lock_list *prev_root,
> @@ -1376,6 +1444,8 @@ print_bad_irq_dependency(struct task_struct *curr,
>        print_stack_trace(forwards_entry->class->usage_traces + bit2, 1);
>
>        printk("\nother info that might help us debug this:\n\n");
> +       print_irq_lock_scenario(backwards_entry, forwards_entry, prev, next);
> +
>        lockdep_print_held_locks(curr);
>
>        printk("\nthe dependencies between %s-irq-safe lock", irqclass);
> --
> 1.7.2.3
>
>
> --
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>



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Re: [PATCH 7/7] lockdep: Remove cmpxchg to update nr_chain_hlocks

[Yong Zhang]

On Thu, Apr 21, 2011 at 9:42 AM, Steven Rostedt <rostedt@goodmis.org> wrote:
> From: Steven Rostedt <srostedt@redhat.com>
>
> For some reason nr_chain_hlocks is updated with cmpxchg, but this
> is performed inside of the lockdep global "grab_lock()", which also
> makes simple modification of this variable atomic.
>
> Remove the cmpxchg logic for updating nr_chain_hlocks and simplify
> the code.
>
> Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
> ---
>  kernel/lockdep.c |   14 ++++----------
>  1 files changed, 4 insertions(+), 10 deletions(-)
>
> diff --git a/kernel/lockdep.c b/kernel/lockdep.c
> index 3b1ac92..60bce52 100644
> --- a/kernel/lockdep.c
> +++ b/kernel/lockdep.c
> @@ -1973,7 +1973,7 @@ static inline int lookup_chain_cache(struct task_struct *curr,
>        struct list_head *hash_head = chainhashentry(chain_key);
>        struct lock_chain *chain;
>        struct held_lock *hlock_curr, *hlock_next;

Maybe we can also remove hlock_next in this function for some cleanup, because
it's just an alias of hlock.

Thanks,
Yong

> -       int i, j, n, cn;
> +       int i, j;
>
>        if (DEBUG_LOCKS_WARN_ON(!irqs_disabled()))
>                return 0;
> @@ -2033,15 +2033,9 @@ cache_hit:
>        }
>        i++;
>        chain->depth = curr->lockdep_depth + 1 - i;
> -       cn = nr_chain_hlocks;
> -       while (cn + chain->depth <= MAX_LOCKDEP_CHAIN_HLOCKS) {
> -               n = cmpxchg(&nr_chain_hlocks, cn, cn + chain->depth);
> -               if (n == cn)
> -                       break;
> -               cn = n;
> -       }
> -       if (likely(cn + chain->depth <= MAX_LOCKDEP_CHAIN_HLOCKS)) {
> -               chain->base = cn;
> +       if (likely(nr_chain_hlocks + chain->depth <= MAX_LOCKDEP_CHAIN_HLOCKS)) {
> +               chain->base = nr_chain_hlocks;
> +               nr_chain_hlocks += chain->depth;
>                for (j = 0; j < chain->depth - 1; j++, i++) {
>                        int lock_id = curr->held_locks[i].class_idx - 1;
>                        chain_hlocks[chain->base + j] = lock_id;
> --
> 1.7.2.3
>
>
> --
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
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>



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Re: [PATCH 1/7] lockdep: Print a nice description of an irq locking issue

[Yong Zhang]

On Thu, Apr 21, 2011 at 9:41 AM, Steven Rostedt <rostedt@goodmis.org> wrote:
>  ---
>
> The above is the case when the unsafe lock is taken while holding
> a lock taken in irq context. But when a lock is taken that also
> grabs a unsafe lock, the call chain is shown:
>
>  ---
> other info that might help us debug this:
>
> Chain exists of:
>  &rq->lock --> lockA --> lockC
>
>  Possible interrupt unsafe locking scenario:
>
>       CPU0                    CPU1
>       ----                    ----
>  lock(lockC);
>                               local_irq_disable();
>                               lock(&rq->lock);
>                               lock(lockA);
>  <Interrupt>
>    lock(&rq->lock);
>
>  *** DEADLOCK ***

Or we could show this:
Chain exists of:
&rq->lock --> lockA --> lockC

 Possible interrupt unsafe locking scenario:

      CPU0                    CPU1                           CPU2
      ----                    ----                                     ----
 lock(lockC);
                              local_irq_disable();
                              lock(&rq->lock);            lock(lockA);
                              lock(lockA);                   lock(lockC);
 <Interrupt>
   lock(&rq->lock);

 *** DEADLOCK ***

Thanks,
Yong

>
> Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
> ---
>  kernel/lockdep.c |   70 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 files changed, 70 insertions(+), 0 deletions(-)
>
> diff --git a/kernel/lockdep.c b/kernel/lockdep.c
> index 0d2058d..bb77c030 100644
> --- a/kernel/lockdep.c
> +++ b/kernel/lockdep.c
> @@ -490,6 +490,18 @@ void get_usage_chars(struct lock_class *class, char usage[LOCK_USAGE_CHARS])
>        usage[i] = '\0';
>  }
>
> +static int __print_lock_name(struct lock_class *class)
> +{
> +       char str[KSYM_NAME_LEN];
> +       const char *name;
> +
> +       name = class->name;
> +       if (!name)
> +               name = __get_key_name(class->key, str);
> +
> +       return printk("%s", name);
> +}
> +
>  static void print_lock_name(struct lock_class *class)
>  {
>        char str[KSYM_NAME_LEN], usage[LOCK_USAGE_CHARS];
> @@ -1325,6 +1337,62 @@ print_shortest_lock_dependencies(struct lock_list *leaf,
>        return;
>  }
>
> +static void
> +print_irq_lock_scenario(struct lock_list *safe_entry,
> +                       struct lock_list *unsafe_entry,
> +                       struct held_lock *prev,
> +                       struct held_lock *next)
> +{
> +       struct lock_class *safe_class = safe_entry->class;
> +       struct lock_class *unsafe_class = unsafe_entry->class;
> +       struct lock_class *middle_class = hlock_class(prev);
> +
> +       if (middle_class == safe_class)
> +               middle_class = hlock_class(next);
> +
> +       /*
> +        * A direct locking problem where unsafe_class lock is taken
> +        * directly by safe_class lock, then all we need to show
> +        * is the deadlock scenario, as it is obvious that the
> +        * unsafe lock is taken under the safe lock.
> +        *
> +        * But if there is a chain instead, where the safe lock takes
> +        * an intermediate lock (middle_class) where this lock is
> +        * not the same as the safe lock, then the lock chain is
> +        * used to describe the problem. Otherwise we would need
> +        * to show a different CPU case for each link in the chain
> +        * from the safe_class lock to the unsafe_class lock.
> +        */
> +       if (middle_class != unsafe_class) {
> +               printk("Chain exists of:\n  ");
> +               __print_lock_name(safe_class);
> +               printk(" --> ");
> +               __print_lock_name(middle_class);
> +               printk(" --> ");
> +               __print_lock_name(unsafe_class);
> +               printk("\n\n");
> +       }
> +
> +       printk(" Possible interrupt unsafe locking scenario:\n\n");
> +       printk("       CPU0                    CPU1\n");
> +       printk("       ----                    ----\n");
> +       printk("  lock(");
> +       __print_lock_name(unsafe_class);
> +       printk(");\n");
> +       printk("                               local_irq_disable();\n");
> +       printk("                               lock(");
> +       __print_lock_name(safe_class);
> +       printk(");\n");
> +       printk("                               lock(");
> +       __print_lock_name(middle_class);
> +       printk(");\n");
> +       printk("  <Interrupt>\n");
> +       printk("    lock(");
> +       __print_lock_name(safe_class);
> +       printk(");\n");
> +       printk("\n *** DEADLOCK ***\n\n");
> +}
> +
>  static int
>  print_bad_irq_dependency(struct task_struct *curr,
>                         struct lock_list *prev_root,
> @@ -1376,6 +1444,8 @@ print_bad_irq_dependency(struct task_struct *curr,
>        print_stack_trace(forwards_entry->class->usage_traces + bit2, 1);
>
>        printk("\nother info that might help us debug this:\n\n");
> +       print_irq_lock_scenario(backwards_entry, forwards_entry, prev, next);
> +
>        lockdep_print_held_locks(curr);
>
>        printk("\nthe dependencies between %s-irq-safe lock", irqclass);
> --
> 1.7.2.3
>
>
> --
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> the body of a message to majordomo@vger.kernel.org
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>



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Re: [PATCH 1/7] lockdep: Print a nice description of an irq locking issue

[Yong Zhang]

On Thu, Apr 21, 2011 at 3:02 PM, Yong Zhang <yong.zhang0@gmail.com> wrote:
> On Thu, Apr 21, 2011 at 9:41 AM, Steven Rostedt <rostedt@goodmis.org> wrote:
>>  ---
>>
>> The above is the case when the unsafe lock is taken while holding
>> a lock taken in irq context. But when a lock is taken that also
>> grabs a unsafe lock, the call chain is shown:
>>
>>  ---
>> other info that might help us debug this:
>>
>> Chain exists of:
>>  &rq->lock --> lockA --> lockC
>>
>>  Possible interrupt unsafe locking scenario:
>>
>>       CPU0                    CPU1
>>       ----                    ----
>>  lock(lockC);
>>                               local_irq_disable();
>>                               lock(&rq->lock);
>>                               lock(lockA);
>>  <Interrupt>
>>    lock(&rq->lock);
>>
>>  *** DEADLOCK ***
>
> Or we could show this:
> Chain exists of:
> &rq->lock --> lockA --> lockC
>
>  Possible interrupt unsafe locking scenario:
>
>      CPU0                    CPU1                           CPU2
>      ----                    ----                                     ----
>  lock(lockC);
>                              local_irq_disable();

                                                                Forget
 local_irq_disable(); here :)

>                              lock(&rq->lock);            lock(lockA);
>                              lock(lockA);                   lock(lockC);
>  <Interrupt>
>   lock(&rq->lock);
>
>  *** DEADLOCK ***
>
> Thanks,
> Yong
>
>>
>> Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
>> Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
>> ---
>>  kernel/lockdep.c |   70 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
>>  1 files changed, 70 insertions(+), 0 deletions(-)
>>
>> diff --git a/kernel/lockdep.c b/kernel/lockdep.c
>> index 0d2058d..bb77c030 100644
>> --- a/kernel/lockdep.c
>> +++ b/kernel/lockdep.c
>> @@ -490,6 +490,18 @@ void get_usage_chars(struct lock_class *class, char usage[LOCK_USAGE_CHARS])
>>        usage[i] = '\0';
>>  }
>>
>> +static int __print_lock_name(struct lock_class *class)
>> +{
>> +       char str[KSYM_NAME_LEN];
>> +       const char *name;
>> +
>> +       name = class->name;
>> +       if (!name)
>> +               name = __get_key_name(class->key, str);
>> +
>> +       return printk("%s", name);
>> +}
>> +
>>  static void print_lock_name(struct lock_class *class)
>>  {
>>        char str[KSYM_NAME_LEN], usage[LOCK_USAGE_CHARS];
>> @@ -1325,6 +1337,62 @@ print_shortest_lock_dependencies(struct lock_list *leaf,
>>        return;
>>  }
>>
>> +static void
>> +print_irq_lock_scenario(struct lock_list *safe_entry,
>> +                       struct lock_list *unsafe_entry,
>> +                       struct held_lock *prev,
>> +                       struct held_lock *next)
>> +{
>> +       struct lock_class *safe_class = safe_entry->class;
>> +       struct lock_class *unsafe_class = unsafe_entry->class;
>> +       struct lock_class *middle_class = hlock_class(prev);
>> +
>> +       if (middle_class == safe_class)
>> +               middle_class = hlock_class(next);
>> +
>> +       /*
>> +        * A direct locking problem where unsafe_class lock is taken
>> +        * directly by safe_class lock, then all we need to show
>> +        * is the deadlock scenario, as it is obvious that the
>> +        * unsafe lock is taken under the safe lock.
>> +        *
>> +        * But if there is a chain instead, where the safe lock takes
>> +        * an intermediate lock (middle_class) where this lock is
>> +        * not the same as the safe lock, then the lock chain is
>> +        * used to describe the problem. Otherwise we would need
>> +        * to show a different CPU case for each link in the chain
>> +        * from the safe_class lock to the unsafe_class lock.
>> +        */
>> +       if (middle_class != unsafe_class) {
>> +               printk("Chain exists of:\n  ");
>> +               __print_lock_name(safe_class);
>> +               printk(" --> ");
>> +               __print_lock_name(middle_class);
>> +               printk(" --> ");
>> +               __print_lock_name(unsafe_class);
>> +               printk("\n\n");
>> +       }
>> +
>> +       printk(" Possible interrupt unsafe locking scenario:\n\n");
>> +       printk("       CPU0                    CPU1\n");
>> +       printk("       ----                    ----\n");
>> +       printk("  lock(");
>> +       __print_lock_name(unsafe_class);
>> +       printk(");\n");
>> +       printk("                               local_irq_disable();\n");
>> +       printk("                               lock(");
>> +       __print_lock_name(safe_class);
>> +       printk(");\n");
>> +       printk("                               lock(");
>> +       __print_lock_name(middle_class);
>> +       printk(");\n");
>> +       printk("  <Interrupt>\n");
>> +       printk("    lock(");
>> +       __print_lock_name(safe_class);
>> +       printk(");\n");
>> +       printk("\n *** DEADLOCK ***\n\n");
>> +}
>> +
>>  static int
>>  print_bad_irq_dependency(struct task_struct *curr,
>>                         struct lock_list *prev_root,
>> @@ -1376,6 +1444,8 @@ print_bad_irq_dependency(struct task_struct *curr,
>>        print_stack_trace(forwards_entry->class->usage_traces + bit2, 1);
>>
>>        printk("\nother info that might help us debug this:\n\n");
>> +       print_irq_lock_scenario(backwards_entry, forwards_entry, prev, next);
>> +
>>        lockdep_print_held_locks(curr);
>>
>>        printk("\nthe dependencies between %s-irq-safe lock", irqclass);
>> --
>> 1.7.2.3
>>
>>
>> --
>> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
>> the body of a message to majordomo@vger.kernel.org
>> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>> Please read the FAQ at  http://www.tux.org/lkml/
>>
>
>
>
> --
> Only stand for myself
>



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Re: [PATCH 1/7] lockdep: Print a nice description of an irq locking issue

[Steven Rostedt]

On Thu, 2011-04-21 at 15:02 +0800, Yong Zhang wrote:
> On Thu, Apr 21, 2011 at 9:41 AM, Steven Rostedt <rostedt@goodmis.org> wrote:
> >  ---
> >
> > The above is the case when the unsafe lock is taken while holding
> > a lock taken in irq context. But when a lock is taken that also
> > grabs a unsafe lock, the call chain is shown:
> >
> >  ---
> > other info that might help us debug this:
> >
> > Chain exists of:
> >  &rq->lock --> lockA --> lockC
> >
> >  Possible interrupt unsafe locking scenario:
> >
> >       CPU0                    CPU1
> >       ----                    ----
> >  lock(lockC);
> >                               local_irq_disable();
> >                               lock(&rq->lock);
> >                               lock(lockA);
> >  <Interrupt>
> >    lock(&rq->lock);
> >
> >  *** DEADLOCK ***
> 
> Or we could show this:
> Chain exists of:
> &rq->lock --> lockA --> lockC
> 
>  Possible interrupt unsafe locking scenario:
> 
>       CPU0                    CPU1                           CPU2
>       ----                    ----                                     ----
>  lock(lockC);
>                               local_irq_disable();
>                               lock(&rq->lock);            lock(lockA);
>                               lock(lockA);                   lock(lockC);
>  <Interrupt>
>    lock(&rq->lock);
> 
>  *** DEADLOCK ***


We could but I prefer not to ;) We have some chains that are 8 locks
deep. I really don't want to scatter that entirely across the screen.
Hence my "Chain exists.." statement, following an example that any
kernel developer can (with a little thinking) see is a possible
deadlock.

In fact, this code doesn't even look at the full chain, it only examines
3 locks in the chain, and lets the developer figure out the rest. I
could show the entire chain too.

Thanks,

-- Steve

Re: [PATCH 1/7] lockdep: Print a nice description of an irq locking issue

[Yong Zhang]

On Thu, Apr 21, 2011 at 07:40:29AM -0400, Steven Rostedt wrote:
> On Thu, 2011-04-21 at 15:02 +0800, Yong Zhang wrote:
> > On Thu, Apr 21, 2011 at 9:41 AM, Steven Rostedt <rostedt@goodmis.org> wrote:
> > >  ---
> > >
> > > The above is the case when the unsafe lock is taken while holding
> > > a lock taken in irq context. But when a lock is taken that also
> > > grabs a unsafe lock, the call chain is shown:
> > >
> > >  ---
> > > other info that might help us debug this:
> > >
> > > Chain exists of:
> > >  &rq->lock --> lockA --> lockC
> > >
> > >  Possible interrupt unsafe locking scenario:
> > >
> > >       CPU0                    CPU1
> > >       ----                    ----
> > >  lock(lockC);
> > >                               local_irq_disable();
> > >                               lock(&rq->lock);
> > >                               lock(lockA);
> > >  <Interrupt>
> > >    lock(&rq->lock);
> > >
> > >  *** DEADLOCK ***
> > 
> > Or we could show this:
> > Chain exists of:
> > &rq->lock --> lockA --> lockC
> > 
> >  Possible interrupt unsafe locking scenario:
> > 
> >       CPU0                    CPU1                           CPU2
> >       ----                    ----                                     ----
> >  lock(lockC);
> >                               local_irq_disable();
> >                               lock(&rq->lock);            lock(lockA);
> >                               lock(lockA);                   lock(lockC);
> >  <Interrupt>
> >    lock(&rq->lock);
> > 
> >  *** DEADLOCK ***
> 
> 
> We could but I prefer not to ;) We have some chains that are 8 locks
> deep. I really don't want to scatter that entirely across the screen.
> Hence my "Chain exists.." statement, following an example that any
> kernel developer can (with a little thinking) see is a possible
> deadlock.

Yup :)

> 
> In fact, this code doesn't even look at the full chain, it only examines
> 3 locks in the chain, and lets the developer figure out the rest.

OK, fair enough.

> I
> could show the entire chain too.

Sure :)

Thanks,
Yong

Re: [PATCH 1/7] lockdep: Print a nice description of an irq locking issue

[Steven Rostedt]

On Thu, 2011-04-21 at 21:35 +0800, Yong Zhang wrote:
> On Thu, Apr 21, 2011 at 07:40:29AM -0400, Steven Rostedt wrote:

> > I
> > could show the entire chain too.
> 
> Sure :)

I talked a little about this with Peter Zijlstra on IRC, and we agreed
that we'll keep this as is until we notice that kernel developers are
still confused with the output.

The more advanced I make this output, the more likely I'll make a
mistake and show something that is not what actually happens. I wrote a
module to trigger each of these cases and the problems that lockdep can
detect grows exponentially as we make the output more specific.

I probably should modify the lockdep selftests to add an option to show
the output as well as it triggers errors. This would allow me to make
sure the output still works.

But for now, I have other things to work on ;)

-- Steve

Re: [PATCH 1/7] lockdep: Print a nice description of an irq locking issue

[Yong Zhang]

On Thu, Apr 21, 2011 at 10:24 PM, Steven Rostedt <rostedt@goodmis.org> wrote:
> On Thu, 2011-04-21 at 21:35 +0800, Yong Zhang wrote:
>> On Thu, Apr 21, 2011 at 07:40:29AM -0400, Steven Rostedt wrote:
>
>> > I
>> > could show the entire chain too.
>>
>> Sure :)
>
> I talked a little about this with Peter Zijlstra on IRC, and we agreed
> that we'll keep this as is until we notice that kernel developers are
> still confused with the output.

OK :)

>
> The more advanced I make this output, the more likely I'll make a
> mistake and show something that is not what actually happens. I wrote a
> module to trigger each of these cases and the problems that lockdep can
> detect grows exponentially as we make the output more specific.
>
> I probably should modify the lockdep selftests to add an option to show
> the output as well as it triggers errors. This would allow me to make
> sure the output still works.

This does make sense to keep things consistent if we have it.

>
> But for now, I have other things to work on ;)

If you want, you can show more details on your ideas,
maybe I could get some time to finish it :)

Thanks,
Yong



-- 
Only stand for myself

Re: [PATCH 1/7] lockdep: Print a nice description of an irq locking issue

[Steven Rostedt]

On Fri, 2011-04-22 at 09:41 +0800, Yong Zhang wrote:

> > I probably should modify the lockdep selftests to add an option to show
> > the output as well as it triggers errors. This would allow me to make
> > sure the output still works.
> 
> This does make sense to keep things consistent if we have it.

Actually, Peter informed me that we already do this. Just need to boot
with debug_locks_verbose=1.

> 
> >
> > But for now, I have other things to work on ;)
> 
> If you want, you can show more details on your ideas,
> maybe I could get some time to finish it :)

Well, I got pretty much what I wanted out for now. I'll probably go and
look later. But by all means, if you find other ways to make lockdep
more readable. Please, don't let me stop you ;)

-- Steve

Re: [PATCH 1/7] lockdep: Print a nice description of an irq locking issue

[Ingo Molnar]

* Steven Rostedt <rostedt@goodmis.org> wrote:

> On Fri, 2011-04-22 at 09:41 +0800, Yong Zhang wrote:
> 
> > > I probably should modify the lockdep selftests to add an option to show
> > > the output as well as it triggers errors. This would allow me to make
> > > sure the output still works.
> > 
> > This does make sense to keep things consistent if we have it.
> 
> Actually, Peter informed me that we already do this. Just need to boot
> with debug_locks_verbose=1.

Yeah, that's been in lockdep since v1, that's how i prettified the original 
lockdep output.

Thanks,

	Ingo

[tip:core/locking] lockdep: Print a nicer description for irq lock inversions

[tip-bot for Steven Rostedt]

Commit-ID:  3003eba313dd0e0502dd71548c36fe7c19801ce5
Gitweb:     http://git.kernel.org/tip/3003eba313dd0e0502dd71548c36fe7c19801ce5
Author:     Steven Rostedt <srostedt@redhat.com>
AuthorDate: Wed, 20 Apr 2011 21:41:54 -0400
Committer:  Ingo Molnar <mingo@elte.hu>
CommitDate: Fri, 22 Apr 2011 11:06:57 +0200

lockdep: Print a nicer description for irq lock inversions

Locking order inversion due to interrupts is a subtle problem.

When an irq lockiinversion discovered by lockdep it currently
reports something like:

[ INFO: HARDIRQ-safe -> HARDIRQ-unsafe lock order detected ]

... and then prints out the locks that are involved, as back traces.

Judging by lkml feedback developers were routinely confused by what
a HARDIRQ->safe to unsafe issue is all about, and sometimes even
blew it off as a bug in lockdep.

It is not obvious when lockdep prints this message about a lock that
is never taken in interrupt context.

After explaining the problems that lockdep is reporting, I
decided to add a description of the problem in visual form. Now
the following is shown:

 ---
other info that might help us debug this:

 Possible interrupt unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(lockA);
                               local_irq_disable();
                               lock(&rq->lock);
                               lock(lockA);
  <Interrupt>
    lock(&rq->lock);

 *** DEADLOCK ***

 ---

The above is the case when the unsafe lock is taken while
holding a lock taken in irq context. But when a lock is taken
that also grabs a unsafe lock, the call chain is shown:

 ---
other info that might help us debug this:

Chain exists of:
  &rq->lock --> lockA --> lockC

 Possible interrupt unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(lockC);
                               local_irq_disable();
                               lock(&rq->lock);
                               lock(lockA);
  <Interrupt>
    lock(&rq->lock);

 *** DEADLOCK ***

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/20110421014259.132728798@goodmis.org
Signed-off-by: Ingo Molnar <mingo@elte.hu>
---
 kernel/lockdep.c |   70 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 files changed, 70 insertions(+), 0 deletions(-)

diff --git a/kernel/lockdep.c b/kernel/lockdep.c
index 53a6895..7b2ffee 100644
--- a/kernel/lockdep.c
+++ b/kernel/lockdep.c
@@ -490,6 +490,18 @@ void get_usage_chars(struct lock_class *class, char usage[LOCK_USAGE_CHARS])
 	usage[i] = '\0';
 }
 
+static int __print_lock_name(struct lock_class *class)
+{
+	char str[KSYM_NAME_LEN];
+	const char *name;
+
+	name = class->name;
+	if (!name)
+		name = __get_key_name(class->key, str);
+
+	return printk("%s", name);
+}
+
 static void print_lock_name(struct lock_class *class)
 {
 	char str[KSYM_NAME_LEN], usage[LOCK_USAGE_CHARS];
@@ -1325,6 +1337,62 @@ print_shortest_lock_dependencies(struct lock_list *leaf,
 	return;
 }
 
+static void
+print_irq_lock_scenario(struct lock_list *safe_entry,
+			struct lock_list *unsafe_entry,
+			struct held_lock *prev,
+			struct held_lock *next)
+{
+	struct lock_class *safe_class = safe_entry->class;
+	struct lock_class *unsafe_class = unsafe_entry->class;
+	struct lock_class *middle_class = hlock_class(prev);
+
+	if (middle_class == safe_class)
+		middle_class = hlock_class(next);
+
+	/*
+	 * A direct locking problem where unsafe_class lock is taken
+	 * directly by safe_class lock, then all we need to show
+	 * is the deadlock scenario, as it is obvious that the
+	 * unsafe lock is taken under the safe lock.
+	 *
+	 * But if there is a chain instead, where the safe lock takes
+	 * an intermediate lock (middle_class) where this lock is
+	 * not the same as the safe lock, then the lock chain is
+	 * used to describe the problem. Otherwise we would need
+	 * to show a different CPU case for each link in the chain
+	 * from the safe_class lock to the unsafe_class lock.
+	 */
+	if (middle_class != unsafe_class) {
+		printk("Chain exists of:\n  ");
+		__print_lock_name(safe_class);
+		printk(" --> ");
+		__print_lock_name(middle_class);
+		printk(" --> ");
+		__print_lock_name(unsafe_class);
+		printk("\n\n");
+	}
+
+	printk(" Possible interrupt unsafe locking scenario:\n\n");
+	printk("       CPU0                    CPU1\n");
+	printk("       ----                    ----\n");
+	printk("  lock(");
+	__print_lock_name(unsafe_class);
+	printk(");\n");
+	printk("                               local_irq_disable();\n");
+	printk("                               lock(");
+	__print_lock_name(safe_class);
+	printk(");\n");
+	printk("                               lock(");
+	__print_lock_name(middle_class);
+	printk(");\n");
+	printk("  <Interrupt>\n");
+	printk("    lock(");
+	__print_lock_name(safe_class);
+	printk(");\n");
+	printk("\n *** DEADLOCK ***\n\n");
+}
+
 static int
 print_bad_irq_dependency(struct task_struct *curr,
 			 struct lock_list *prev_root,
@@ -1376,6 +1444,8 @@ print_bad_irq_dependency(struct task_struct *curr,
 	print_stack_trace(forwards_entry->class->usage_traces + bit2, 1);
 
 	printk("\nother info that might help us debug this:\n\n");
+	print_irq_lock_scenario(backwards_entry, forwards_entry, prev, next);
+
 	lockdep_print_held_locks(curr);
 
 	printk("\nthe dependencies between %s-irq-safe lock", irqclass);

[tip:core/locking] lockdep: Print a nicer description for normal deadlocks

[tip-bot for Steven Rostedt]

Commit-ID:  f4185812aa046ecb97e8817e10148cacdd7a6baa
Gitweb:     http://git.kernel.org/tip/f4185812aa046ecb97e8817e10148cacdd7a6baa
Author:     Steven Rostedt <srostedt@redhat.com>
AuthorDate: Wed, 20 Apr 2011 21:41:55 -0400
Committer:  Ingo Molnar <mingo@elte.hu>
CommitDate: Fri, 22 Apr 2011 11:06:57 +0200

lockdep: Print a nicer description for normal deadlocks

The lockdep output can be pretty cryptic, having nicer output
can save a lot of head scratching. When a normal deadlock
scenario is detected by lockdep (lock A -> lock B and there
exists a place where lock B -> lock A) we now get the following
new output:

other info that might help us debug this:

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(lockB);
                               lock(lockA);
                               lock(lockB);
  lock(lockA);

 *** DEADLOCK ***

On cases where there's a deeper chair, it shows the partial
chain that can cause the issue:

Chain exists of:
  lockC --> lockA --> lockB

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(lockB);
                               lock(lockA);
                               lock(lockB);
  lock(lockC);

 *** DEADLOCK ***

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/20110421014259.380621789@goodmis.org
Signed-off-by: Ingo Molnar <mingo@elte.hu>
---
 kernel/lockdep.c |   55 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 files changed, 55 insertions(+), 0 deletions(-)

diff --git a/kernel/lockdep.c b/kernel/lockdep.c
index 7b2ffee..73cebd7 100644
--- a/kernel/lockdep.c
+++ b/kernel/lockdep.c
@@ -1065,6 +1065,56 @@ print_circular_bug_entry(struct lock_list *target, int depth)
 	return 0;
 }
 
+static void
+print_circular_lock_scenario(struct held_lock *src,
+			     struct held_lock *tgt,
+			     struct lock_list *prt)
+{
+	struct lock_class *source = hlock_class(src);
+	struct lock_class *target = hlock_class(tgt);
+	struct lock_class *parent = prt->class;
+
+	/*
+	 * A direct locking problem where unsafe_class lock is taken
+	 * directly by safe_class lock, then all we need to show
+	 * is the deadlock scenario, as it is obvious that the
+	 * unsafe lock is taken under the safe lock.
+	 *
+	 * But if there is a chain instead, where the safe lock takes
+	 * an intermediate lock (middle_class) where this lock is
+	 * not the same as the safe lock, then the lock chain is
+	 * used to describe the problem. Otherwise we would need
+	 * to show a different CPU case for each link in the chain
+	 * from the safe_class lock to the unsafe_class lock.
+	 */
+	if (parent != source) {
+		printk("Chain exists of:\n  ");
+		__print_lock_name(source);
+		printk(" --> ");
+		__print_lock_name(parent);
+		printk(" --> ");
+		__print_lock_name(target);
+		printk("\n\n");
+	}
+
+	printk(" Possible unsafe locking scenario:\n\n");
+	printk("       CPU0                    CPU1\n");
+	printk("       ----                    ----\n");
+	printk("  lock(");
+	__print_lock_name(target);
+	printk(");\n");
+	printk("                               lock(");
+	__print_lock_name(parent);
+	printk(");\n");
+	printk("                               lock(");
+	__print_lock_name(target);
+	printk(");\n");
+	printk("  lock(");
+	__print_lock_name(source);
+	printk(");\n");
+	printk("\n *** DEADLOCK ***\n\n");
+}
+
 /*
  * When a circular dependency is detected, print the
  * header first:
@@ -1108,6 +1158,7 @@ static noinline int print_circular_bug(struct lock_list *this,
 {
 	struct task_struct *curr = current;
 	struct lock_list *parent;
+	struct lock_list *first_parent;
 	int depth;
 
 	if (!debug_locks_off_graph_unlock() || debug_locks_silent)
@@ -1121,6 +1172,7 @@ static noinline int print_circular_bug(struct lock_list *this,
 	print_circular_bug_header(target, depth, check_src, check_tgt);
 
 	parent = get_lock_parent(target);
+	first_parent = parent;
 
 	while (parent) {
 		print_circular_bug_entry(parent, --depth);
@@ -1128,6 +1180,9 @@ static noinline int print_circular_bug(struct lock_list *this,
 	}
 
 	printk("\nother info that might help us debug this:\n\n");
+	print_circular_lock_scenario(check_src, check_tgt,
+				     first_parent);
+
 	lockdep_print_held_locks(curr);
 
 	printk("\nstack backtrace:\n");

[tip:core/locking] lockdep: Print a nicer description for simple deadlocks

[tip-bot for Steven Rostedt]

Commit-ID:  48702ecf308e53f176c1f6fdc193d622ded54ac0
Gitweb:     http://git.kernel.org/tip/48702ecf308e53f176c1f6fdc193d622ded54ac0
Author:     Steven Rostedt <srostedt@redhat.com>
AuthorDate: Wed, 20 Apr 2011 21:41:56 -0400
Committer:  Ingo Molnar <mingo@elte.hu>
CommitDate: Fri, 22 Apr 2011 11:06:58 +0200

lockdep: Print a nicer description for simple deadlocks

Lockdep output can be pretty cryptic, having nicer output
can save a lot of head scratching. When a simple deadlock
scenario is detected by lockdep (lock A -> lock A) we now
get the following new output:

other info that might help us debug this:
 Possible unsafe locking scenario:

       CPU0
       ----
  lock(&(lock)->rlock);
  lock(&(lock)->rlock);

 *** DEADLOCK ***

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/20110421014259.643930104@goodmis.org
Signed-off-by: Ingo Molnar <mingo@elte.hu>
---
 kernel/lockdep.c |   21 +++++++++++++++++++++
 1 files changed, 21 insertions(+), 0 deletions(-)

diff --git a/kernel/lockdep.c b/kernel/lockdep.c
index 73cebd7..c4cc5d1 100644
--- a/kernel/lockdep.c
+++ b/kernel/lockdep.c
@@ -1664,6 +1664,26 @@ static inline void inc_chains(void)
 
 #endif
 
+static void
+print_deadlock_scenario(struct held_lock *nxt,
+			     struct held_lock *prv)
+{
+	struct lock_class *next = hlock_class(nxt);
+	struct lock_class *prev = hlock_class(prv);
+
+	printk(" Possible unsafe locking scenario:\n\n");
+	printk("       CPU0\n");
+	printk("       ----\n");
+	printk("  lock(");
+	__print_lock_name(prev);
+	printk(");\n");
+	printk("  lock(");
+	__print_lock_name(next);
+	printk(");\n");
+	printk("\n *** DEADLOCK ***\n\n");
+	printk(" May be due to missing lock nesting notation\n\n");
+}
+
 static int
 print_deadlock_bug(struct task_struct *curr, struct held_lock *prev,
 		   struct held_lock *next)
@@ -1682,6 +1702,7 @@ print_deadlock_bug(struct task_struct *curr, struct held_lock *prev,
 	print_lock(prev);
 
 	printk("\nother info that might help us debug this:\n");
+	print_deadlock_scenario(next, prev);
 	lockdep_print_held_locks(curr);
 
 	printk("\nstack backtrace:\n");

[tip:core/locking] lockdep: Print a nicer description for irq inversion bugs

[tip-bot for Steven Rostedt]

Commit-ID:  dad3d7435e1d8c254d6877dc06852dc00c5da812
Gitweb:     http://git.kernel.org/tip/dad3d7435e1d8c254d6877dc06852dc00c5da812
Author:     Steven Rostedt <srostedt@redhat.com>
AuthorDate: Wed, 20 Apr 2011 21:41:57 -0400
Committer:  Ingo Molnar <mingo@elte.hu>
CommitDate: Fri, 22 Apr 2011 11:06:58 +0200

lockdep: Print a nicer description for irq inversion bugs

Irq inversion and irq dependency bugs are only subtly
different. The diffenerence lies where the interrupt occurred.

For irq dependency:

	irq_disable
	lock(A)
	lock(B)
	unlock(B)
	unlock(A)
	irq_enable

	lock(B)
	unlock(B)

 	<interrupt>
	  lock(A)

The interrupt comes in after it has been established that lock A
can be held when taking an irq unsafe lock. Lockdep detects the
problem when taking lock A in interrupt context.

With the irq_inversion the irq happens before it is established
and lockdep detects the problem with the taking of lock B:

 	<interrupt>
	  lock(A)

	irq_disable
	lock(A)
	lock(B)
	unlock(B)
	unlock(A)
	irq_enable

	lock(B)
	unlock(B)

Since the problem with the locking logic for both of these issues
is in actuality the same, they both should report the same scenario.
This patch implements that and prints this:

other info that might help us debug this:

Chain exists of:
  &rq->lock --> lockA --> lockC

 Possible interrupt unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(lockC);
                               local_irq_disable();
                               lock(&rq->lock);
                               lock(lockA);
  <Interrupt>
    lock(&rq->lock);

 *** DEADLOCK ***

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/20110421014259.910720381@goodmis.org
Signed-off-by: Ingo Molnar <mingo@elte.hu>
---
 kernel/lockdep.c |   34 +++++++++++++++++++++++++++++-----
 1 files changed, 29 insertions(+), 5 deletions(-)

diff --git a/kernel/lockdep.c b/kernel/lockdep.c
index c4cc5d1..0b497dd 100644
--- a/kernel/lockdep.c
+++ b/kernel/lockdep.c
@@ -1395,15 +1395,15 @@ print_shortest_lock_dependencies(struct lock_list *leaf,
 static void
 print_irq_lock_scenario(struct lock_list *safe_entry,
 			struct lock_list *unsafe_entry,
-			struct held_lock *prev,
-			struct held_lock *next)
+			struct lock_class *prev_class,
+			struct lock_class *next_class)
 {
 	struct lock_class *safe_class = safe_entry->class;
 	struct lock_class *unsafe_class = unsafe_entry->class;
-	struct lock_class *middle_class = hlock_class(prev);
+	struct lock_class *middle_class = prev_class;
 
 	if (middle_class == safe_class)
-		middle_class = hlock_class(next);
+		middle_class = next_class;
 
 	/*
 	 * A direct locking problem where unsafe_class lock is taken
@@ -1499,7 +1499,8 @@ print_bad_irq_dependency(struct task_struct *curr,
 	print_stack_trace(forwards_entry->class->usage_traces + bit2, 1);
 
 	printk("\nother info that might help us debug this:\n\n");
-	print_irq_lock_scenario(backwards_entry, forwards_entry, prev, next);
+	print_irq_lock_scenario(backwards_entry, forwards_entry,
+				hlock_class(prev), hlock_class(next));
 
 	lockdep_print_held_locks(curr);
 
@@ -2219,6 +2220,10 @@ print_irq_inversion_bug(struct task_struct *curr,
 			struct held_lock *this, int forwards,
 			const char *irqclass)
 {
+	struct lock_list *entry = other;
+	struct lock_list *middle = NULL;
+	int depth;
+
 	if (!debug_locks_off_graph_unlock() || debug_locks_silent)
 		return 0;
 
@@ -2237,6 +2242,25 @@ print_irq_inversion_bug(struct task_struct *curr,
 	printk("\n\nand interrupts could create inverse lock ordering between them.\n\n");
 
 	printk("\nother info that might help us debug this:\n");
+
+	/* Find a middle lock (if one exists) */
+	depth = get_lock_depth(other);
+	do {
+		if (depth == 0 && (entry != root)) {
+			printk("lockdep:%s bad path found in chain graph\n", __func__);
+			break;
+		}
+		middle = entry;
+		entry = get_lock_parent(entry);
+		depth--;
+	} while (entry && entry != root && (depth >= 0));
+	if (forwards)
+		print_irq_lock_scenario(root, other,
+			middle ? middle->class : root->class, other->class);
+	else
+		print_irq_lock_scenario(other, root,
+			middle ? middle->class : other->class, root->class);
+
 	lockdep_print_held_locks(curr);
 
 	printk("\nthe shortest dependencies between 2nd lock and 1st lock:\n");

[tip:core/locking] lockdep: Replace "Bad BFS generated tree" message with something less cryptic

[tip-bot for Steven Rostedt]

Commit-ID:  6be8c3935b914dfbc24b27c91c2b6d583645e61a
Gitweb:     http://git.kernel.org/tip/6be8c3935b914dfbc24b27c91c2b6d583645e61a
Author:     Steven Rostedt <srostedt@redhat.com>
AuthorDate: Wed, 20 Apr 2011 21:41:58 -0400
Committer:  Ingo Molnar <mingo@elte.hu>
CommitDate: Fri, 22 Apr 2011 11:06:59 +0200

lockdep: Replace "Bad BFS generated tree" message with something less cryptic

The message of "Bad BFS generated tree" is a bit confusing.
Replace it with a more sane error message.

Thanks to Peter Zijlstra for helping me come up with a better
message.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/20110421014300.135521252@goodmis.org
Signed-off-by: Ingo Molnar <mingo@elte.hu>
---
 kernel/lockdep.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/kernel/lockdep.c b/kernel/lockdep.c
index 0b497dd..270cfa4 100644
--- a/kernel/lockdep.c
+++ b/kernel/lockdep.c
@@ -1381,7 +1381,7 @@ print_shortest_lock_dependencies(struct lock_list *leaf,
 		printk("\n");
 
 		if (depth == 0 && (entry != root)) {
-			printk("lockdep:%s bad BFS generated tree\n", __func__);
+			printk("lockdep:%s bad path found in chain graph\n", __func__);
 			break;
 		}
 

[tip:core/locking] lockdep: Print a nicer description for simple irq lock inversions

[tip-bot for Steven Rostedt]

Commit-ID:  282b5c2f6f663c008444321fd8fcdd374596046b
Gitweb:     http://git.kernel.org/tip/282b5c2f6f663c008444321fd8fcdd374596046b
Author:     Steven Rostedt <srostedt@redhat.com>
AuthorDate: Wed, 20 Apr 2011 21:41:59 -0400
Committer:  Ingo Molnar <mingo@elte.hu>
CommitDate: Fri, 22 Apr 2011 11:06:59 +0200

lockdep: Print a nicer description for simple irq lock inversions

Lockdep output can be pretty cryptic, having nicer output
can save a lot of head scratching. When a simple irq inversion
scenario is detected by lockdep (lock A taken in interrupt
context but also in thread context without disabling interrupts)
we now get the following (hopefully more informative) output:

other info that might help us debug this:
 Possible unsafe locking scenario:

       CPU0
       ----
  lock(lockA);
  <Interrupt>
    lock(lockA);

 *** DEADLOCK ***

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/20110421014300.436140880@goodmis.org
Signed-off-by: Ingo Molnar <mingo@elte.hu>
---
 kernel/lockdep.c |   20 ++++++++++++++++++++
 1 files changed, 20 insertions(+), 0 deletions(-)

diff --git a/kernel/lockdep.c b/kernel/lockdep.c
index 270cfa4..27c609f 100644
--- a/kernel/lockdep.c
+++ b/kernel/lockdep.c
@@ -2158,6 +2158,24 @@ static void check_chain_key(struct task_struct *curr)
 #endif
 }
 
+static void
+print_usage_bug_scenario(struct held_lock *lock)
+{
+	struct lock_class *class = hlock_class(lock);
+
+	printk(" Possible unsafe locking scenario:\n\n");
+	printk("       CPU0\n");
+	printk("       ----\n");
+	printk("  lock(");
+	__print_lock_name(class);
+	printk(");\n");
+	printk("  <Interrupt>\n");
+	printk("    lock(");
+	__print_lock_name(class);
+	printk(");\n");
+	printk("\n *** DEADLOCK ***\n\n");
+}
+
 static int
 print_usage_bug(struct task_struct *curr, struct held_lock *this,
 		enum lock_usage_bit prev_bit, enum lock_usage_bit new_bit)
@@ -2186,6 +2204,8 @@ print_usage_bug(struct task_struct *curr, struct held_lock *this,
 
 	print_irqtrace_events(curr);
 	printk("\nother info that might help us debug this:\n");
+	print_usage_bug_scenario(this);
+
 	lockdep_print_held_locks(curr);
 
 	printk("\nstack backtrace:\n");

[tip:core/locking] lockdep: Remove cmpxchg to update nr_chain_hlocks

[tip-bot for Steven Rostedt]

Commit-ID:  e0944ee63f7249802be74454cef81c97630ae1cd
Gitweb:     http://git.kernel.org/tip/e0944ee63f7249802be74454cef81c97630ae1cd
Author:     Steven Rostedt <srostedt@redhat.com>
AuthorDate: Wed, 20 Apr 2011 21:42:00 -0400
Committer:  Ingo Molnar <mingo@elte.hu>
CommitDate: Fri, 22 Apr 2011 11:06:59 +0200

lockdep: Remove cmpxchg to update nr_chain_hlocks

For some reason nr_chain_hlocks is updated with cmpxchg, but
this is performed inside of the lockdep global "grab_lock()",
which also makes simple modification of this variable atomic.

Remove the cmpxchg logic for updating nr_chain_hlocks and
simplify the code.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/20110421014300.727863282@goodmis.org
Signed-off-by: Ingo Molnar <mingo@elte.hu>
---
 kernel/lockdep.c |   14 ++++----------
 1 files changed, 4 insertions(+), 10 deletions(-)

diff --git a/kernel/lockdep.c b/kernel/lockdep.c
index 27c609f..63437d0 100644
--- a/kernel/lockdep.c
+++ b/kernel/lockdep.c
@@ -1973,7 +1973,7 @@ static inline int lookup_chain_cache(struct task_struct *curr,
 	struct list_head *hash_head = chainhashentry(chain_key);
 	struct lock_chain *chain;
 	struct held_lock *hlock_curr, *hlock_next;
-	int i, j, n, cn;
+	int i, j;
 
 	if (DEBUG_LOCKS_WARN_ON(!irqs_disabled()))
 		return 0;
@@ -2033,15 +2033,9 @@ cache_hit:
 	}
 	i++;
 	chain->depth = curr->lockdep_depth + 1 - i;
-	cn = nr_chain_hlocks;
-	while (cn + chain->depth <= MAX_LOCKDEP_CHAIN_HLOCKS) {
-		n = cmpxchg(&nr_chain_hlocks, cn, cn + chain->depth);
-		if (n == cn)
-			break;
-		cn = n;
-	}
-	if (likely(cn + chain->depth <= MAX_LOCKDEP_CHAIN_HLOCKS)) {
-		chain->base = cn;
+	if (likely(nr_chain_hlocks + chain->depth <= MAX_LOCKDEP_CHAIN_HLOCKS)) {
+		chain->base = nr_chain_hlocks;
+		nr_chain_hlocks += chain->depth;
 		for (j = 0; j < chain->depth - 1; j++, i++) {
 			int lock_id = curr->held_locks[i].class_idx - 1;
 			chain_hlocks[chain->base + j] = lock_id;