Re: [PATCH RT V2] sched: Prevent task state corruption by spurious lock wakeup

Sebastian Sewior
2017-06-07T20:18:12Z

On 2017-06-06 14:20:37 [+0200], Thomas Gleixner wrote:
> Mathias and others reported GDB failures on RT.
>
> The following scenario leads to task state corruption:
>
> CPU0 CPU1
>
> T1->state = TASK_XXX;
> spin_lock(&lock)
> rt_spin_lock_slowlock(&lock->rtmutex)
> raw_spin_lock(&rtm->wait_lock);
> T1->saved_state = current->state;
> T1->state = TASK_UNINTERRUPTIBLE;
> spin_unlock(&lock)
> task_blocks_on_rt_mutex(rtm) rt_spin_lock_slowunlock(&lock->rtmutex)
> queue_waiter(rtm) raw_spin_lock(&rtm->wait_lock);
> pi_chain_walk(rtm)
> raw_spin_unlock(&rtm->wait_lock);
> wake_top_waiter(T1)
>
> raw_spin_lock(&rtm->wait_lock);
>
>
> for (;;) {
> if (__try_to_take_rt_mutex()) <- Succeeds
> break;
> ...
> }
>
> T1->state = T1->saved_state;
> try_to_wake_up(T1)
> ttwu_do_wakeup(T1)
> T1->state = TASK_RUNNING;
>
> In most cases this is harmless because waiting for some event, which is the
> usual reason for TASK_[UN]INTERRUPTIBLE has to be safe against other forms
> of spurious wakeups anyway.
>
> But in case of TASK_TRACED this is actually fatal, because the task loses
> the TASK_TRACED state. In consequence it fails to consume SIGSTOP which was
> sent from the debugger and actually delivers SIGSTOP to the task which
> breaks the ptrace mechanics and brings the debugger into an unexpected
> state.
>
> The TASK_TRACED state should prevent getting there due to the state
> matching logic in try_to_wake_up(). But that's not true because
> wake_up_lock_sleeper() uses TASK_ALL as state mask. That's bogus because
> lock sleepers always use TASK_UNINTERRUPTIBLE, so the wakeup should use
> that as well.
>
> The cure is way simpler as figuring it out:
>
> Change the mask used in wake_up_lock_sleeper() from TASK_ALL to
> TASK_UNINTERRUPTIBLE.
>
> Reported-by: Mathias Koehrer <mathias.koehrer@etas.com>
> Reported-by: David Hauck <davidh@netacquire.com>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Cc: stable-rt@vger.kernel.org

Applied

Sebastian