[PATCH 26/76] tty: Add a kref count

Alan Cox
2008-10-05T16:13:59Z

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>
---

drivers/char/tty_io.c | 54 ++++++++++++++++++++++++++++++++++++++++++++-----
include/linux/tty.h | 18 ++++++++++++++++
kernel/fork.c | 5 ++++-
kernel/sys.c | 4 +---
4 files changed, 71 insertions(+), 10 deletions(-)

diff --git a/drivers/char/tty_io.c b/drivers/char/tty_io.c
index 7323168..310e070 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 e3612c3..b6e6c26 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 7ce2ebe..30de644 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 038a7bc..234d945 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;