[PATCH v3 5/6] pid: perform free_pid() calls outside of tasklist_lock

[Mateusz Guzik]

As the clone side already executes pid allocation with only pidmap_lock
held, issuing free_pid() while still holding tasklist_lock exacerbates
total hold time of the latter.

Signed-off-by: Mateusz Guzik <mjguzik@xxxxxxxxx>
---
 include/linux/pid.h |  7 ++++---
 kernel/exit.c       | 15 +++++++++------
 kernel/pid.c        | 44 ++++++++++++++++++++++----------------------
 kernel/sys.c        | 14 +++++++++-----
 4 files changed, 44 insertions(+), 36 deletions(-)

diff --git a/include/linux/pid.h b/include/linux/pid.h
index 98837a1ff0f3..311ecebd7d56 100644
--- a/include/linux/pid.h
+++ b/include/linux/pid.h
@@ -101,9 +101,9 @@ extern struct pid *get_task_pid(struct task_struct *task, enum pid_type type);
  * these helpers must be called with the tasklist_lock write-held.
  */
 extern void attach_pid(struct task_struct *task, enum pid_type);
-extern void detach_pid(struct task_struct *task, enum pid_type);
-extern void change_pid(struct task_struct *task, enum pid_type,
-			struct pid *pid);
+void detach_pid(struct pid **pids, struct task_struct *task, enum pid_type);
+void change_pid(struct pid **pids, struct task_struct *task, enum pid_type,
+		struct pid *pid);
 extern void exchange_tids(struct task_struct *task, struct task_struct *old);
 extern void transfer_pid(struct task_struct *old, struct task_struct *new,
 			 enum pid_type);
@@ -129,6 +129,7 @@ extern struct pid *find_ge_pid(int nr, struct pid_namespace *);
 extern struct pid *alloc_pid(struct pid_namespace *ns, pid_t *set_tid,
 			     size_t set_tid_size);
 extern void free_pid(struct pid *pid);
+void free_pids(struct pid **pids);
 extern void disable_pid_allocation(struct pid_namespace *ns);
 
 /*
diff --git a/kernel/exit.c b/kernel/exit.c
index d2c74f93f7d2..a90a0d159570 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -127,16 +127,18 @@ late_initcall(kernel_exit_sysfs_init);
  */
 struct release_task_post {
 	struct tty_struct *tty;
+	struct pid *pids[PIDTYPE_MAX];
 };
 
-static void __unhash_process(struct task_struct *p, bool group_dead)
+static void __unhash_process(struct release_task_post *post, struct task_struct *p,
+			     bool group_dead)
 {
 	nr_threads--;
-	detach_pid(p, PIDTYPE_PID);
+	detach_pid(post->pids, p, PIDTYPE_PID);
 	if (group_dead) {
-		detach_pid(p, PIDTYPE_TGID);
-		detach_pid(p, PIDTYPE_PGID);
-		detach_pid(p, PIDTYPE_SID);
+		detach_pid(post->pids, p, PIDTYPE_TGID);
+		detach_pid(post->pids, p, PIDTYPE_PGID);
+		detach_pid(post->pids, p, PIDTYPE_SID);
 
 		list_del_rcu(&p->tasks);
 		list_del_init(&p->sibling);
@@ -200,7 +202,7 @@ static void __exit_signal(struct release_task_post *post, struct task_struct *ts
 	task_io_accounting_add(&sig->ioac, &tsk->ioac);
 	sig->sum_sched_runtime += tsk->se.sum_exec_runtime;
 	sig->nr_threads--;
-	__unhash_process(tsk, group_dead);
+	__unhash_process(post, tsk, group_dead);
 	write_sequnlock(&sig->stats_lock);
 
 	/*
@@ -285,6 +287,7 @@ void release_task(struct task_struct *p)
 	put_pid(thread_pid);
 	add_device_randomness((const void*) &p->se.sum_exec_runtime,
 			      sizeof(unsigned long long));
+	free_pids(post.pids);
 	release_thread(p);
 	put_task_struct_rcu_user(p);
 	tty_kref_put(post.tty);
diff --git a/kernel/pid.c b/kernel/pid.c
index 2ae872f689a7..73625f28c166 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -88,20 +88,6 @@ struct pid_namespace init_pid_ns = {
 };
 EXPORT_SYMBOL_GPL(init_pid_ns);
 
-/*
- * Note: disable interrupts while the pidmap_lock is held as an
- * interrupt might come in and do read_lock(&tasklist_lock).
- *
- * If we don't disable interrupts there is a nasty deadlock between
- * detach_pid()->free_pid() and another cpu that does
- * spin_lock(&pidmap_lock) followed by an interrupt routine that does
- * read_lock(&tasklist_lock);
- *
- * After we clean up the tasklist_lock and know there are no
- * irq handlers that take it we can leave the interrupts enabled.
- * For now it is easier to be safe than to prove it can't happen.
- */
-
 static  __cacheline_aligned_in_smp DEFINE_SPINLOCK(pidmap_lock);
 seqcount_spinlock_t pidmap_lock_seq = SEQCNT_SPINLOCK_ZERO(pidmap_lock_seq, &pidmap_lock);
 
@@ -128,10 +114,11 @@ static void delayed_put_pid(struct rcu_head *rhp)
 
 void free_pid(struct pid *pid)
 {
-	/* We can be called with write_lock_irq(&tasklist_lock) held */
 	int i;
 	unsigned long flags;
 
+	lockdep_assert_not_held(&tasklist_lock);
+
 	spin_lock_irqsave(&pidmap_lock, flags);
 	for (i = 0; i <= pid->level; i++) {
 		struct upid *upid = pid->numbers + i;
@@ -160,6 +147,18 @@ void free_pid(struct pid *pid)
 	call_rcu(&pid->rcu, delayed_put_pid);
 }
 
+void free_pids(struct pid **pids)
+{
+	int tmp;
+
+	/*
+	 * This can batch pidmap_lock.
+	 */
+	for (tmp = PIDTYPE_MAX; --tmp >= 0; )
+		if (pids[tmp])
+			free_pid(pids[tmp]);
+}
+
 struct pid *alloc_pid(struct pid_namespace *ns, pid_t *set_tid,
 		      size_t set_tid_size)
 {
@@ -347,8 +346,8 @@ void attach_pid(struct task_struct *task, enum pid_type type)
 	hlist_add_head_rcu(&task->pid_links[type], &pid->tasks[type]);
 }
 
-static void __change_pid(struct task_struct *task, enum pid_type type,
-			struct pid *new)
+static void __change_pid(struct pid **pids, struct task_struct *task,
+			 enum pid_type type, struct pid *new)
 {
 	struct pid **pid_ptr, *pid;
 	int tmp;
@@ -370,18 +369,19 @@ static void __change_pid(struct task_struct *task, enum pid_type type,
 		if (pid_has_task(pid, tmp))
 			return;
 
-	free_pid(pid);
+	WARN_ON(pids[type]);
+	pids[type] = pid;
 }
 
-void detach_pid(struct task_struct *task, enum pid_type type)
+void detach_pid(struct pid **pids, struct task_struct *task, enum pid_type type)
 {
-	__change_pid(task, type, NULL);
+	__change_pid(pids, task, type, NULL);
 }
 
-void change_pid(struct task_struct *task, enum pid_type type,
+void change_pid(struct pid **pids, struct task_struct *task, enum pid_type type,
 		struct pid *pid)
 {
-	__change_pid(task, type, pid);
+	__change_pid(pids, task, type, pid);
 	attach_pid(task, type);
 }
 
diff --git a/kernel/sys.c b/kernel/sys.c
index cb366ff8703a..4efca8a97d62 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -1085,6 +1085,7 @@ SYSCALL_DEFINE2(setpgid, pid_t, pid, pid_t, pgid)
 {
 	struct task_struct *p;
 	struct task_struct *group_leader = current->group_leader;
+	struct pid *pids[PIDTYPE_MAX] = { 0 };
 	struct pid *pgrp;
 	int err;
 
@@ -1142,13 +1143,14 @@ SYSCALL_DEFINE2(setpgid, pid_t, pid, pid_t, pgid)
 		goto out;
 
 	if (task_pgrp(p) != pgrp)
-		change_pid(p, PIDTYPE_PGID, pgrp);
+		change_pid(pids, p, PIDTYPE_PGID, pgrp);
 
 	err = 0;
 out:
 	/* All paths lead to here, thus we are safe. -DaveM */
 	write_unlock_irq(&tasklist_lock);
 	rcu_read_unlock();
+	free_pids(pids);
 	return err;
 }
 
@@ -1222,21 +1224,22 @@ SYSCALL_DEFINE1(getsid, pid_t, pid)
 	return retval;
 }
 
-static void set_special_pids(struct pid *pid)
+static void set_special_pids(struct pid **pids, struct pid *pid)
 {
 	struct task_struct *curr = current->group_leader;
 
 	if (task_session(curr) != pid)
-		change_pid(curr, PIDTYPE_SID, pid);
+		change_pid(pids, curr, PIDTYPE_SID, pid);
 
 	if (task_pgrp(curr) != pid)
-		change_pid(curr, PIDTYPE_PGID, pid);
+		change_pid(pids, curr, PIDTYPE_PGID, pid);
 }
 
 int ksys_setsid(void)
 {
 	struct task_struct *group_leader = current->group_leader;
 	struct pid *sid = task_pid(group_leader);
+	struct pid *pids[PIDTYPE_MAX] = { 0 };
 	pid_t session = pid_vnr(sid);
 	int err = -EPERM;
 
@@ -1252,13 +1255,14 @@ int ksys_setsid(void)
 		goto out;
 
 	group_leader->signal->leader = 1;
-	set_special_pids(sid);
+	set_special_pids(pids, sid);
 
 	proc_clear_tty(group_leader);
 
 	err = session;
 out:
 	write_unlock_irq(&tasklist_lock);
+	free_pids(pids);
 	if (err > 0) {
 		proc_sid_connector(group_leader);
 		sched_autogroup_create_attach(group_leader);
-- 
2.43.0