[PATCH RT 0/4] Linux v4.19.255-rt114-rc2

[PATCH RT 0/4] Linux v4.19.255-rt114-rc2

[Daniel Wagner]

Dear RT Folks,

This is the RT stable review cycle of patch 4.19.255-rt114-rc2.

Please scream at me if I messed something up. Please test the patches
too.

The -rc release will be uploaded to kernel.org and will be deleted
when the final release is out. This is just a review release (or
release candidate).

The pre-releases will not be pushed to the git repository, only the
final release is.

If all goes well, this patch will be converted to the next main
release on 2022-11-10.

To build 4.19.255-rt114-rc2 directly, the following patches should be applied:

  https://www.kernel.org/pub/linux/kernel/v4.x/linux-4.19.tar.xz

  https://www.kernel.org/pub/linux/kernel/v4.x/patch-4.19.255.xz

  https://www.kernel.org/pub/linux/kernel/projects/rt/4.19/older/patch-4.19.255-rt114-rc2.patch.xz

Signing key fingerprint:

  5BF6 7BC5 0826 72CA BB45  ACAE 587C 5ECA 5D0A 306C

All keys used for the above files and repositories can be found on the
following git repository:

   git://git.kernel.org/pub/scm/docs/kernel/pgpkeys.git

Enjoy!
Daniel

Changes from v4.19.255-rt114-rc1:


Anna-Maria Gleixner (1):
  timers: Prepare support for PREEMPT_RT

Daniel Wagner (1):
  Linux 4.19.255-rt114-rc2

Sebastian Andrzej Siewior (1):
  timers: Don't block on ->expiry_lock for TIMER_IRQSAFE timers

Thomas Gleixner (1):
  timers: Move clearing of base::timer_running under base:: Lock

 kernel/time/timer.c | 137 +++++++++++++++++++++++++++++++-------------
 localversion-rt     |   2 +-
 2 files changed, 98 insertions(+), 41 deletions(-)

-- 
2.38.0

[PATCH RT 1/4] timers: Prepare support for PREEMPT_RT

[Daniel Wagner]

From: Anna-Maria Gleixner <anna-maria@linutronix.de>

v4.19.255-rt114-rc2 stable review patch.
If anyone has any objections, please let me know.

-----------


Upstream commit 030dcdd197d77374879bb5603d091eee7d8aba80

When PREEMPT_RT is enabled, the soft interrupt thread can be preempted.  If
the soft interrupt thread is preempted in the middle of a timer callback,
then calling del_timer_sync() can lead to two issues:

  - If the caller is on a remote CPU then it has to spin wait for the timer
    handler to complete. This can result in unbound priority inversion.

  - If the caller originates from the task which preempted the timer
    handler on the same CPU, then spin waiting for the timer handler to
    complete is never going to end.

To avoid these issues, add a new lock to the timer base which is held
around the execution of the timer callbacks. If del_timer_sync() detects
that the timer callback is currently running, it blocks on the expiry
lock. When the callback is finished, the expiry lock is dropped by the
softirq thread which wakes up the waiter and the system makes progress.

This addresses both the priority inversion and the life lock issues.

This mechanism is not used for timers which are marked IRQSAFE as for those
preemption is disabled accross the callback and therefore this situation
cannot happen. The callbacks for such timers need to be individually
audited for RT compliance.

The same issue can happen in virtual machines when the vCPU which runs a
timer callback is scheduled out. If a second vCPU of the same guest calls
del_timer_sync() it will spin wait for the other vCPU to be scheduled back
in. The expiry lock mechanism would avoid that. It'd be trivial to enable
this when paravirt spinlocks are enabled in a guest, but it's not clear
whether this is an actual problem in the wild, so for now it's an RT only
mechanism.

As the softirq thread can be preempted with PREEMPT_RT=y, the SMP variant
of del_timer_sync() needs to be used on UP as well.

[ tglx: Refactored it for mainline ]

Signed-off-by: Anna-Maria Gleixner <anna-maria@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20190726185753.832418500@linutronix.de
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Daniel Wagner <wagi@monom.org>
---
 kernel/time/timer.c | 130 ++++++++++++++++++++++++++++++--------------
 1 file changed, 88 insertions(+), 42 deletions(-)

diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index 0a6d60b3e67c..b859ecf6424b 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -198,7 +198,10 @@ EXPORT_SYMBOL(jiffies_64);
 struct timer_base {
 	raw_spinlock_t		lock;
 	struct timer_list	*running_timer;
+#ifdef CONFIG_PREEMPT_RT
 	spinlock_t		expiry_lock;
+	atomic_t		timer_waiters;
+#endif
 	unsigned long		clk;
 	unsigned long		next_expiry;
 	unsigned int		cpu;
@@ -1227,8 +1230,14 @@ int del_timer(struct timer_list *timer)
 }
 EXPORT_SYMBOL(del_timer);
 
-static int __try_to_del_timer_sync(struct timer_list *timer,
-				   struct timer_base **basep)
+/**
+ * try_to_del_timer_sync - Try to deactivate a timer
+ * @timer: timer to delete
+ *
+ * This function tries to deactivate a timer. Upon successful (ret >= 0)
+ * exit the timer is not queued and the handler is not running on any CPU.
+ */
+int try_to_del_timer_sync(struct timer_list *timer)
 {
 	struct timer_base *base;
 	unsigned long flags;
@@ -1236,7 +1245,7 @@ static int __try_to_del_timer_sync(struct timer_list *timer,
 
 	debug_assert_init(timer);
 
-	*basep = base = lock_timer_base(timer, &flags);
+	base = lock_timer_base(timer, &flags);
 
 	if (base->running_timer != timer)
 		ret = detach_if_pending(timer, base, true);
@@ -1245,45 +1254,80 @@ static int __try_to_del_timer_sync(struct timer_list *timer,
 
 	return ret;
 }
+EXPORT_SYMBOL(try_to_del_timer_sync);
 
-/**
- * try_to_del_timer_sync - Try to deactivate a timer
- * @timer: timer to delete
- *
- * This function tries to deactivate a timer. Upon successful (ret >= 0)
- * exit the timer is not queued and the handler is not running on any CPU.
- */
-int try_to_del_timer_sync(struct timer_list *timer)
+#ifdef CONFIG_PREEMPT_RT
+static __init void timer_base_init_expiry_lock(struct timer_base *base)
 {
-	struct timer_base *base;
+	spin_lock_init(&base->expiry_lock);
+}
 
-	return __try_to_del_timer_sync(timer, &base);
+static inline void timer_base_lock_expiry(struct timer_base *base)
+{
+	spin_lock(&base->expiry_lock);
 }
-EXPORT_SYMBOL(try_to_del_timer_sync);
 
-#if defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT_FULL)
-static int __del_timer_sync(struct timer_list *timer)
+static inline void timer_base_unlock_expiry(struct timer_base *base)
 {
-	struct timer_base *base;
-	int ret;
+	spin_unlock(&base->expiry_lock);
+}
 
-	for (;;) {
-		ret = __try_to_del_timer_sync(timer, &base);
-		if (ret >= 0)
-			return ret;
+/*
+ * The counterpart to del_timer_wait_running().
+ *
+ * If there is a waiter for base->expiry_lock, then it was waiting for the
+ * timer callback to finish. Drop expiry_lock and reaquire it. That allows
+ * the waiter to acquire the lock and make progress.
+ */
+static void timer_sync_wait_running(struct timer_base *base)
+{
+	if (atomic_read(&base->timer_waiters)) {
+		spin_unlock(&base->expiry_lock);
+		spin_lock(&base->expiry_lock);
+	}
+}
 
-		if (READ_ONCE(timer->flags) & TIMER_IRQSAFE)
-			continue;
+/*
+ * This function is called on PREEMPT_RT kernels when the fast path
+ * deletion of a timer failed because the timer callback function was
+ * running.
+ *
+ * This prevents priority inversion, if the softirq thread on a remote CPU
+ * got preempted, and it prevents a life lock when the task which tries to
+ * delete a timer preempted the softirq thread running the timer callback
+ * function.
+ */
+static void del_timer_wait_running(struct timer_list *timer)
+{
+	u32 tf;
+
+	tf = READ_ONCE(timer->flags);
+	if (!(tf & TIMER_MIGRATING)) {
+		struct timer_base *base = get_timer_base(tf);
 
 		/*
-		 * When accessing the lock, timers of base are no longer expired
-		 * and so timer is no longer running.
+		 * Mark the base as contended and grab the expiry lock,
+		 * which is held by the softirq across the timer
+		 * callback. Drop the lock immediately so the softirq can
+		 * expire the next timer. In theory the timer could already
+		 * be running again, but that's more than unlikely and just
+		 * causes another wait loop.
 		 */
-		spin_lock(&base->expiry_lock);
-		spin_unlock(&base->expiry_lock);
+		atomic_inc(&base->timer_waiters);
+		spin_lock_bh(&base->expiry_lock);
+		atomic_dec(&base->timer_waiters);
+		spin_unlock_bh(&base->expiry_lock);
 	}
 }
+#else
+static inline void timer_base_init_expiry_lock(struct timer_base *base) { }
+static inline void timer_base_lock_expiry(struct timer_base *base) { }
+static inline void timer_base_unlock_expiry(struct timer_base *base) { }
+static inline void timer_sync_wait_running(struct timer_base *base) { }
+static inline void del_timer_wait_running(struct timer_list *timer) { }
+#endif
 
+#if defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT_FULL)
 /**
  * del_timer_sync - deactivate a timer and wait for the handler to finish.
  * @timer: the timer to be deactivated
@@ -1322,6 +1366,8 @@ static int __del_timer_sync(struct timer_list *timer)
  */
 int del_timer_sync(struct timer_list *timer)
 {
+	int ret;
+
 #ifdef CONFIG_LOCKDEP
 	unsigned long flags;
 
@@ -1339,14 +1385,17 @@ int del_timer_sync(struct timer_list *timer)
 	 * could lead to deadlock.
 	 */
 	WARN_ON(in_irq() && !(timer->flags & TIMER_IRQSAFE));
-	/*
-	 * Must be able to sleep on PREEMPT_RT because of the slowpath in
-	 * __del_timer_sync().
-	 */
-	if (IS_ENABLED(CONFIG_PREEMPT_RT) && !(timer->flags & TIMER_IRQSAFE))
-		might_sleep();
 
-	return __del_timer_sync(timer);
+	do {
+		ret = try_to_del_timer_sync(timer);
+
+		if (unlikely(ret < 0)) {
+			del_timer_wait_running(timer);
+			cpu_relax();
+		}
+	} while (ret < 0);
+
+	return ret;
 }
 EXPORT_SYMBOL(del_timer_sync);
 #endif
@@ -1410,15 +1459,12 @@ static void expire_timers(struct timer_base *base, struct hlist_head *head)
 			raw_spin_unlock(&base->lock);
 			call_timer_fn(timer, fn);
 			base->running_timer = NULL;
-			spin_unlock(&base->expiry_lock);
-			spin_lock(&base->expiry_lock);
 			raw_spin_lock(&base->lock);
 		} else {
 			raw_spin_unlock_irq(&base->lock);
 			call_timer_fn(timer, fn);
 			base->running_timer = NULL;
-			spin_unlock(&base->expiry_lock);
-			spin_lock(&base->expiry_lock);
+			timer_sync_wait_running(base);
 			raw_spin_lock_irq(&base->lock);
 		}
 	}
@@ -1715,7 +1761,7 @@ static inline void __run_timers(struct timer_base *base)
 	if (!time_after_eq(jiffies, base->clk))
 		return;
 
-	spin_lock(&base->expiry_lock);
+	timer_base_lock_expiry(base);
 	raw_spin_lock_irq(&base->lock);
 
 	/*
@@ -1743,7 +1789,7 @@ static inline void __run_timers(struct timer_base *base)
 			expire_timers(base, heads + levels);
 	}
 	raw_spin_unlock_irq(&base->lock);
-	spin_unlock(&base->expiry_lock);
+	timer_base_unlock_expiry(base);
 }
 
 /*
@@ -1990,7 +2036,7 @@ static void __init init_timer_cpu(int cpu)
 		base->cpu = cpu;
 		raw_spin_lock_init(&base->lock);
 		base->clk = jiffies;
-		spin_lock_init(&base->expiry_lock);
+		timer_base_init_expiry_lock(base);
 	}
 }
 
-- 
2.38.0

[PATCH RT 2/4] timers: Move clearing of base::timer_running under base:: Lock

[Daniel Wagner]

From: Thomas Gleixner <tglx@linutronix.de>

v4.19.255-rt114-rc2 stable review patch.
If anyone has any objections, please let me know.

-----------


Upstream commit bb7262b295472eb6858b5c49893954794027cd84

syzbot reported KCSAN data races vs. timer_base::timer_running being set to
NULL without holding base::lock in expire_timers().

This looks innocent and most reads are clearly not problematic, but
Frederic identified an issue which is:

 int data = 0;

 void timer_func(struct timer_list *t)
 {
    data = 1;
 }

 CPU 0                                            CPU 1
 ------------------------------                   --------------------------
 base = lock_timer_base(timer, &flags);           raw_spin_unlock(&base->lock);
 if (base->running_timer != timer)                call_timer_fn(timer, fn, baseclk);
   ret = detach_if_pending(timer, base, true);    base->running_timer = NULL;
 raw_spin_unlock_irqrestore(&base->lock, flags);  raw_spin_lock(&base->lock);

 x = data;

If the timer has previously executed on CPU 1 and then CPU 0 can observe
base->running_timer == NULL and returns, assuming the timer has completed,
but it's not guaranteed on all architectures. The comment for
del_timer_sync() makes that guarantee. Moving the assignment under
base->lock prevents this.

For non-RT kernel it's performance wise completely irrelevant whether the
store happens before or after taking the lock. For an RT kernel moving the
store under the lock requires an extra unlock/lock pair in the case that
there is a waiter for the timer, but that's not the end of the world.

Reported-by: syzbot+aa7c2385d46c5eba0b89@syzkaller.appspotmail.com
Reported-by: syzbot+abea4558531bae1ba9fe@syzkaller.appspotmail.com
Fixes: 030dcdd197d7 ("timers: Prepare support for PREEMPT_RT")
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://lore.kernel.org/r/87lfea7gw8.fsf@nanos.tec.linutronix.de
Cc: stable@vger.kernel.org
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Daniel Wagner <wagi@monom.org>
---
 kernel/time/timer.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index b859ecf6424b..603985720f54 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -1282,8 +1282,10 @@ static inline void timer_base_unlock_expiry(struct timer_base *base)
 static void timer_sync_wait_running(struct timer_base *base)
 {
 	if (atomic_read(&base->timer_waiters)) {
+		raw_spin_unlock_irq(&base->lock);
 		spin_unlock(&base->expiry_lock);
 		spin_lock(&base->expiry_lock);
+		raw_spin_lock_irq(&base->lock);
 	}
 }
 
@@ -1458,14 +1460,14 @@ static void expire_timers(struct timer_base *base, struct hlist_head *head)
 		if (timer->flags & TIMER_IRQSAFE) {
 			raw_spin_unlock(&base->lock);
 			call_timer_fn(timer, fn);
-			base->running_timer = NULL;
 			raw_spin_lock(&base->lock);
+			base->running_timer = NULL;
 		} else {
 			raw_spin_unlock_irq(&base->lock);
 			call_timer_fn(timer, fn);
+			raw_spin_lock_irq(&base->lock);
 			base->running_timer = NULL;
 			timer_sync_wait_running(base);
-			raw_spin_lock_irq(&base->lock);
 		}
 	}
 }
-- 
2.38.0

[PATCH RT 3/4] timers: Don't block on ->expiry_lock for TIMER_IRQSAFE timers

[Daniel Wagner]

From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

v4.19.255-rt114-rc2 stable review patch.
If anyone has any objections, please let me know.

-----------


Upstream commit c725dafc95f1b37027840aaeaa8b7e4e9cd20516

PREEMPT_RT does not spin and wait until a running timer completes its
callback but instead it blocks on a sleeping lock to prevent a livelock in
the case that the task waiting for the callback completion preempted the
callback.

This cannot be done for timers flagged with TIMER_IRQSAFE. These timers can
be canceled from an interrupt disabled context even on RT kernels.

The expiry callback of such timers is invoked with interrupts disabled so
there is no need to use the expiry lock mechanism because obviously the
callback cannot be preempted even on RT kernels.

Do not use the timer_base::expiry_lock mechanism when waiting for a running
callback to complete if the timer is flagged with TIMER_IRQSAFE.

Also add a lockdep assertion for RT kernels to validate that the expiry
lock mechanism is always invoked in preemptible context.

[ bigeasy: Dropping that lockdep_assert_preemption_enabled() check in
           backport ]

Reported-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20201103190937.hga67rqhvknki3tp@linutronix.de
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Daniel Wagner <wagi@monom.org>
---
 kernel/time/timer.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index 603985720f54..8c7bfcee9609 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -1304,7 +1304,7 @@ static void del_timer_wait_running(struct timer_list *timer)
 	u32 tf;
 
 	tf = READ_ONCE(timer->flags);
-	if (!(tf & TIMER_MIGRATING)) {
+	if (!(tf & (TIMER_MIGRATING | TIMER_IRQSAFE))) {
 		struct timer_base *base = get_timer_base(tf);
 
 		/*
@@ -1388,6 +1388,15 @@ int del_timer_sync(struct timer_list *timer)
 	 */
 	WARN_ON(in_irq() && !(timer->flags & TIMER_IRQSAFE));
 
+	/*
+	 * Must be able to sleep on PREEMPT_RT because of the slowpath in
+	 * del_timer_wait_running().
+	 */
+#if 0
+	if (IS_ENABLED(CONFIG_PREEMPT_RT) && !(timer->flags & TIMER_IRQSAFE))
+		lockdep_assert_preemption_enabled();
+#endif
+
 	do {
 		ret = try_to_del_timer_sync(timer);
 
-- 
2.38.0

[PATCH RT 4/4] Linux 4.19.255-rt114-rc2

[Daniel Wagner]

v4.19.255-rt114-rc2 stable review patch.
If anyone has any objections, please let me know.

-----------


Signed-off-by: Daniel Wagner <wagi@monom.org>
---
 localversion-rt | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/localversion-rt b/localversion-rt
index fdcd9167ca0b..02781f8a2f80 100644
--- a/localversion-rt
+++ b/localversion-rt
@@ -1 +1 @@
--rt113
+-rt114-rc2
-- 
2.38.0