[Devel] [PATCH RHEL9 COMMIT] vtty: fix slave peer lockdep annotation

[Konstantin Khorenko]

The commit is pushed to "branch-rh9-5.14.vz9.1.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh9-5.14.0-4.vz9.10.41
------>
commit 557770d3cbda6b6ad9bb30409db991aa5403f013
Author: Cyrill Gorcunov <gorcunov at gmail.com>
Date:   Mon Jan 17 14:38:42 2022 +0300

    vtty: fix slave peer lockdep annotation
    
    vtty driver is a tricky one -- we keep slave peers inside container
    available for console output even if master peer is not used (unlike
    traditional unix terminals where one have to open master peer first
    to be able to communicate via multiple slave peers).
    
    Thus our procedure of the vtty driver initialization is quite the
    reverse with compare to PTYs: tty_init_dev() is called with
    @vttys_driver in first place. What is more interesting is tty_release()
    procedure: it locks ttys via tty_lock() call like
    
    tty_release()
      tty_lock(tty)
      // o_tty = tty->link;
      tty_lock_slave(o_tty);
    
    and here we need to annotate slave peer mutex that its locking is
    nested. The annotation is done via tty_set_lock_subclass() call
    but it must be called with pure tty instance never locked before.
    
    So we have a calling sequence
    
    tty_init_dev()
       tty = alloc_tty_struct(driver, idx);
       // the driver is pointing to @vttys_driver instance, ie slave peer
       tty_lock(tty);
       // and here we lock the slave peer with mutex without annotation
    
    For regular PTY case it is fine, because the PTYs are always created
    with master driver first but for our needs this won't work.
    
    Thus before the first tty_lock() call we need to annotate the mutex
    and for this sake we introduce vtty_alloc_tty_struct() helper which
    get called right after tty instance is allocated.
    
    Without annotation the warning appears:
      =============================================
      [ INFO: possible recursive locking detected ]
      3.10.0-1160.42.2.ovz.184.7 #37 Tainted: G         C     ------------
      ---------------------------------------------
      ffff9521b6bc4010 vzctl/8093 is trying to acquire lock:
       (&tty->legacy_mutex){+.+.+.}, at: [<ffffffff8658fec6>] tty_lock+0x86/0xf0
    
      but task is already holding lock:
       (&tty->legacy_mutex){+.+.+.}, at: [<ffffffff8658fec6>] tty_lock+0x86/0xf0
    
    Note: that lack of proper annotation simply makes lockdep grumpy but there
    is no real error.
    
    https://jira.sw.ru/browse/PSBM-136773
    
    Signed-off-by: Cyrill Gorcunov <gorcunov at virtuozzo.com>
---
 drivers/tty/pty.c    | 11 ++++++++++-
 drivers/tty/tty_io.c |  3 +++
 include/linux/ve.h   |  2 ++
 3 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/pty.c b/drivers/tty/pty.c
index 7d33bce95a36..c9151c12480c 100644
--- a/drivers/tty/pty.c
+++ b/drivers/tty/pty.c
@@ -1021,6 +1021,16 @@ static int __init vtty_init(void)
 	return 0;
 }
 
+void vtty_alloc_tty_struct(const struct tty_driver *driver,
+			   struct tty_struct *tty)
+{
+	if (driver != vttys_driver)
+		return;
+
+	tty_set_lock_subclass(tty);
+	lockdep_set_subclass(&tty->termios_rwsem, TTY_LOCK_SLAVE);
+}
+
 int vtty_open_master(envid_t veid, int idx)
 {
 	struct tty_struct *tty;
@@ -1071,7 +1081,6 @@ int vtty_open_master(envid_t veid, int idx)
 		}
 		tty->count--;
 		tty_unlock(tty);
-		tty_set_lock_subclass(tty);
 		tty = tty->link;
 	}
 
diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index b41ee2d7b33e..cc4550714b7d 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -3222,6 +3222,9 @@ struct tty_struct *alloc_tty_struct(struct tty_driver *driver, int idx)
 	tty_line_name(driver, idx, tty->name);
 	tty->dev = tty_get_device(tty);
 
+#ifdef CONFIG_TTY
+	vtty_alloc_tty_struct(driver, tty);
+#endif
 	return tty;
 }
 
diff --git a/include/linux/ve.h b/include/linux/ve.h
index 4c8f7d308829..ca5cc31a0910 100644
--- a/include/linux/ve.h
+++ b/include/linux/ve.h
@@ -207,6 +207,8 @@ extern int vtty_open_master(envid_t veid, int idx);
 extern void vtty_release(struct tty_struct *tty, struct tty_struct *o_tty,
 			int *tty_closing, int *o_tty_closing);
 extern bool vtty_is_master(struct tty_struct *tty);
+extern void vtty_alloc_tty_struct(const struct tty_driver *driver,
+				  struct tty_struct *tty);
 #endif /* CONFIG_TTY */
 
 extern struct cgroup *cgroup_ve_root1(struct cgroup *cgrp);