posix-cpu-timers: Implement the missing timer_wait_running callback

For some unknown reason the introduction of the timer_wait_running callback
missed to fixup posix CPU timers, which went unnoticed for almost four years.
Marco reported recently that the WARN_ON() in timer_wait_running()
triggers with a posix CPU timer test case.

Posix CPU timers have two execution models for expiring timers depending on
CONFIG_POSIX_CPU_TIMERS_TASK_WORK:

1) If not enabled, the expiry happens in hard interrupt context so
   spin waiting on the remote CPU is reasonably time bound.

   Implement an empty stub function for that case.

2) If enabled, the expiry happens in task work before returning to user
   space or guest mode. The expired timers are marked as firing and moved
   from the timer queue to a local list head with sighand lock held. Once
   the timers are moved, sighand lock is dropped and the expiry happens in
   fully preemptible context. That means the expiring task can be scheduled
   out, migrated, interrupted etc. So spin waiting on it is more than
   suboptimal.

   The timer wheel has a timer_wait_running() mechanism for RT, which uses
   a per CPU timer-base expiry lock which is held by the expiry code and the
   task waiting for the timer function to complete blocks on that lock.

   This does not work in the same way for posix CPU timers as there is no
   timer base and expiry for process wide timers can run on any task
   belonging to that process, but the concept of waiting on an expiry lock
   can be used too in a slightly different way:

    - Add a mutex to struct posix_cputimers_work. This struct is per task
      and used to schedule the expiry task work from the timer interrupt.

    - Add a task_struct pointer to struct cpu_timer which is used to store
      a the task which runs the expiry. That's filled in when the task
      moves the expired timers to the local expiry list. That's not
      affecting the size of the k_itimer union as there are bigger union
      members already

    - Let the task take the expiry mutex around the expiry function

    - Let the waiter acquire a task reference with rcu_read_lock() held and
      block on the expiry mutex

   This avoids spin-waiting on a task which might not even be on a CPU and
   works nicely for RT too.

Fixes: ec8f954a40da ("posix-timers: Use a callback for cancel synchronization on PREEMPT_RT")
Reported-by: Marco Elver <elver@google.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Marco Elver <elver@google.com>
Tested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/87zg764ojw.ffs@tglx

1 files changed, 11 insertions, 6 deletions

diff --git a/include/linux/posix-timers.h b/include/linux/posix-timers.h
index 2c6e99ca48af..d607f51404fc 100644
--- a/include/linux/posix-timers.h
+++ b/include/linux/posix-timers.h
@@ -4,6 +4,7 @@
 
 #include <linux/spinlock.h>
 #include <linux/list.h>
+#include <linux/mutex.h>
 #include <linux/alarmtimer.h>
 #include <linux/timerqueue.h>
 
@@ -62,16 +63,18 @@ static inline int clockid_to_fd(const clockid_t clk)
  * cpu_timer - Posix CPU timer representation for k_itimer
  * @node:	timerqueue node to queue in the task/sig
  * @head:	timerqueue head on which this timer is queued
- * @task:	Pointer to target task
+ * @pid:	Pointer to target task PID
  * @elist:	List head for the expiry list
  * @firing:	Timer is currently firing
+ * @handling:	Pointer to the task which handles expiry
  */
 struct cpu_timer {
-	struct timerqueue_node	node;
-	struct timerqueue_head	*head;
-	struct pid		*pid;
-	struct list_head	elist;
-	int			firing;
+	struct timerqueue_node		node;
+	struct timerqueue_head		*head;
+	struct pid			*pid;
+	struct list_head		elist;
+	int				firing;
+	struct task_struct __rcu	*handling;
 };
 
 static inline bool cpu_timer_enqueue(struct timerqueue_head *head,
@@ -135,10 +138,12 @@ struct posix_cputimers {
 /**
  * posix_cputimers_work - Container for task work based posix CPU timer expiry
  * @work:	The task work to be scheduled
+ * @mutex:	Mutex held around expiry in context of this task work
  * @scheduled:  @work has been scheduled already, no further processing
  */
 struct posix_cputimers_work {
 	struct callback_head	work;
+	struct mutex		mutex;
 	unsigned int		scheduled;
 };