[tip: timers/core] posix-cpu-timers: Assert task sighand is locked while starting cputime counter

[tip-bot2 for Frederic Weisbecker]

The following commit has been merged into the timers/core branch of tip:

Commit-ID:     a5dec9f82ab2ae486119f0b0820ea16db3e522c3
Gitweb:        https://git.kernel.org/tip/a5dec9f82ab2ae486119f0b0820ea16db3e522c3
Author:        Frederic Weisbecker <frederic@xxxxxxxxxx>
AuthorDate:    Mon, 26 Jul 2021 14:55:08 +02:00
Committer:     Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitterDate: Tue, 10 Aug 2021 17:09:58 +02:00

posix-cpu-timers: Assert task sighand is locked while starting cputime counter

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

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

Suggested-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20210726125513.271824-2-frederic@xxxxxxxxxx

---
 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 b9126fe..0310a5a 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 a3229ad..52b6abe 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 517be7f..4693d3c 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;