[PATCH 3/4] threadgroup: extend threadgroup_lock() to cover exit and exec

[Tejun Heo <htejun@gmail.com>]

From: Tejun Heo <tj@kernel.org>

threadgroup_lock() protected only protected against new addition to
the threadgroup, which was inherently somewhat incomplete and
problematic for its only user cgroup.  On-going migration could race
against exec and exit leading to interesting problems - the symmetry
between various attach methods, task exiting during method execution,
->exit() racing against attach methods, migrating task switching basic
properties during exec and so on.

This patch extends threadgroup_lock() such that it protects against
all three threadgroup altering operations - fork, exit and exec.  For
exit, threadgroup_change_begin/end() calls are added to exit path.
For exec, threadgroup_[un]lock() are updated to also grab and release
cred_guard_mutex.

With this change, threadgroup_lock() guarantees that the target
threadgroup will remain stable - no new task will be added, no new
PF_EXITING will be set and exec won't happen.

The next patch will update cgroup so that it can take full advantage
of this change.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Paul Menage <menage@google.com>
Cc: Li Zefan <lizf@cn.fujitsu.com>
---
 include/linux/sched.h |   32 ++++++++++++++++++++++++++------
 kernel/exit.c         |   16 ++++++++++++----
 2 files changed, 38 insertions(+), 10 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index da74d6f..179cbdc 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -635,11 +635,12 @@ struct signal_struct {
 #endif
 #ifdef CONFIG_CGROUPS
 	/*
-	 * The group_rwsem prevents threads from forking with
-	 * CLONE_THREAD while held for writing. Use this for fork-sensitive
-	 * threadgroup-wide operations. It's taken for reading in fork.c in
-	 * copy_process().
-	 * Currently only needed write-side by cgroups.
+	 * group_rwsem prevents new tasks from entering the threadgroup and
+	 * member tasks from exiting.  fork and exit paths are protected
+	 * with this rwsem using threadgroup_change_begin/end().  Users
+	 * which require threadgroup to remain stable should use
+	 * threadgroup_[un]lock() which also takes care of exec path.
+	 * Currently, cgroup is the only user.
 	 */
 	struct rw_semaphore group_rwsem;
 #endif
@@ -2364,7 +2365,6 @@ static inline void unlock_task_sighand(struct task_struct *tsk,
 	spin_unlock_irqrestore(&tsk->sighand->siglock, *flags);
 }
 
-/* See the declaration of group_rwsem in signal_struct. */
 #ifdef CONFIG_CGROUPS
 static inline void threadgroup_change_begin(struct task_struct *tsk)
 {
@@ -2374,13 +2374,33 @@ static inline void threadgroup_change_done(struct task_struct *tsk)
 {
 	up_read(&tsk->signal->group_rwsem);
 }
+
+/**
+ * threadgroup_lock - lock threadgroup
+ * @tsk: member task of the threadgroup to lock
+ *
+ * Lock the threadgroup @tsk belongs to.  No new task is allowed to enter
+ * and member tasks aren't allowed to exit (as indicated by PF_EXITING) or
+ * perform exec.  This is useful for cases where the threadgroup needs to
+ * stay stable across blockable operations.
+ */
 static inline void threadgroup_lock(struct task_struct *tsk)
 {
+	/* exec uses exit for de-threading, grab cred_guard_mutex first */
+	mutex_lock(&tsk->signal->cred_guard_mutex);
 	down_write(&tsk->signal->group_rwsem);
 }
+
+/**
+ * threadgroup_unlock - unlock threadgroup
+ * @tsk: member task of the threadgroup to unlock
+ *
+ * Reverse threadgroup_lock().
+ */
 static inline void threadgroup_unlock(struct task_struct *tsk)
 {
 	up_write(&tsk->signal->group_rwsem);
+	mutex_unlock(&tsk->signal->cred_guard_mutex);
 }
 #else
 static inline void threadgroup_change_begin(struct task_struct *tsk) {}
diff --git a/kernel/exit.c b/kernel/exit.c
index 7b6c4fa..a5b40b3 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -936,6 +936,12 @@ NORET_TYPE void do_exit(long code)
 		schedule();
 	}
 
+	/*
+	 * @tsk's threadgroup is going through changes - lock out users
+	 * which expect stable threadgroup.
+	 */
+	threadgroup_change_begin(tsk);
+
 	exit_irq_thread();
 
 	exit_signals(tsk);  /* sets PF_EXITING */
@@ -1018,10 +1024,6 @@ NORET_TYPE void do_exit(long code)
 		kfree(current->pi_state_cache);
 #endif
 	/*
-	 * Make sure we are holding no locks:
-	 */
-	debug_check_no_locks_held(tsk);
-	/*
 	 * We can do this unlocked here. The futex code uses this flag
 	 * just to verify whether the pi state cleanup has been done
 	 * or not. In the worst case it loops once more.
@@ -1039,6 +1041,12 @@ NORET_TYPE void do_exit(long code)
 	preempt_disable();
 	exit_rcu();
 
+	/*
+	 * Release threadgroup and make sure we are holding no locks.
+	 */
+	threadgroup_change_done(tsk);
+	debug_check_no_locks_held(tsk);
+
 	/* this task is now dead and freezer should ignore it */
 	current->flags |= PF_NOFREEZE;
 
-- 
1.7.6