[PATCH RFC 08/13] core: Use kin_lock synchronizations between parent and child and for thread group

[Kirill Tkhai]

real_parent->kin_lock protects:
	child's threads, place in sibling list etc.

parent->kin_lock protects:
	ptrace operations

Note: sighand is still stable under tasklist_lock.

Signed-off-by: Kirill Tkhai <ktkhai@xxxxxxxx>
---
 fs/exec.c                |   16 ++++++++---
 fs/proc/array.c          |    4 +--
 kernel/exit.c            |   68 +++++++++++++++++++++++++++++++++++++---------
 kernel/fork.c            |    4 +++
 kernel/ptrace.c          |   45 ++++++++++++++++++++++++++----
 kernel/signal.c          |    9 ++++++
 kernel/sys.c             |    8 ++++-
 mm/oom_kill.c            |    9 ++++--
 security/keys/keyctl.c   |    4 +--
 security/selinux/hooks.c |    4 +--
 10 files changed, 136 insertions(+), 35 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index 1977c2a..4de7770 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -932,14 +932,16 @@ static int de_thread(struct task_struct *tsk)
 		for (;;) {
 			threadgroup_change_begin(tsk);
 			write_lock_irq(&tasklist_lock);
+			write_real_parent_lock(tsk);
 			/*
-			 * Do this under tasklist_lock to ensure that
-			 * exit_notify() can't miss ->group_exit_task
+			 * Do this under real_parent's kin_lock to ensure
+			 * that exit_notify() can't miss ->group_exit_task.
 			 */
 			sig->notify_count = -1;
 			if (likely(leader->exit_state))
 				break;
 			__set_current_state(TASK_KILLABLE);
+			write_real_parent_unlock(tsk);
 			write_unlock_irq(&tasklist_lock);
 			threadgroup_change_end(tsk);
 			schedule();
@@ -995,9 +997,13 @@ static int de_thread(struct task_struct *tsk)
 		 * We are going to release_task()->ptrace_unlink() silently,
 		 * the tracer can sleep in do_wait(). EXIT_DEAD guarantees
 		 * the tracer wont't block again waiting for this thread.
+		 *
+		 * Also, we not need hold leader->parent->kin_lock, because
+		 * it can't change till we release real_parent->kin_lock.
 		 */
 		if (unlikely(leader->ptrace))
 			__wake_up_parent(leader, leader->parent);
+		write_real_parent_unlock(tsk);
 		write_unlock_irq(&tasklist_lock);
 		threadgroup_change_end(tsk);
 
@@ -1029,9 +1035,11 @@ static int de_thread(struct task_struct *tsk)
 		       sizeof(newsighand->action));
 
 		write_lock_irq(&tasklist_lock);
+		write_parent_and_real_parent_lock(tsk);
 		spin_lock(&oldsighand->siglock);
 		rcu_assign_pointer(tsk->sighand, newsighand);
 		spin_unlock(&oldsighand->siglock);
+		write_parent_and_real_parent_unlock(tsk);
 		write_unlock_irq(&tasklist_lock);
 
 		__cleanup_sighand(oldsighand);
@@ -1042,10 +1050,10 @@ static int de_thread(struct task_struct *tsk)
 
 killed:
 	/* protects against exit_notify() and __exit_signal() */
-	read_lock(&tasklist_lock);
+	read_real_parent_lock(tsk);
 	sig->group_exit_task = NULL;
 	sig->notify_count = 0;
-	read_unlock(&tasklist_lock);
+	read_real_parent_unlock(tsk);
 	return -EAGAIN;
 }
 
diff --git a/fs/proc/array.c b/fs/proc/array.c
index e2f21c0..820611cc 100644
--- a/fs/proc/array.c
+++ b/fs/proc/array.c
@@ -588,7 +588,7 @@ get_children_pid(struct inode *inode, struct pid *pid_prev, loff_t pos)
 	if (!task)
 		goto put_start;
 
-	read_lock(&tasklist_lock);
+	read_lock(&start->kin_lock);
 	if (task->real_parent == start && !(list_empty(&task->sibling))) {
 		put_task_struct(task);
 		if (!list_is_last(&task->sibling, &start->children)) {
@@ -623,7 +623,7 @@ get_children_pid(struct inode *inode, struct pid *pid_prev, loff_t pos)
 	}
 
 unlock:
-	read_unlock(&tasklist_lock);
+	read_unlock(&start->kin_lock);
 put_start:
 	put_task_struct(start);
 out:
diff --git a/kernel/exit.c b/kernel/exit.c
index 5e82787..a268093 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -180,6 +180,7 @@ void release_task(struct task_struct *p)
 	proc_flush_task(p);
 
 	write_lock_irq(&tasklist_lock);
+	write_parent_and_real_parent_lock(p);
 	ptrace_release_task(p);
 	__exit_signal(p);
 
@@ -202,6 +203,7 @@ void release_task(struct task_struct *p)
 			leader->exit_state = EXIT_DEAD;
 	}
 
+	write_parent_and_real_parent_unlock(p);
 	write_unlock_irq(&tasklist_lock);
 	release_thread(p);
 	call_rcu(&p->rcu, delayed_put_task_struct);
@@ -317,43 +319,50 @@ void mm_update_next_owner(struct mm_struct *mm)
 		return;
 	}
 
-	read_lock(&tasklist_lock);
 	/*
 	 * Search in the children
 	 */
+	read_lock(&p->kin_lock);
 	list_for_each_entry(c, &p->children, sibling) {
 		if (c->mm == mm) {
 			get_task_struct(c);
+			read_unlock(&p->kin_lock);
 			goto assign_new_owner;
 		}
 	}
+	read_unlock(&p->kin_lock);
 
 	/*
 	 * Search in the siblings
 	 */
+	read_real_parent_lock(p);
 	list_for_each_entry(c, &p->real_parent->children, sibling) {
 		if (c->mm == mm) {
 			get_task_struct(c);
+			read_real_parent_unlock(p);
 			goto assign_new_owner;
 		}
 	}
+	read_real_parent_unlock(p);
 
 	/*
 	 * Search through everything else, we should not get here often.
 	 */
+	rcu_read_lock();
 	for_each_process(g) {
 		if (g->flags & PF_KTHREAD)
 			continue;
 		for_each_thread(g, c) {
 			if (c->mm == mm) {
 				get_task_struct(c);
+				rcu_read_unlock();
 				goto assign_new_owner;
 			}
 			if (c->mm)
 				break;
 		}
 	}
-	read_unlock(&tasklist_lock);
+	rcu_read_unlock();
 	/*
 	 * We found no owner yet mm_users > 1: this implies that we are
 	 * most likely racing with swapoff (try_to_unuse()) or /proc or
@@ -369,11 +378,6 @@ void mm_update_next_owner(struct mm_struct *mm)
 	 * We always want mm->owner->mm == mm
 	 */
 	task_lock(c);
-	/*
-	 * Delay read_unlock() till we have the task_lock()
-	 * to ensure that c does not slip away underneath us
-	 */
-	read_unlock(&tasklist_lock);
 	if (c->mm != mm) {
 		task_unlock(c);
 		put_task_struct(c);
@@ -470,9 +474,13 @@ static void check_pid_ns_reaper_exit(struct task_struct *father)
 		return;
 
 	write_lock(&pid_ns->cr_lock);
+	read_real_parent_lock(father);
+
 	reaper = find_alive_thread(father);
 	if (reaper)
 		pid_ns->child_reaper = reaper;
+
+	read_real_parent_unlock(father);
 	write_unlock(&pid_ns->cr_lock);
 
 	if (reaper)
@@ -494,13 +502,22 @@ static void check_pid_ns_reaper_exit(struct task_struct *father)
  *    child_subreaper for its children (like a service manager)
  * 3. give it to the init process (PID 1) in our pid namespace
  */
-static struct task_struct *find_new_reaper(struct task_struct *father)
+static struct task_struct *find_double_lock_new_reaper(struct task_struct *father)
 {
 	struct task_struct *thread, *reaper, *child_reaper;
 
+	rcu_read_lock();
+again:
 	thread = find_alive_thread(father);
-	if (thread)
+	if (thread) {
+		double_write_lock(&father->kin_lock, &thread->kin_lock);
+		if (thread->flags & PF_EXITING) {
+			double_write_unlock(&father->kin_lock, &thread->kin_lock);
+			goto again;
+		}
+		rcu_read_unlock();
 		return thread;
+	}
 
 	child_reaper = task_active_pid_ns(father)->child_reaper;
 
@@ -518,12 +535,26 @@ static struct task_struct *find_new_reaper(struct task_struct *father)
 				break;
 			if (!reaper->signal->is_child_subreaper)
 				continue;
+find_again:
 			thread = find_alive_thread(reaper);
-			if (thread)
-				return thread;
+			if (thread) {
+				double_write_lock(&father->kin_lock,
+						  &thread->kin_lock);
+				if (!(thread->flags & PF_EXITING)) {
+					rcu_read_unlock();
+					return thread;
+				}
+				double_write_unlock(&father->kin_lock,
+						    &thread->kin_lock);
+				goto find_again;
+			}
 		}
 	}
 
+	rcu_read_unlock();
+
+	double_write_lock(&child_reaper->kin_lock, &father->kin_lock);
+
 	return child_reaper;
 }
 
@@ -569,11 +600,18 @@ static void forget_original_parent(struct task_struct *father,
 
 	/* Can drop and reacquire tasklist_lock */
 	check_pid_ns_reaper_exit(father);
-	if (list_empty(&father->children))
+
+	/* read_lock() guarantees concurrent thread sees our PF_EXITING */
+	read_lock(&father->kin_lock);
+	if (list_empty(&father->children)) {
+		read_unlock(&father->kin_lock);
 		return;
+	}
+	read_unlock(&father->kin_lock);
 
 	read_lock(&task_active_pid_ns(father)->cr_lock);
-	reaper = find_new_reaper(father);
+	reaper = find_double_lock_new_reaper(father);
+
 	list_for_each_entry(p, &father->children, sibling) {
 		for_each_thread(p, t) {
 			t->real_parent = reaper;
@@ -592,6 +630,8 @@ static void forget_original_parent(struct task_struct *father,
 			reparent_leader(father, p, dead);
 	}
 	list_splice_tail_init(&father->children, &reaper->children);
+
+	double_write_unlock(&reaper->kin_lock, &father->kin_lock);
 	read_unlock(&task_active_pid_ns(father)->cr_lock);
 }
 
@@ -608,6 +648,7 @@ static void exit_notify(struct task_struct *tsk, int group_dead)
 	write_lock_irq(&tasklist_lock);
 	forget_original_parent(tsk, &dead);
 
+	write_parent_and_real_parent_lock(tsk);
 	if (group_dead)
 		kill_orphaned_pgrp(tsk->group_leader, NULL);
 
@@ -631,6 +672,7 @@ static void exit_notify(struct task_struct *tsk, int group_dead)
 	/* mt-exec, de_thread() is waiting for group leader */
 	if (unlikely(tsk->signal->notify_count < 0))
 		wake_up_process(tsk->signal->group_exit_task);
+	write_parent_and_real_parent_unlock(tsk);
 	write_unlock_irq(&tasklist_lock);
 
 	list_for_each_entry_safe(p, n, &dead, ptrace_entry) {
diff --git a/kernel/fork.c b/kernel/fork.c
index ee389ba..63e225b 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1524,9 +1524,11 @@ static struct task_struct *copy_process(unsigned long clone_flags,
 
 	/* CLONE_PARENT re-uses the old parent */
 	if (clone_flags & (CLONE_PARENT|CLONE_THREAD)) {
+		write_real_parent_lock(current);
 		p->real_parent = current->real_parent;
 		p->parent_exec_id = current->parent_exec_id;
 	} else {
+		write_lock(&current->kin_lock);
 		p->real_parent = current;
 		p->parent_exec_id = current->self_exec_id;
 	}
@@ -1550,6 +1552,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
 	recalc_sigpending();
 	if (signal_pending(current)) {
 		spin_unlock(&current->sighand->siglock);
+		write_real_parent_unlock(p);
 		write_unlock_irq(&tasklist_lock);
 		retval = -ERESTARTNOINTR;
 		goto bad_fork_free_pid;
@@ -1591,6 +1594,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
 
 	total_forks++;
 	spin_unlock(&current->sighand->siglock);
+	write_real_parent_unlock(p);
 	syscall_tracepoint_update(p);
 	write_unlock_irq(&tasklist_lock);
 
diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index c8e0e05..817288d 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -181,6 +181,7 @@ static int ptrace_check_attach(struct task_struct *child, bool ignore_state)
 	 * be changed by us so it's not changing right after this.
 	 */
 	read_lock(&tasklist_lock);
+	read_parent_lock(child);
 	if (child->ptrace && child->parent == current) {
 		WARN_ON(child->state == __TASK_TRACED);
 		/*
@@ -190,6 +191,7 @@ static int ptrace_check_attach(struct task_struct *child, bool ignore_state)
 		if (ignore_state || ptrace_freeze_traced(child))
 			ret = 0;
 	}
+	read_parent_unlock(child);
 	read_unlock(&tasklist_lock);
 
 	if (!ret && !ignore_state) {
@@ -275,6 +277,7 @@ static int ptrace_attach(struct task_struct *task, long request,
 			 unsigned long flags)
 {
 	bool seize = (request == PTRACE_SEIZE);
+	struct task_struct *old_parent;
 	int retval;
 
 	retval = -EIO;
@@ -312,11 +315,13 @@ static int ptrace_attach(struct task_struct *task, long request,
 		goto unlock_creds;
 
 	write_lock_irq(&tasklist_lock);
+	write_parent_and_given_lock(task, current);
+	old_parent = task->parent;
 	retval = -EPERM;
-	if (unlikely(task->exit_state))
-		goto unlock_tasklist;
-	if (task->ptrace)
-		goto unlock_tasklist;
+	if (unlikely(task->exit_state) || task->ptrace) {
+		write_unlock(&old_parent->kin_lock);
+		goto unlock_current;
+	}
 
 	if (seize)
 		flags |= PT_SEIZED;
@@ -327,6 +332,7 @@ static int ptrace_attach(struct task_struct *task, long request,
 	task->ptrace = flags;
 
 	__ptrace_link(task, current);
+	write_unlock(&old_parent->kin_lock);
 
 	/* SEIZE doesn't trap tracee on attach */
 	if (!seize)
@@ -358,7 +364,8 @@ static int ptrace_attach(struct task_struct *task, long request,
 	spin_unlock(&task->sighand->siglock);
 
 	retval = 0;
-unlock_tasklist:
+unlock_current:
+	write_unlock(&current->kin_lock);
 	write_unlock_irq(&tasklist_lock);
 unlock_creds:
 	mutex_unlock(&task->signal->cred_guard_mutex);
@@ -383,6 +390,7 @@ static int ptrace_traceme(void)
 	int ret = -EPERM;
 
 	write_lock_irq(&tasklist_lock);
+	write_parent_lock(current);
 	/* Are we already being traced? */
 	if (!current->ptrace) {
 		ret = security_ptrace_traceme(current->parent);
@@ -392,10 +400,12 @@ static int ptrace_traceme(void)
 		 * pretend ->real_parent untraces us right after return.
 		 */
 		if (!ret && !(current->real_parent->flags & PF_EXITING)) {
+			BUG_ON(current->parent != current->real_parent);
 			current->ptrace = PT_PTRACED;
 			__ptrace_link(current, current->real_parent);
 		}
 	}
+	write_parent_unlock(current);
 	write_unlock_irq(&tasklist_lock);
 
 	return ret;
@@ -456,6 +466,7 @@ static bool __ptrace_detach(struct task_struct *tracer, struct task_struct *p)
 
 static int ptrace_detach(struct task_struct *child, unsigned int data)
 {
+	struct task_struct *old_parent;
 	if (!valid_signal(data))
 		return -EIO;
 
@@ -464,6 +475,8 @@ static int ptrace_detach(struct task_struct *child, unsigned int data)
 	clear_tsk_thread_flag(child, TIF_SYSCALL_TRACE);
 
 	write_lock_irq(&tasklist_lock);
+	write_parent_and_real_parent_lock(child);
+	old_parent = child->parent;
 	/*
 	 * We rely on ptrace_freeze_traced(). It can't be killed and
 	 * untraced by another thread, it can't be a zombie.
@@ -475,6 +488,9 @@ static int ptrace_detach(struct task_struct *child, unsigned int data)
 	 */
 	child->exit_code = data;
 	__ptrace_detach(current, child);
+	if (old_parent != child->real_parent)
+		write_unlock(&old_parent->kin_lock);
+	write_real_parent_unlock(child);
 	write_unlock_irq(&tasklist_lock);
 
 	proc_ptrace_connector(child, PTRACE_DETACH);
@@ -488,15 +504,30 @@ static int ptrace_detach(struct task_struct *child, unsigned int data)
  */
 void exit_ptrace(struct task_struct *tracer, struct list_head *dead)
 {
-	struct task_struct *p, *n;
+	struct task_struct *p;
+
+	rcu_read_lock();
+	for (;;) {
+		p = list_first_or_null_rcu(&tracer->ptraced, struct task_struct,
+					   ptrace_entry);
+		if (!p)
+			break;
+
+		write_parent_and_real_parent_lock(p);
+		if (p->parent != tracer) {
+			write_parent_and_real_parent_unlock(p);
+			continue;
+		}
 
-	list_for_each_entry_safe(p, n, &tracer->ptraced, ptrace_entry) {
 		if (unlikely(p->ptrace & PT_EXITKILL))
 			send_sig_info(SIGKILL, SEND_SIG_FORCED, p);
 
 		if (__ptrace_detach(tracer, p))
 			list_add(&p->ptrace_entry, dead);
+
+		write_parent_and_real_parent_unlock(p);
 	}
+	rcu_read_unlock();
 }
 
 int ptrace_readdata(struct task_struct *tsk, unsigned long src, char __user *dst, int len)
diff --git a/kernel/signal.c b/kernel/signal.c
index f19833b..8288019 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1882,6 +1882,7 @@ static void ptrace_stop(int exit_code, int why, int clear_code, siginfo_t *info)
 
 	spin_unlock_irq(&current->sighand->siglock);
 	read_lock(&tasklist_lock);
+	read_parent_and_real_parent_lock(current);
 	if (may_ptrace_stop()) {
 		/*
 		 * Notify parents of the stop.
@@ -1904,6 +1905,7 @@ static void ptrace_stop(int exit_code, int why, int clear_code, siginfo_t *info)
 		 * XXX: implement read_unlock_no_resched().
 		 */
 		preempt_disable();
+		read_parent_and_real_parent_unlock(current);
 		read_unlock(&tasklist_lock);
 		preempt_enable_no_resched();
 		freezable_schedule();
@@ -1925,6 +1927,7 @@ static void ptrace_stop(int exit_code, int why, int clear_code, siginfo_t *info)
 		__set_current_state(TASK_RUNNING);
 		if (clear_code)
 			current->exit_code = 0;
+		read_parent_and_real_parent_unlock(current);
 		read_unlock(&tasklist_lock);
 	}
 
@@ -2079,7 +2082,9 @@ static bool do_signal_stop(int signr)
 		 */
 		if (notify) {
 			read_lock(&tasklist_lock);
+			read_parent_and_real_parent_lock(current);
 			do_notify_parent_cldstop(current, false, notify);
+			read_parent_and_real_parent_unlock(current);
 			read_unlock(&tasklist_lock);
 		}
 
@@ -2225,11 +2230,13 @@ int get_signal(struct ksignal *ksig)
 		 * a duplicate.
 		 */
 		read_lock(&tasklist_lock);
+		read_parent_and_real_parent_lock(current->group_leader);
 		do_notify_parent_cldstop(current, false, why);
 
 		if (ptrace_reparented(current->group_leader))
 			do_notify_parent_cldstop(current->group_leader,
 						true, why);
+		read_parent_and_real_parent_unlock(current->group_leader);
 		read_unlock(&tasklist_lock);
 
 		goto relock;
@@ -2476,7 +2483,9 @@ void exit_signals(struct task_struct *tsk)
 	 */
 	if (unlikely(group_stop)) {
 		read_lock(&tasklist_lock);
+		read_parent_and_real_parent_lock(tsk);
 		do_notify_parent_cldstop(tsk, false, group_stop);
+		read_parent_and_real_parent_unlock(tsk);
 		read_unlock(&tasklist_lock);
 	}
 }
diff --git a/kernel/sys.c b/kernel/sys.c
index a4e372b..783aec4 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -939,7 +939,9 @@ SYSCALL_DEFINE2(setpgid, pid_t, pid, pid_t, pgid)
 	err = -ESRCH;
 	p = find_task_by_vpid(pid);
 	if (!p)
-		goto out;
+		goto out2;
+
+	write_real_parent_lock(p);
 
 	err = -EINVAL;
 	if (!thread_group_leader(p))
@@ -981,7 +983,9 @@ SYSCALL_DEFINE2(setpgid, pid_t, pid, pid_t, pgid)
 
 	err = 0;
 out:
-	/* All paths lead to here, thus we are safe. -DaveM */
+	/* All (almost) paths lead to here, thus we are safe. -DaveM */
+	write_real_parent_unlock(p);
+out2:
 	write_unlock_irq(&tasklist_lock);
 	rcu_read_unlock();
 	return err;
diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 2b665da..aa89beb 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -538,13 +538,14 @@ void oom_kill_process(struct task_struct *p, gfp_t gfp_mask, int order,
 	 * parent.  This attempts to lose the minimal amount of work done while
 	 * still freeing memory.
 	 */
-	read_lock(&tasklist_lock);
+	rcu_read_lock();
 	for_each_thread(p, t) {
+		read_lock(&t->kin_lock);
 		list_for_each_entry(child, &t->children, sibling) {
 			unsigned int child_points;
 
 			if (child->mm == p->mm)
-				continue;
+				goto unlock;
 			/*
 			 * oom_badness() returns 0 if the thread is unkillable
 			 */
@@ -557,8 +558,10 @@ void oom_kill_process(struct task_struct *p, gfp_t gfp_mask, int order,
 				get_task_struct(victim);
 			}
 		}
+unlock:
+		read_unlock(&t->kin_lock);
 	}
-	read_unlock(&tasklist_lock);
+	rcu_read_unlock();
 
 	p = find_lock_task_mm(victim);
 	if (!p) {
diff --git a/security/keys/keyctl.c b/security/keys/keyctl.c
index 0b9ec78..64eea7b 100644
--- a/security/keys/keyctl.c
+++ b/security/keys/keyctl.c
@@ -1491,7 +1491,7 @@ long keyctl_session_to_parent(void)
 
 	me = current;
 	rcu_read_lock();
-	write_lock_irq(&tasklist_lock);
+	write_real_parent_lock_irq(me);
 
 	ret = -EPERM;
 	oldwork = NULL;
@@ -1540,7 +1540,7 @@ long keyctl_session_to_parent(void)
 	if (!ret)
 		newwork = NULL;
 unlock:
-	write_unlock_irq(&tasklist_lock);
+	write_real_parent_unlock_irq(me);
 	rcu_read_unlock();
 	if (oldwork)
 		put_cred(container_of(oldwork, struct cred, rcu));
diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 7dade28..0feda4b 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -2461,9 +2461,9 @@ static void selinux_bprm_committed_creds(struct linux_binprm *bprm)
 
 	/* Wake up the parent if it is waiting so that it can recheck
 	 * wait permission to the new task SID. */
-	read_lock(&tasklist_lock);
+	read_real_parent_lock(current);
 	__wake_up_parent(current, current->real_parent);
-	read_unlock(&tasklist_lock);
+	read_real_parent_unlock(current);
 }
 
 /* superblock security operations */



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