From: Thomas Gleixner tglx@linutronix.de
[ Upstream commit 9d9e522010eb5685d8b53e8a24320653d9d4cbbf ]
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: ec8f954a40da ("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 Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/time/posix-timers.c | 43 +++++++++++++++++++++++++++++++------- 1 file changed, 35 insertions(+), 8 deletions(-)
diff --git a/kernel/time/posix-timers.c b/kernel/time/posix-timers.c index 808a247205a9a..ed3c4a9543982 100644 --- a/kernel/time/posix-timers.c +++ b/kernel/time/posix-timers.c @@ -1037,27 +1037,52 @@ SYSCALL_DEFINE1(timer_delete, timer_t, timer_id) }
/* - * 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);