*/
static int posix_cpu_timer_create(struct k_itimer *new_timer)
{
+ static struct lock_class_key posix_cpu_timers_key;
struct pid *pid;
rcu_read_lock();
return -EINVAL;
}
+ /*
+ * If posix timer expiry is handled in task work context then
+ * timer::it_lock can be taken without disabling interrupts as all
+ * other locking happens in task context. This requires a seperate
+ * lock class key otherwise regular posix timer expiry would record
+ * the lock class being taken in interrupt context and generate a
+ * false positive warning.
+ */
+ if (IS_ENABLED(CONFIG_POSIX_CPU_TIMERS_TASK_WORK))
+ lockdep_set_class(&new_timer->it_lock, &posix_cpu_timers_key);
+
new_timer->kclock = &clock_posix_cpu;
timerqueue_init(&new_timer->it.cpu.node);
new_timer->it.cpu.pid = get_pid(pid);
return false;
}
-static void __run_posix_cpu_timers(struct task_struct *tsk)
+static void handle_posix_cpu_timers(struct task_struct *tsk);
+
+#ifdef CONFIG_POSIX_CPU_TIMERS_TASK_WORK
+static void posix_cpu_timers_work(struct callback_head *work)
+{
+ handle_posix_cpu_timers(current);
+}
+
+/*
+ * Initialize posix CPU timers task work in init task. Out of line to
+ * keep the callback static and to avoid header recursion hell.
+ */
+void __init posix_cputimers_init_work(void)
+{
+ init_task_work(¤t->posix_cputimers_work.work,
+ posix_cpu_timers_work);
+}
+
+/*
+ * Note: All operations on tsk->posix_cputimer_work.scheduled happen either
+ * in hard interrupt context or in task context with interrupts
+ * disabled. Aside of that the writer/reader interaction is always in the
+ * context of the current task, which means they are strict per CPU.
+ */
+static inline bool posix_cpu_timers_work_scheduled(struct task_struct *tsk)
+{
+ return tsk->posix_cputimers_work.scheduled;
+}
+
+static inline void __run_posix_cpu_timers(struct task_struct *tsk)
+{
+ if (WARN_ON_ONCE(tsk->posix_cputimers_work.scheduled))
+ return;
+
+ /* Schedule task work to actually expire the timers */
+ tsk->posix_cputimers_work.scheduled = true;
+ task_work_add(tsk, &tsk->posix_cputimers_work.work, TWA_RESUME);
+}
+
+static inline bool posix_cpu_timers_enable_work(struct task_struct *tsk,
+ unsigned long start)
+{
+ bool ret = true;
+
+ /*
+ * On !RT kernels interrupts are disabled while collecting expired
+ * timers, so no tick can happen and the fast path check can be
+ * reenabled without further checks.
+ */
+ if (!IS_ENABLED(CONFIG_PREEMPT_RT)) {
+ tsk->posix_cputimers_work.scheduled = false;
+ return true;
+ }
+
+ /*
+ * On RT enabled kernels ticks can happen while the expired timers
+ * are collected under sighand lock. But any tick which observes
+ * the CPUTIMERS_WORK_SCHEDULED bit set, does not run the fastpath
+ * checks. So reenabling the tick work has do be done carefully:
+ *
+ * Disable interrupts and run the fast path check if jiffies have
+ * advanced since the collecting of expired timers started. If
+ * jiffies have not advanced or the fast path check did not find
+ * newly expired timers, reenable the fast path check in the timer
+ * interrupt. If there are newly expired timers, return false and
+ * let the collection loop repeat.
+ */
+ local_irq_disable();
+ if (start != jiffies && fastpath_timer_check(tsk))
+ ret = false;
+ else
+ tsk->posix_cputimers_work.scheduled = false;
+ local_irq_enable();
+
+ return ret;
+}
+#else /* CONFIG_POSIX_CPU_TIMERS_TASK_WORK */
+static inline void __run_posix_cpu_timers(struct task_struct *tsk)
+{
+ lockdep_posixtimer_enter();
+ handle_posix_cpu_timers(tsk);
+ lockdep_posixtimer_exit();
+}
+
+static inline bool posix_cpu_timers_work_scheduled(struct task_struct *tsk)
+{
+ return false;
+}
+
+static inline bool posix_cpu_timers_enable_work(struct task_struct *tsk,
+ unsigned long start)
+{
+ return true;
+}
+#endif /* CONFIG_POSIX_CPU_TIMERS_TASK_WORK */
+
+static void handle_posix_cpu_timers(struct task_struct *tsk)
{
struct k_itimer *timer, *next;
- unsigned long flags;
+ unsigned long flags, start;
LIST_HEAD(firing);
if (!lock_task_sighand(tsk, &flags))
return;
- /*
- * Here we take off tsk->signal->cpu_timers[N] and
- * tsk->cpu_timers[N] all the timers that are firing, and
- * put them on the firing list.
- */
- check_thread_timers(tsk, &firing);
+ do {
+ /*
+ * On RT locking sighand lock does not disable interrupts,
+ * so this needs to be careful vs. ticks. Store the current
+ * jiffies value.
+ */
+ start = READ_ONCE(jiffies);
+ barrier();
- check_process_timers(tsk, &firing);
+ /*
+ * Here we take off tsk->signal->cpu_timers[N] and
+ * tsk->cpu_timers[N] all the timers that are firing, and
+ * put them on the firing list.
+ */
+ check_thread_timers(tsk, &firing);
+
+ check_process_timers(tsk, &firing);
+
+ /*
+ * The above timer checks have updated the exipry cache and
+ * because nothing can have queued or modified timers after
+ * sighand lock was taken above it is guaranteed to be
+ * consistent. So the next timer interrupt fastpath check
+ * will find valid data.
+ *
+ * If timer expiry runs in the timer interrupt context then
+ * the loop is not relevant as timers will be directly
+ * expired in interrupt context. The stub function below
+ * returns always true which allows the compiler to
+ * optimize the loop out.
+ *
+ * If timer expiry is deferred to task work context then
+ * the following rules apply:
+ *
+ * - On !RT kernels no tick can have happened on this CPU
+ * after sighand lock was acquired because interrupts are
+ * disabled. So reenabling task work before dropping
+ * sighand lock and reenabling interrupts is race free.
+ *
+ * - On RT kernels ticks might have happened but the tick
+ * work ignored posix CPU timer handling because the
+ * CPUTIMERS_WORK_SCHEDULED bit is set. Reenabling work
+ * must be done very carefully including a check whether
+ * ticks have happened since the start of the timer
+ * expiry checks. posix_cpu_timers_enable_work() takes
+ * care of that and eventually lets the expiry checks
+ * run again.
+ */
+ } while (!posix_cpu_timers_enable_work(tsk, start));
/*
- * We must release these locks before taking any timer's lock.
+ * We must release sighand lock before taking any timer's lock.
* There is a potential race with timer deletion here, as the
* siglock now protects our private firing list. We have set
* the firing flag in each timer, so that a deletion attempt
list_for_each_entry_safe(timer, next, &firing, it.cpu.elist) {
int cpu_firing;
+ /*
+ * spin_lock() is sufficient here even independent of the
+ * expiry context. If expiry happens in hard interrupt
+ * context it's obvious. For task work context it's safe
+ * because all other operations on timer::it_lock happen in
+ * task context (syscall or exit).
+ */
spin_lock(&timer->it_lock);
list_del_init(&timer->it.cpu.elist);
cpu_firing = timer->it.cpu.firing;
lockdep_assert_irqs_disabled();
/*
+ * If the actual expiry is deferred to task work context and the
+ * work is already scheduled there is no point to do anything here.
+ */
+ if (posix_cpu_timers_work_scheduled(tsk))
+ return;
+
+ /*
* The fast path checks that there are no expired thread or thread
* group timers. If that's so, just return.
*/
if (!fastpath_timer_check(tsk))
return;
- lockdep_posixtimer_enter();
__run_posix_cpu_timers(tsk);
- lockdep_posixtimer_exit();
}
/*
projects / platform / kernel / linux-rpi.git / commitdiff