Message-ID: <20230509164640.-aaZNrjH@linutronix.de>
Date:   Tue, 9 May 2023 18:46:40 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        linux-rt-users@...r.kernel.org,
        Steven Rostedt <rostedt@...dmis.org>
Subject: [ANNOUNCE] v6.3.1-rt13

Dear RT folks!

I'm pleased to announce the v6.3.1-rt13 patch set. 

Changes since v6.3.1-rt12:

  - Two posix-timers picked-up from the list. They are scheduled for
    upstream inclusion. One of prevents a livelock on PREEMPT_RT in
    itimer_delete(). Patches by Thomas Gleixner.

  - A softirq handling patch from the list 'revert: "softirq: Let
    ksoftirqd do its job' from Paolo Abeni. This revert should reduce a
    lot of trouble which start once ksoftirqd is woken up.
    The 6.1-RT series has the ktimersd thread which mitigates some of
    the pain. This patch should render the patch obsolete.
    Should everything work out as expected I intend to backport this
    patch the earlier RT series and revert the ktimersd patch in the
    v6.1 series.

Known issues
     None

The delta patch against v6.3.1-rt12 is appended below and can be found here:
 
     https://cdn.kernel.org/pub/linux/kernel/projects/rt/6.3/incr/patch-6.3.1-rt12-rt13.patch.xz

You can get this release via the git tree at:

    git://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git v6.3.1-rt13

The RT patch against v6.3.1 can be found here:

    https://cdn.kernel.org/pub/linux/kernel/projects/rt/6.3/older/patch-6.3.1-rt13.patch.xz

The split quilt queue is available at:

    https://cdn.kernel.org/pub/linux/kernel/projects/rt/6.3/older/patches-6.3.1-rt13.tar.xz

Sebastian

diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
index a39be9f9ba966..b38ce53576000 100644
--- a/include/linux/sched/signal.h
+++ b/include/linux/sched/signal.h
@@ -136,7 +136,7 @@ struct signal_struct {
 #ifdef CONFIG_POSIX_TIMERS
 
 	/* POSIX.1b Interval Timers */
-	int			posix_timer_id;
+	unsigned int		next_posix_timer_id;
 	struct list_head	posix_timers;
 
 	/* ITIMER_REAL timer for the process */
diff --git a/kernel/softirq.c b/kernel/softirq.c
index 82f3e68fbe220..af9e879bbbf75 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -80,21 +80,6 @@ static void wakeup_softirqd(void)
 		wake_up_process(tsk);
 }
 
-/*
- * If ksoftirqd is scheduled, we do not want to process pending softirqs
- * right now. Let ksoftirqd handle this at its own rate, to get fairness,
- * unless we're doing some of the synchronous softirqs.
- */
-#define SOFTIRQ_NOW_MASK ((1 << HI_SOFTIRQ) | (1 << TASKLET_SOFTIRQ))
-static bool ksoftirqd_running(unsigned long pending)
-{
-	struct task_struct *tsk = __this_cpu_read(ksoftirqd);
-
-	if (pending & SOFTIRQ_NOW_MASK)
-		return false;
-	return tsk && task_is_running(tsk) && !__kthread_should_park(tsk);
-}
-
 #ifdef CONFIG_TRACE_IRQFLAGS
 DEFINE_PER_CPU(int, hardirqs_enabled);
 DEFINE_PER_CPU(int, hardirq_context);
@@ -236,7 +221,7 @@ void __local_bh_enable_ip(unsigned long ip, unsigned int cnt)
 		goto out;
 
 	pending = local_softirq_pending();
-	if (!pending || ksoftirqd_running(pending))
+	if (!pending)
 		goto out;
 
 	/*
@@ -432,9 +417,6 @@ static inline bool should_wake_ksoftirqd(void)
 
 static inline void invoke_softirq(void)
 {
-	if (ksoftirqd_running(local_softirq_pending()))
-		return;
-
 	if (!force_irqthreads() || !__this_cpu_read(ksoftirqd)) {
 #ifdef CONFIG_HAVE_IRQ_EXIT_ON_IRQ_STACK
 		/*
@@ -468,7 +450,7 @@ asmlinkage __visible void do_softirq(void)
 
 	pending = local_softirq_pending();
 
-	if (pending && !ksoftirqd_running(pending))
+	if (pending)
 		do_softirq_own_stack();
 
 	local_irq_restore(flags);
diff --git a/kernel/time/posix-timers.c b/kernel/time/posix-timers.c
index 0c8a87a11b39d..863b8ac073c30 100644
--- a/kernel/time/posix-timers.c
+++ b/kernel/time/posix-timers.c
@@ -140,25 +140,29 @@ static struct k_itimer *posix_timer_by_id(timer_t id)
 static int posix_timer_add(struct k_itimer *timer)
 {
 	struct signal_struct *sig = current->signal;
-	int first_free_id = sig->posix_timer_id;
 	struct hlist_head *head;
-	int ret = -ENOENT;
+	unsigned int start, id;
 
-	do {
+	/* Can be written by a different task concurrently in the loop below */
+	start = READ_ONCE(sig->next_posix_timer_id);
+
+	for (id = ~start; start != id; id++) {
 		spin_lock(&hash_lock);
-		head = &posix_timers_hashtable[hash(sig, sig->posix_timer_id)];
-		if (!__posix_timers_find(head, sig, sig->posix_timer_id)) {
+		id = sig->next_posix_timer_id;
+
+		/* Write the next ID back. Clamp it to the positive space */
+		WRITE_ONCE(sig->next_posix_timer_id, (id + 1) & INT_MAX);
+
+		head = &posix_timers_hashtable[hash(sig, id)];
+		if (!__posix_timers_find(head, sig, id)) {
 			hlist_add_head_rcu(&timer->t_hash, head);
-			ret = sig->posix_timer_id;
+			spin_unlock(&hash_lock);
+			return id;
 		}
-		if (++sig->posix_timer_id < 0)
-			sig->posix_timer_id = 0;
-		if ((sig->posix_timer_id == first_free_id) && (ret == -ENOENT))
-			/* Loop over all possible ids completed */
-			ret = -EAGAIN;
 		spin_unlock(&hash_lock);
-	} while (ret == -ENOENT);
-	return ret;
+	}
+	/* POSIX return code when no timer ID could be allocated */
+	return -EAGAIN;
 }
 
 static inline void unlock_timer(struct k_itimer *timr, unsigned long flags)
@@ -1033,27 +1037,59 @@ SYSCALL_DEFINE1(timer_delete, timer_t, timer_id)
 }
 
 /*
- * return timer owned by the process, used by exit_itimers
+ * Delete a timer if it is armed, remove it from the hash and schedule it
+ * for RCU freeing.
  */
 static void itimer_delete(struct k_itimer *timer)
 {
-retry_delete:
-	spin_lock_irq(&timer->it_lock);
+	unsigned long flags;
 
+retry_delete:
+	/*
+	 * irqsave is required to make timer_wait_running() work.
+	 */
+	spin_lock_irqsave(&timer->it_lock, flags);
+
+	/*
+	 * Even if the timer is not longer accessible from other tasks
+	 * it still might be armed and queued in the underlying timer
+	 * mechanism. Worse, that timer mechanism might run the expiry
+	 * function concurrently.
+	 */
 	if (timer_delete_hook(timer) == TIMER_RETRY) {
-		spin_unlock_irq(&timer->it_lock);
+		/*
+		 * Timer is expired concurrently, prevent livelocks
+		 * and pointless spinning on RT.
+		 *
+		 * The CONFIG_POSIX_CPU_TIMERS_TASK_WORK=y case is
+		 * irrelevant here because obviously the exiting task
+		 * cannot be expiring timer in task work concurrently.
+		 * Ditto for CONFIG_POSIX_CPU_TIMERS_TASK_WORK=n as the
+		 * tick interrupt cannot run on this CPU because the above
+		 * spin_lock disabled interrupts.
+		 *
+		 * timer_wait_running() drops timer::it_lock, which opens
+		 * the possibility for another task to delete the timer.
+		 *
+		 * That's not possible here because this is invoked from
+		 * do_exit() only for the last thread of the thread group.
+		 * So no other task can access that timer.
+		 */
+		if (WARN_ON_ONCE(timer_wait_running(timer, &flags) != timer))
+			return;
+
 		goto retry_delete;
 	}
 	list_del(&timer->list);
 
-	spin_unlock_irq(&timer->it_lock);
+	spin_unlock_irqrestore(&timer->it_lock, flags);
 	release_posix_timer(timer, IT_ID_SET);
 }
 
 /*
- * This is called by do_exit or de_thread, only when nobody else can
- * modify the signal->posix_timers list. Yet we need sighand->siglock
- * to prevent the race with /proc/pid/timers.
+ * Invoked from do_exit() when the last thread of a thread group exits.
+ * At that point no other task can access the timers of the dying
+ * task anymore.
  */
 void exit_itimers(struct task_struct *tsk)
 {
@@ -1063,10 +1099,12 @@ void exit_itimers(struct task_struct *tsk)
 	if (list_empty(&tsk->signal->posix_timers))
 		return;
 
+	/* Protect against concurrent read via /proc/$PID/timers */
 	spin_lock_irq(&tsk->sighand->siglock);
 	list_replace_init(&tsk->signal->posix_timers, &timers);
 	spin_unlock_irq(&tsk->sighand->siglock);
 
+	/* The timers are not longer accessible via tsk::signal */
 	while (!list_empty(&timers)) {
 		tmr = list_first_entry(&timers, struct k_itimer, list);
 		itimer_delete(tmr);
diff --git a/localversion-rt b/localversion-rt
index 6e44e540b927b..9f7d0bdbffb18 100644
--- a/localversion-rt
+++ b/localversion-rt
@@ -1 +1 @@
--rt12
+-rt13

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