signals: annotate lock_task_sighand()

lock_task_sighand() grabs sighand->siglock in case of returning non-NULL
but unlock_task_sighand() releases it unconditionally.  This leads sparse
to complain about the lock context imbalance.  Rename and wrap
lock_task_sighand() using __cond_lock() macro to make sparse happy.

Suggested-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Namhyung Kim <namhyung@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/sched.h

@@ -2236,9 +2236,16 @@ static inline void task_unlock(struct task_struct *p)
 	spin_unlock(&p->alloc_lock);
 }
 
-extern struct sighand_struct *lock_task_sighand(struct task_struct *tsk,
+extern struct sighand_struct *__lock_task_sighand(struct task_struct *tsk,
 							unsigned long *flags);
 
+#define lock_task_sighand(tsk, flags)					\
+({	struct sighand_struct *__ss;					\
+	__cond_lock(&(tsk)->sighand->siglock,				\
+		    (__ss = __lock_task_sighand(tsk, flags)));		\
+	__ss;								\
+})									\
+
 static inline void unlock_task_sighand(struct task_struct *tsk,
 						unsigned long *flags)
 {

kernel/signal.c

@@ -1105,7 +1105,8 @@ int zap_other_threads(struct task_struct *p)
 	return count;
 }
 
-struct sighand_struct *lock_task_sighand(struct task_struct *tsk, unsigned long *flags)
+struct sighand_struct *__lock_task_sighand(struct task_struct *tsk,
+					   unsigned long *flags)
 {
 	struct sighand_struct *sighand;