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posix-timers: Prevent RT livelock in itimer_delete()

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itimer_delete() has a retry loop when the timer is concurrently expired. On
non-RT kernels this just spin-waits until the timer callback has completed,
except for posix CPU timers which have HAVE_POSIX_CPU_TIMERS_TASK_WORK
enabled.

In that case and on RT kernels the existing task could live lock when
preempting the task which does the timer delivery.

Replace spin_unlock() with an invocation of timer_wait_running() to handle
it the same way as the other retry loops in the posix timer code.

Fixes: ec8f954a40 ("posix-timers: Use a callback for cancel synchronization on PREEMPT_RT")
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://lore.kernel.org/r/87v8g7c50d.ffs@tglx
This commit is contained in:
Thomas Gleixner 2023-06-01 22:16:34 +02:00
parent b7a7ce1bb7
commit 9d9e522010
1 changed files with 35 additions and 8 deletions
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43
kernel/time/posix-timers.c
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@ -1037,27 +1037,52 @@ retry_delete:
}
/*
* 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;
/*
* irqsave is required to make timer_wait_running() work.
*/
spin_lock_irqsave(&timer->it_lock, flags);
retry_delete:
/*
* 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.
*
* 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 and delete 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)
{
@ -1067,10 +1092,12 @@ 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);