posix-timers: Use a callback for cancel synchronization on PREEMPT_RT

Posix timer delete retry loops are affected by the same priority inversion
and live lock issues as the other timers.

Provide a RT specific synchronization function which keeps a reference to
the timer by holding rcu read lock to prevent the timer from being freed,
dropping the timer lock and invoking the timer specific wait function via a
new callback.

This does not yet cover posix CPU timers because they need more special
treatment on PREEMPT_RT.

[ This is folded into the original attempt which did not use a callback. ]

Originally-by: Anna-Maria Gleixenr <anna-maria@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://lkml.kernel.org/r/20190819143801.656864506@linutronix.de

diff --git a/kernel/time/alarmtimer.c b/kernel/time/alarmtimer.c
index 3694744..ec32876 100644 (file)
--- a/kernel/time/alarmtimer.c
+++ b/kernel/time/alarmtimer.c
@@ -606,6 +606,19 @@ static int alarm_timer_try_to_cancel(struct k_itimer *timr)
 }
 
 /**
+ * alarm_timer_wait_running - Posix timer callback to wait for a timer
+ * @timr:      Pointer to the posixtimer data struct
+ *
+ * Called from the core code when timer cancel detected that the callback
+ * is running. @timr is unlocked and rcu read lock is held to prevent it
+ * from being freed.
+ */
+static void alarm_timer_wait_running(struct k_itimer *timr)
+{
+       hrtimer_cancel_wait_running(&timr->it.alarm.alarmtimer.timer);
+}
+
+/**
  * alarm_timer_arm - Posix timer callback to arm a timer
  * @timr:      Pointer to the posixtimer data struct
  * @expires:   The new expiry time
@@ -834,6 +847,7 @@ const struct k_clock alarm_clock = {
        .timer_forward          = alarm_timer_forward,
        .timer_remaining        = alarm_timer_remaining,
        .timer_try_to_cancel    = alarm_timer_try_to_cancel,
+       .timer_wait_running     = alarm_timer_wait_running,
        .nsleep                 = alarm_timer_nsleep,
 };
 #endif /* CONFIG_POSIX_TIMERS */

diff --git a/kernel/time/posix-timers.c b/kernel/time/posix-timers.c
index 3e663f9..9e37783 100644 (file)
--- a/kernel/time/posix-timers.c
+++ b/kernel/time/posix-timers.c
@@ -805,13 +805,25 @@ static int common_hrtimer_try_to_cancel(struct k_itimer *timr)
        return hrtimer_try_to_cancel(&timr->it.real.timer);
 }
 
+static void common_timer_wait_running(struct k_itimer *timer)
+{
+       hrtimer_cancel_wait_running(&timer->it.real.timer);
+}
+
 static struct k_itimer *timer_wait_running(struct k_itimer *timer,
                                           unsigned long *flags)
 {
+       const struct k_clock *kc = READ_ONCE(timer->kclock);
        timer_t timer_id = READ_ONCE(timer->it_id);
 
+       /* Prevent kfree(timer) after dropping the lock */
+       rcu_read_lock();
        unlock_timer(timer, *flags);
-       cpu_relax();
+
+       if (!WARN_ON_ONCE(!kc->timer_wait_running))
+               kc->timer_wait_running(timer);
+
+       rcu_read_unlock();
        /* Relock the timer. It might be not longer hashed. */
        return lock_timer(timer_id, flags);
 }
@@ -1255,6 +1267,7 @@ static const struct k_clock clock_realtime = {
        .timer_forward          = common_hrtimer_forward,
        .timer_remaining        = common_hrtimer_remaining,
        .timer_try_to_cancel    = common_hrtimer_try_to_cancel,
+       .timer_wait_running     = common_timer_wait_running,
        .timer_arm              = common_hrtimer_arm,
 };
 
@@ -1270,6 +1283,7 @@ static const struct k_clock clock_monotonic = {
        .timer_forward          = common_hrtimer_forward,
        .timer_remaining        = common_hrtimer_remaining,
        .timer_try_to_cancel    = common_hrtimer_try_to_cancel,
+       .timer_wait_running     = common_timer_wait_running,
        .timer_arm              = common_hrtimer_arm,
 };
 
@@ -1300,6 +1314,7 @@ static const struct k_clock clock_tai = {
        .timer_forward          = common_hrtimer_forward,
        .timer_remaining        = common_hrtimer_remaining,
        .timer_try_to_cancel    = common_hrtimer_try_to_cancel,
+       .timer_wait_running     = common_timer_wait_running,
        .timer_arm              = common_hrtimer_arm,
 };
 
@@ -1315,6 +1330,7 @@ static const struct k_clock clock_boottime = {
        .timer_forward          = common_hrtimer_forward,
        .timer_remaining        = common_hrtimer_remaining,
        .timer_try_to_cancel    = common_hrtimer_try_to_cancel,
+       .timer_wait_running     = common_timer_wait_running,
        .timer_arm              = common_hrtimer_arm,
 };
 

diff --git a/kernel/time/posix-timers.h b/kernel/time/posix-timers.h
index de5daa6..897c29e 100644 (file)
--- a/kernel/time/posix-timers.h
+++ b/kernel/time/posix-timers.h
@@ -24,6 +24,7 @@ struct k_clock {
        int     (*timer_try_to_cancel)(struct k_itimer *timr);
        void    (*timer_arm)(struct k_itimer *timr, ktime_t expires,
                             bool absolute, bool sigev_none);
+       void    (*timer_wait_running)(struct k_itimer *timr);
 };
 
 extern const struct k_clock clock_posix_cpu;