Re: [patch 01/20] posix-timers: Prevent RT livelock in itimer_delete()

Thomas Gleixner
2023-05-04T18:20:37Z

On Thu, May 04 2023 at 19:06, Frederic Weisbecker wrote:
> Le Tue, Apr 25, 2023 at 08:48:56PM +0200, Thomas Gleixner a écrit :
>> 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. On RT kernels this is a potential livelock when the exiting task
>> preempted the hrtimer soft interrupt.
>>
>> This only affects hrtimer based timers as Posix CPU timers cannot be
>> concurrently expired. For CONFIG_POSIX_CPU_TIMERS_TASK_WORK=y this is
>> obviously impossible as the task cannot run task work and exit at the same
>> time. The CONFIG_POSIX_CPU_TIMERS_TASK_WORK=n (only non-RT) is prevented
>> because interrupts are disabled.
>
> But the owner of the timer is not the same as the target of the timer, right?
>
> Though I seem to remember that we forbid setting a timer to a target outside
> the current process, in which case the owner and the target are the same at
> this exit stage. But I can't remember what enforces that permission in
> pid_for_clock()..

The owner of the timer is always the one which needs to find the entity
to synchronize on, whether that's the right hrtimer base or the task
which runs the expiry code.

wait_for_running_timer() is taking that into account:

- The hrtimer timer based posix timers lock the hrtimer base expiry
lock on the base to which the timer is currently associated

- Posix CPU timers can be armed on a differnet process (only per
thread timers are restricted to currents threadgroup) but the
wait_for_running() callback "knows" how to find that process:

When the timer is moved to the expiry list it gets:

cputimer->firing = 1;
rcu_assign_pointer(ctmr->handling, current);

and the wait for running side does:

rcu_read_lock()
tsk = rcu_dereference(timr->it.cpu.handling);
....
mutex_lock(&tsk->posix_cputimers_work.mutex);

See collect_timerqueue(), handle_posix_cpu_timers() and
posix_cpu_timer_wait_running() for details.

commit f7abf14f0001 ("posix-cpu-timers: Implement the missing
timer_wait_running callback") has quite some prose in the changelog.

Thanks,

tglx