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-rw-r--r--drivers/char/tty_ldisc.c30
-rw-r--r--include/linux/tty.h1
2 files changed, 22 insertions, 9 deletions
diff --git a/drivers/char/tty_ldisc.c b/drivers/char/tty_ldisc.c
index f307f135cbfb..7a84b406a952 100644
--- a/drivers/char/tty_ldisc.c
+++ b/drivers/char/tty_ldisc.c
@@ -316,8 +316,7 @@ struct tty_ldisc *tty_ldisc_ref_wait(struct tty_struct *tty)
316{ 316{
317 /* wait_event is a macro */ 317 /* wait_event is a macro */
318 wait_event(tty_ldisc_wait, tty_ldisc_try(tty)); 318 wait_event(tty_ldisc_wait, tty_ldisc_try(tty));
319 if (tty->ldisc.refcount == 0) 319 WARN_ON(tty->ldisc.refcount == 0);
320 printk(KERN_ERR "tty_ldisc_ref_wait\n");
321 return &tty->ldisc; 320 return &tty->ldisc;
322} 321}
323 322
@@ -376,15 +375,17 @@ EXPORT_SYMBOL_GPL(tty_ldisc_deref);
376 * @tty: terminal to activate ldisc on 375 * @tty: terminal to activate ldisc on
377 * 376 *
378 * Set the TTY_LDISC flag when the line discipline can be called 377 * Set the TTY_LDISC flag when the line discipline can be called
379 * again. Do necessary wakeups for existing sleepers. 378 * again. Do necessary wakeups for existing sleepers. Clear the LDISC
379 * changing flag to indicate any ldisc change is now over.
380 * 380 *
381 * Note: nobody should set this bit except via this function. Clearing 381 * Note: nobody should set the TTY_LDISC bit except via this function.
382 * directly is allowed. 382 * Clearing directly is allowed.
383 */ 383 */
384 384
385void tty_ldisc_enable(struct tty_struct *tty) 385void tty_ldisc_enable(struct tty_struct *tty)
386{ 386{
387 set_bit(TTY_LDISC, &tty->flags); 387 set_bit(TTY_LDISC, &tty->flags);
388 clear_bit(TTY_LDISC_CHANGING, &tty->flags);
388 wake_up(&tty_ldisc_wait); 389 wake_up(&tty_ldisc_wait);
389} 390}
390 391
@@ -496,7 +497,14 @@ restart:
496 * reference to the line discipline. The TTY_LDISC bit 497 * reference to the line discipline. The TTY_LDISC bit
497 * prevents anyone taking a reference once it is clear. 498 * prevents anyone taking a reference once it is clear.
498 * We need the lock to avoid racing reference takers. 499 * We need the lock to avoid racing reference takers.
500 *
501 * We must clear the TTY_LDISC bit here to avoid a livelock
502 * with a userspace app continually trying to use the tty in
503 * parallel to the change and re-referencing the tty.
499 */ 504 */
505 clear_bit(TTY_LDISC, &tty->flags);
506 if (o_tty)
507 clear_bit(TTY_LDISC, &o_tty->flags);
500 508
501 spin_lock_irqsave(&tty_ldisc_lock, flags); 509 spin_lock_irqsave(&tty_ldisc_lock, flags);
502 if (tty->ldisc.refcount || (o_tty && o_tty->ldisc.refcount)) { 510 if (tty->ldisc.refcount || (o_tty && o_tty->ldisc.refcount)) {
@@ -528,7 +536,7 @@ restart:
528 * If the TTY_LDISC bit is set, then we are racing against 536 * If the TTY_LDISC bit is set, then we are racing against
529 * another ldisc change 537 * another ldisc change
530 */ 538 */
531 if (!test_bit(TTY_LDISC, &tty->flags)) { 539 if (test_bit(TTY_LDISC_CHANGING, &tty->flags)) {
532 struct tty_ldisc *ld; 540 struct tty_ldisc *ld;
533 spin_unlock_irqrestore(&tty_ldisc_lock, flags); 541 spin_unlock_irqrestore(&tty_ldisc_lock, flags);
534 tty_ldisc_put(new_ldisc.ops); 542 tty_ldisc_put(new_ldisc.ops);
@@ -536,10 +544,14 @@ restart:
536 tty_ldisc_deref(ld); 544 tty_ldisc_deref(ld);
537 goto restart; 545 goto restart;
538 } 546 }
539 547 /*
540 clear_bit(TTY_LDISC, &tty->flags); 548 * This flag is used to avoid two parallel ldisc changes. Once
549 * open and close are fine grained locked this may work better
550 * as a mutex shared with the open/close/hup paths
551 */
552 set_bit(TTY_LDISC_CHANGING, &tty->flags);
541 if (o_tty) 553 if (o_tty)
542 clear_bit(TTY_LDISC, &o_tty->flags); 554 set_bit(TTY_LDISC_CHANGING, &o_tty->flags);
543 spin_unlock_irqrestore(&tty_ldisc_lock, flags); 555 spin_unlock_irqrestore(&tty_ldisc_lock, flags);
544 556
545 /* 557 /*
diff --git a/include/linux/tty.h b/include/linux/tty.h
index f88169787a5f..bbbeaef99626 100644
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -301,6 +301,7 @@ struct tty_struct {
301#define TTY_PUSH 6 /* n_tty private */ 301#define TTY_PUSH 6 /* n_tty private */
302#define TTY_CLOSING 7 /* ->close() in progress */ 302#define TTY_CLOSING 7 /* ->close() in progress */
303#define TTY_LDISC 9 /* Line discipline attached */ 303#define TTY_LDISC 9 /* Line discipline attached */
304#define TTY_LDISC_CHANGING 10 /* Line discipline changing */
304#define TTY_HW_COOK_OUT 14 /* Hardware can do output cooking */ 305#define TTY_HW_COOK_OUT 14 /* Hardware can do output cooking */
305#define TTY_HW_COOK_IN 15 /* Hardware can do input cooking */ 306#define TTY_HW_COOK_IN 15 /* Hardware can do input cooking */
306#define TTY_PTY_LOCK 16 /* pty private */ 307#define TTY_PTY_LOCK 16 /* pty private */