[PATCH 1/6] posix-cpu-timers: Assert task sighand is locked while starting cputime counter

[Frederic Weisbecker <frederic@kernel.org>]

Starting the process wide cputime counter needs to be done in the same
sighand locking sequence than actually arming the related timer
otherwise we risk races against concurrent timers setting/expiring
in the same threadgroup.

Detecting that we start the cputime counter without holding the sighand
lock is a first step toward debugging such situations.

Suggested-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Eric W. Biederman <ebiederm@xmission.com>
---
 include/linux/sched/signal.h   |  6 ++++++
 kernel/signal.c                | 15 +++++++++++++++
 kernel/time/posix-cpu-timers.c |  2 ++
 3 files changed, 23 insertions(+)

diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
index b9126fe06c3f..0310a5add9ab 100644
--- a/include/linux/sched/signal.h
+++ b/include/linux/sched/signal.h
@@ -714,6 +714,12 @@ static inline void unlock_task_sighand(struct task_struct *task,
 	spin_unlock_irqrestore(&task->sighand->siglock, *flags);
 }
 
+#ifdef CONFIG_LOCKDEP
+extern void lockdep_assert_task_sighand_held(struct task_struct *task);
+#else
+static inline void lockdep_assert_task_sighand_held(struct task_struct *task) { }
+#endif
+
 static inline unsigned long task_rlimit(const struct task_struct *task,
 		unsigned int limit)
 {
diff --git a/kernel/signal.c b/kernel/signal.c
index a3229add4455..52b6abec0ff8 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1413,6 +1413,21 @@ struct sighand_struct *__lock_task_sighand(struct task_struct *tsk,
 	return sighand;
 }
 
+#ifdef CONFIG_LOCKDEP
+void lockdep_assert_task_sighand_held(struct task_struct *task)
+{
+	struct sighand_struct *sighand;
+
+	rcu_read_lock();
+	sighand = rcu_dereference(task->sighand);
+	if (sighand)
+		lockdep_assert_held(&sighand->siglock);
+	else
+		WARN_ON_ONCE(1);
+	rcu_read_unlock();
+}
+#endif
+
 /*
  * send signal info to all the members of a group
  */
diff --git a/kernel/time/posix-cpu-timers.c b/kernel/time/posix-cpu-timers.c
index 517be7fd175e..4693d3c71e7e 100644
--- a/kernel/time/posix-cpu-timers.c
+++ b/kernel/time/posix-cpu-timers.c
@@ -291,6 +291,8 @@ static void thread_group_start_cputime(struct task_struct *tsk, u64 *samples)
 	struct thread_group_cputimer *cputimer = &tsk->signal->cputimer;
 	struct posix_cputimers *pct = &tsk->signal->posix_cputimers;
 
+	lockdep_assert_task_sighand_held(tsk);
+
 	/* Check if cputimer isn't running. This is accessed without locking. */
 	if (!READ_ONCE(pct->timers_active)) {
 		struct task_cputime sum;
-- 
2.25.1