From: Shailabh Nagar <nagar@watson.ibm.com>

Use irqsave variants of spin_lock for newly introduced
tsk->signal->stats_lock The lock is nested within tasklist_lock as part of
release_task() and hence there is possibility of a AB-BA deadlock if a
timer interrupt occurs while stats_lock is held.

Thanks to Arjan van de Ven for pointing out the lock bug (akpm: found by the
locking validator).

The patch conservatively converts all use of stats_lock to irqsave variant
rather than only the call within taskstats_tgid_free which is the function
called within tasklist_lock protection.

Signed-off-by: Shailabh Nagar <nagar@watson.ibm.com>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
---

 include/linux/taskstats_kern.h |   11 +++++++----
 kernel/taskstats.c             |   16 ++++++++++------
 2 files changed, 17 insertions(+), 10 deletions(-)

diff -puN include/linux/taskstats_kern.h~delay-accounting-taskstats-interface-send-tgid-once-locking include/linux/taskstats_kern.h
--- a/include/linux/taskstats_kern.h~delay-accounting-taskstats-interface-send-tgid-once-locking
+++ a/include/linux/taskstats_kern.h
@@ -39,17 +39,18 @@ static inline void taskstats_tgid_init(s
 static inline void taskstats_tgid_alloc(struct signal_struct *sig)
 {
 	struct taskstats *stats;
+	unsigned long flags;
 
 	stats = kmem_cache_zalloc(taskstats_cache, SLAB_KERNEL);
 	if (!stats)
 		return;
 
-	spin_lock(&sig->stats_lock);
+	spin_lock_irqsave(&sig->stats_lock, flags);
 	if (!sig->stats) {
 		sig->stats = stats;
 		stats = NULL;
 	}
-	spin_unlock(&sig->stats_lock);
+	spin_unlock_irqrestore(&sig->stats_lock, flags);
 
 	if (stats)
 		kmem_cache_free(taskstats_cache, stats);
@@ -58,12 +59,14 @@ static inline void taskstats_tgid_alloc(
 static inline void taskstats_tgid_free(struct signal_struct *sig)
 {
 	struct taskstats *stats = NULL;
-	spin_lock(&sig->stats_lock);
+	unsigned long flags;
+
+	spin_lock_irqsave(&sig->stats_lock, flags);
 	if (sig->stats) {
 		stats = sig->stats;
 		sig->stats = NULL;
 	}
-	spin_unlock(&sig->stats_lock);
+	spin_unlock_irqrestore(&sig->stats_lock, flags);
 	if (stats)
 		kmem_cache_free(taskstats_cache, stats);
 }
diff -puN kernel/taskstats.c~delay-accounting-taskstats-interface-send-tgid-once-locking kernel/taskstats.c
--- a/kernel/taskstats.c~delay-accounting-taskstats-interface-send-tgid-once-locking
+++ a/kernel/taskstats.c
@@ -133,6 +133,7 @@ static int fill_tgid(pid_t tgid, struct 
 		struct taskstats *stats)
 {
 	struct task_struct *tsk, *first;
+	unsigned long flags;
 
 	/*
 	 * Add additional stats from live tasks except zombie thread group
@@ -152,10 +153,10 @@ static int fill_tgid(pid_t tgid, struct 
 		get_task_struct(first);
 
 	/* Start with stats from dead tasks */
-	spin_lock(&first->signal->stats_lock);
+	spin_lock_irqsave(&first->signal->stats_lock, flags);
 	if (first->signal->stats)
 		memcpy(stats, first->signal->stats, sizeof(*stats));
-	spin_unlock(&first->signal->stats_lock);
+	spin_unlock_irqrestore(&first->signal->stats_lock, flags);
 
 	tsk = first;
 	read_lock(&tasklist_lock);
@@ -185,7 +186,9 @@ static int fill_tgid(pid_t tgid, struct 
 
 static void fill_tgid_exit(struct task_struct *tsk)
 {
-	spin_lock(&tsk->signal->stats_lock);
+	unsigned long flags;
+
+	spin_lock_irqsave(&tsk->signal->stats_lock, flags);
 	if (!tsk->signal->stats)
 		goto ret;
 
@@ -197,7 +200,7 @@ static void fill_tgid_exit(struct task_s
 	 */
 	delayacct_add_tsk(tsk->signal->stats, tsk);
 ret:
-	spin_unlock(&tsk->signal->stats_lock);
+	spin_unlock_irqrestore(&tsk->signal->stats_lock, flags);
 	return;
 }
 
@@ -268,13 +271,14 @@ void taskstats_exit_send(struct task_str
 	size_t size;
 	int is_thread_group;
 	struct nlattr *na;
+	unsigned long flags;
 
 	if (!family_registered || !tidstats)
 		return;
 
-	spin_lock(&tsk->signal->stats_lock);
+	spin_lock_irqsave(&tsk->signal->stats_lock, flags);
 	is_thread_group = tsk->signal->stats ? 1 : 0;
-	spin_unlock(&tsk->signal->stats_lock);
+	spin_unlock_irqrestore(&tsk->signal->stats_lock, flags);
 
 	rc = 0;
 	/*
_