linux-kernel - [tip: timers/core] posix-timers: Annotate concurrent access to k_itimer:: It

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Message-ID: <168597771061.404.17723098484982029633.tip-bot2@tip-bot2>
Date:   Mon, 05 Jun 2023 15:08:30 -0000
From:   "tip-bot2 for Thomas Gleixner" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Frederic Weisbecker <frederic@...nel.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: timers/core] posix-timers: Annotate concurrent access to
 k_itimer:: It_signal

The following commit has been merged into the timers/core branch of tip:

Commit-ID:     d69e873ee50459e3b77019263cf95236f063a97b
Gitweb:        https://git.kernel.org/tip/d69e873ee50459e3b77019263cf95236f063a97b
Author:        Thomas Gleixner <tglx@...utronix.de>
AuthorDate:    Tue, 25 Apr 2023 20:49:05 +02:00
Committer:     Thomas Gleixner <tglx@...utronix.de>
CommitterDate: Mon, 05 Jun 2023 17:03:37 +02:00

posix-timers: Annotate concurrent access to k_itimer:: It_signal

k_itimer::it_signal is read lockless in the RCU protected hash lookup, but
it can be written concurrently in the timer_create() and timer_delete()
path. Annotate these places with READ_ONCE() and WRITE_ONCE()

Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Reviewed-by: Frederic Weisbecker <frederic@...nel.org>
Link: https://lore.kernel.org/r/20230425183313.143596887@linutronix.de

---
 kernel/time/posix-timers.c | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/kernel/time/posix-timers.c b/kernel/time/posix-timers.c
index f6650c1..4333334 100644
--- a/kernel/time/posix-timers.c
+++ b/kernel/time/posix-timers.c
@@ -109,9 +109,9 @@ static struct k_itimer *__posix_timers_find(struct hlist_head *head,
 {
 	struct k_itimer *timer;
 
-	hlist_for_each_entry_rcu(timer, head, t_hash,
-				 lockdep_is_held(&hash_lock)) {
-		if ((timer->it_signal == sig) && (timer->it_id == id))
+	hlist_for_each_entry_rcu(timer, head, t_hash, lockdep_is_held(&hash_lock)) {
+		/* timer->it_signal can be set concurrently */
+		if ((READ_ONCE(timer->it_signal) == sig) && (timer->it_id == id))
 			return timer;
 	}
 	return NULL;
@@ -558,7 +558,7 @@ static int do_timer_create(clockid_t which_clock, struct sigevent *event,
 
 	spin_lock_irq(&current->sighand->siglock);
 	/* This makes the timer valid in the hash table */
-	new_timer->it_signal = current->signal;
+	WRITE_ONCE(new_timer->it_signal, current->signal);
 	list_add(&new_timer->list, &current->signal->posix_timers);
 	spin_unlock_irq(&current->sighand->siglock);
 
@@ -1052,10 +1052,10 @@ retry_delete:
 	list_del(&timer->list);
 	spin_unlock(&current->sighand->siglock);
 	/*
-	 * This keeps any tasks waiting on the spin lock from thinking
-	 * they got something (see the lock code above).
+	 * A concurrent lookup could check timer::it_signal lockless. It
+	 * will reevaluate with timer::it_lock held and observe the NULL.
 	 */
-	timer->it_signal = NULL;
+	WRITE_ONCE(timer->it_signal, NULL);
 
 	unlock_timer(timer, flags);
 	release_posix_timer(timer, IT_ID_SET);