Re: rt_mutex_timed_lock() vs hrtimer_wakeup() race ?

Thomas Gleixner
2006-08-01T07:58:33Z

On Sun, 2006-07-30 at 08:36 +0400, Oleg Nesterov wrote:
> Another question, task_blocks_on_rt_mutex() does get_task_struct() and checks
> owner->pi_blocked_on != NULL under owner->pi_lock. Why ? RT_MUTEX_HAS_WAITERS
> bit is set, we are holding ->wait_lock, so the 'owner' can't go away until
> we drop ->wait_lock. I think we can drop owner->pi_lock right after
> __rt_mutex_adjust_prio(owner), we can't miss owner->pi_blocked_on != NULL
> if it was true before we take owner->pi_lock, and this is the case we should
> worry about, yes?

No.

We hold lock->wait_lock. The owner of this lock can be blocked itself,
which makes it necessary to do the chain walk. The indicator is
owner->pi_blocked_on. This field is only protected by owner->pi_lock.

If we look at this field outside of owner->pi_lock, then we might miss a
chain walk.

CPU 0 CPU 1

lock lock->wait_lock

block_on_rt_mutex()

lock current->pi_lock
current->pi_blocked_on = waiter(lock)
unlock current->pi_lock

boost = owner->pi_blocked_on

owner blocks on lock2
lock owner->pi_lock
owner->pi_blocked_on = waiter(lock2)
unlock owner->pi_lock

lock owner->pi_lock
enqueue waiter
adjust prio
unlock owner->pi_lock
unlock lock->wait_lock

if boost
do_chain_walk()

->pi_blocked_on is protected by ->pi_lock and has to be read/modified
under this lock. That way we can not miss a chain walk:

CPU 0 CPU 1

lock lock->wait_lock

block_on_rt_mutex()

lock current->pi_lock
current->pi_blocked_on = waiter(lock)
unlock current->pi_lock

owner blocks on lock2
lock owner->pi_lock
owner->pi_blocked_on = waiter(lock2)
unlock owner->pi_lock

lock owner->pi_lock
enqueue waiter
adjust prio
boost = owner->pi_blocked_on
unlock owner->pi_lock
unlock lock->wait_lock

if boost
do_chain_walk()

CPU 0 CPU 1

lock lock->wait_lock

block_on_rt_mutex()

lock current->pi_lock
current->pi_blocked_on = waiter(lock)
unlock current->pi_lock

lock owner->pi_lock
enqueue waiter
adjust prio of owner
boost = owner->pi_blocked_on
unlock owner->pi_lock
unlock lock->wait_lock

owner blocks on lock2
lock owner->pi_lock
owner->pi_blocked_on = waiter(lock2)
unlock owner->pi_lock
if boost
do_chain_walk()

owner propagates the priority to owner(lock2)

get_task_struct() is there for a different reason. We have to protect
the gap where we hold _no_ lock in rt_mutex_adjust_prio_chain:

retry:
lock task->pi_lock
block = task->pi_blocked_on->lock
if (! trylock block->wait_lock) {
unlock task->pi_lock

-> task could go away right here, if we do not hold a reference

cpu_relax()
goto retry
}

tglx