linux-kernel - [PATCH 3/3] tty: Move task_pgrp() after tty->ctrl.lock for consistency

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Date:   Thu, 31 Aug 2023 18:50:31 -0700
From:   Earl Chew <earl.chew@...oo.ca>
To:     linux-kernel@...r.kernel.org, gregkh@...uxfoundation.org,
        jirislaby@...nel.org
Cc:     peter@...leysoftware.com, earl.chew@...oo.ca
Subject: [PATCH 3/3] tty: Move task_pgrp() after tty->ctrl.lock for consistency

Refactor __tty_check_change_locked() for consistency
with __proc_set_tty() which calls task_pgrp() after
tty->ctrl.lock is acquired.

In addition, spin_lock_irqsave() can block, while
task_pgrp() cannot block. Fetching the process
group immediately before it is used reduces the
window for inconsistency, and improves clarity.

Signed-off-by: Earl Chew <earl.chew@...oo.ca>
---
 drivers/tty/tty_jobctrl.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/tty/tty_jobctrl.c b/drivers/tty/tty_jobctrl.c
index 462fdf7b2..b4d37cad9 100644
--- a/drivers/tty/tty_jobctrl.c
+++ b/drivers/tty/tty_jobctrl.c
@@ -43,13 +43,13 @@ static int __tty_check_change_locked(struct tty_struct *tty, int sig,
 	if (current->signal->tty != tty)
 		return 0;
 
-	rcu_read_lock();
-	pgrp = task_pgrp(current);
-
 	if (!ctrl_lock)
 		spin_lock_irqsave(&tty->ctrl.lock, flags);
 	tty_pgrp = tty->ctrl.pgrp;
 
+	rcu_read_lock();
+	pgrp = task_pgrp(current);
+
 	if (tty_pgrp && pgrp != tty_pgrp) {
 		if (is_ignored(sig)) {
 			if (sig == SIGTTIN)
@@ -62,9 +62,9 @@ static int __tty_check_change_locked(struct tty_struct *tty, int sig,
 			ret = -ERESTARTSYS;
 		}
 	}
+	rcu_read_unlock();
 	if (!ctrl_lock)
 		spin_unlock_irqrestore(&tty->ctrl.lock, flags);
-	rcu_read_unlock();
 
 	if (!tty_pgrp)
 		tty_warn(tty, "sig=%d, tty->pgrp == NULL!\n", sig);
-- 
2.39.1