tty: Add a kref count

Introduce a kref to the tty structure and use it to protect the tty->signal
tty references. For now we don't introduce it for anything else.

Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

4 files changed, 71 insertions, 10 deletions

diff --git a/drivers/char/tty_io.c b/drivers/char/tty_io.c
index 732316899ca4..310e0703e4a1 100644
--- a/drivers/char/tty_io.c
+++ b/drivers/char/tty_io.c
@@ -559,6 +559,7 @@ static void do_tty_hangup(struct work_struct *work)
         struct tty_ldisc *ld;
         int    closecount = 0, n;
         unsigned long flags;
+        int refs = 0;
 
         if (!tty)
                 return;
@@ -625,8 +626,12 @@ static void do_tty_hangup(struct work_struct *work)
         if (tty->session) {
                 do_each_pid_task(tty->session, PIDTYPE_SID, p) {
                         spin_lock_irq(&p->sighand->siglock);
-                        if (p->signal->tty == tty)
+                        if (p->signal->tty == tty) {
                                 p->signal->tty = NULL;
+                                /* We defer the dereferences outside fo
+                                   the tasklist lock */
+                                refs++;
+                        }
                         if (!p->signal->leader) {
                                 spin_unlock_irq(&p->sighand->siglock);
                                 continue;
@@ -652,6 +657,10 @@ static void do_tty_hangup(struct work_struct *work)
         tty->ctrl_status = 0;
         spin_unlock_irqrestore(&tty->ctrl_lock, flags);
 
+        /* Account for the p->signal references we killed */
+        while (refs--)
+                tty_kref_put(tty);
+
         /*
          * If one of the devices matches a console pointer, we
          * cannot just call hangup() because that will cause
@@ -1424,6 +1433,7 @@ release_mem_out:
 
 /**
  *      release_one_tty         -       release tty structure memory
+ *      @kref: kref of tty we are obliterating
  *
  *      Releases memory associated with a tty structure, and clears out the
  *      driver table slots. This function is called when a device is no longer
@@ -1433,17 +1443,19 @@ release_mem_out:
  *              tty_mutex - sometimes only
  *              takes the file list lock internally when working on the list
  *      of ttys that the driver keeps.
- *              FIXME: should we require tty_mutex is held here ??
  */
-static void release_one_tty(struct tty_struct *tty, int idx)
+static void release_one_tty(struct kref *kref)
 {
+        struct tty_struct *tty = container_of(kref, struct tty_struct, kref);
         int devpts = tty->driver->flags & TTY_DRIVER_DEVPTS_MEM;
         struct ktermios *tp;
+        int idx = tty->index;
 
         if (!devpts)
                 tty->driver->ttys[idx] = NULL;
 
         if (tty->driver->flags & TTY_DRIVER_RESET_TERMIOS) {
+                /* FIXME: Locking on ->termios array */
                 tp = tty->termios;
                 if (!devpts)
                         tty->driver->termios[idx] = NULL;
@@ -1457,6 +1469,7 @@ static void release_one_tty(struct tty_struct *tty, int idx)
 
 
         tty->magic = 0;
+        /* FIXME: locking on tty->driver->refcount */
         tty->driver->refcount--;
 
         file_list_lock();
@@ -1467,6 +1480,21 @@ static void release_one_tty(struct tty_struct *tty, int idx)
 }
 
 /**
+ *      tty_kref_put            -       release a tty kref
+ *      @tty: tty device
+ *
+ *      Release a reference to a tty device and if need be let the kref
+ *      layer destruct the object for us
+ */
+
+void tty_kref_put(struct tty_struct *tty)
+{
+        if (tty)
+                kref_put(&tty->kref, release_one_tty);
+}
+EXPORT_SYMBOL(tty_kref_put);
+
+/**
  *      release_tty             -       release tty structure memory
  *
  *      Release both @tty and a possible linked partner (think pty pair),
@@ -1477,14 +1505,20 @@ static void release_one_tty(struct tty_struct *tty, int idx)
  *              takes the file list lock internally when working on the list
  *      of ttys that the driver keeps.
  *              FIXME: should we require tty_mutex is held here ??
+ *
+ *      FIXME: We want to defer the module put of the driver to the
+ *      destructor.
  */
 static void release_tty(struct tty_struct *tty, int idx)
 {
         struct tty_driver *driver = tty->driver;
 
+        /* This should always be true but check for the moment */
+        WARN_ON(tty->index != idx);
+
         if (tty->link)
-                release_one_tty(tty->link, idx);
-        release_one_tty(tty, idx);
+                tty_kref_put(tty->link);
+        tty_kref_put(tty);
         module_put(driver->owner);
 }
 
@@ -2798,6 +2832,7 @@ EXPORT_SYMBOL(do_SAK);
 static void initialize_tty_struct(struct tty_struct *tty)
 {
         memset(tty, 0, sizeof(struct tty_struct));
+        kref_init(&tty->kref);
         tty->magic = TTY_MAGIC;
         tty_ldisc_init(tty);
         tty->session = NULL;
@@ -3053,9 +3088,12 @@ EXPORT_SYMBOL(tty_devnum);
 
 void proc_clear_tty(struct task_struct *p)
 {
+        struct tty_struct *tty;
         spin_lock_irq(&p->sighand->siglock);
+        tty = p->signal->tty;
         p->signal->tty = NULL;
         spin_unlock_irq(&p->sighand->siglock);
+        tty_kref_put(tty);
 }
 
 /* Called under the sighand lock */
@@ -3071,9 +3109,13 @@ static void __proc_set_tty(struct task_struct *tsk, struct tty_struct *tty)
                 tty->pgrp = get_pid(task_pgrp(tsk));
                 spin_unlock_irqrestore(&tty->ctrl_lock, flags);
                 tty->session = get_pid(task_session(tsk));
+                if (tsk->signal->tty) {
+                        printk(KERN_DEBUG "tty not NULL!!\n");
+                        tty_kref_put(tsk->signal->tty);
+                }
         }
         put_pid(tsk->signal->tty_old_pgrp);
-        tsk->signal->tty = tty;
+        tsk->signal->tty = tty_kref_get(tty);
         tsk->signal->tty_old_pgrp = NULL;
 }
 
diff --git a/include/linux/tty.h b/include/linux/tty.h
index e3612c3ac194..b6e6c26883ee 100644
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -209,6 +209,7 @@ struct tty_operations;
 
 struct tty_struct {
         int     magic;
+        struct kref kref;
         struct tty_driver *driver;
         const struct tty_operations *ops;
         int index;
@@ -311,6 +312,23 @@ extern int kmsg_redirect;
 extern void console_init(void);
 extern int vcs_init(void);
 
+/**
+ *      tty_kref_get            -       get a tty reference
+ *      @tty: tty device
+ *
+ *      Return a new reference to a tty object. The caller must hold
+ *      sufficient locks/counts to ensure that their existing reference cannot
+ *      go away
+ */
+
+extern inline struct tty_struct *tty_kref_get(struct tty_struct *tty)
+{
+        if (tty)
+                kref_get(&tty->kref);
+        return tty;
+}
+extern void tty_kref_put(struct tty_struct *tty);
+
 extern int tty_paranoia_check(struct tty_struct *tty, struct inode *inode,
                               const char *routine);
 extern char *tty_name(struct tty_struct *tty, char *buf);
diff --git a/kernel/fork.c b/kernel/fork.c
index 7ce2ebe84796..30de644a40c4 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -802,6 +802,7 @@ static int copy_signal(unsigned long clone_flags, struct task_struct *tsk)
 
         sig->leader = 0;        /* session leadership doesn't inherit */
         sig->tty_old_pgrp = NULL;
+        sig->tty = NULL;
 
         sig->utime = sig->stime = sig->cutime = sig->cstime = cputime_zero;
         sig->gtime = cputime_zero;
@@ -838,6 +839,7 @@ static int copy_signal(unsigned long clone_flags, struct task_struct *tsk)
 void __cleanup_signal(struct signal_struct *sig)
 {
         exit_thread_group_keys(sig);
+        tty_kref_put(sig->tty);
         kmem_cache_free(signal_cachep, sig);
 }
 
@@ -1227,7 +1229,8 @@ static struct task_struct *copy_process(unsigned long clone_flags,
                                 p->nsproxy->pid_ns->child_reaper = p;
 
                         p->signal->leader_pid = pid;
-                        p->signal->tty = current->signal->tty;
+                        tty_kref_put(p->signal->tty);
+                        p->signal->tty = tty_kref_get(current->signal->tty);
                         set_task_pgrp(p, task_pgrp_nr(current));
                         set_task_session(p, task_session_nr(current));
                         attach_pid(p, PIDTYPE_PGID, task_pgrp(current));
diff --git a/kernel/sys.c b/kernel/sys.c
index 038a7bc0901d..234d9454294e 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -1060,9 +1060,7 @@ asmlinkage long sys_setsid(void)
         group_leader->signal->leader = 1;
         __set_special_pids(sid);
 
-        spin_lock(&group_leader->sighand->siglock);
-        group_leader->signal->tty = NULL;
-        spin_unlock(&group_leader->sighand->siglock);
+        proc_clear_tty(group_leader);
 
         err = session;
 out: