Messages in this thread | | | Date | Mon, 23 Sep 2019 16:09:00 +0200 (CEST) | From | Thomas Gleixner <> | Subject | Re: [PATCH] futex: robust futex maybe never be awaked, on rare situation. |
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On Mon, 23 Sep 2019, Peter Zijlstra wrote: > On Mon, Sep 23, 2019 at 11:18:20AM +0800, Yunfeng Cui wrote: > > I use model checker find a issue of robust and pi futex. On below > > situation, the owner can't find something in pi_state_list, while > > the requester will be blocked, never be awaked. > > > > CPU0 CPU1 > > futex_lock_pi > > /*some cs code*/ > > futex_lock_pi > > futex_lock_pi_atomic > > ... > > newval = uval | FUTEX_WAITERS; > > ret = lock_pi_update_atomic(uaddr, uval, newval); > > ... > > attach_to_pi_owner > > .... > > p = find_get_task_by_vpid(pid); > > if (!p) > > return handle_exit_race(uaddr, uval, NULL); > > .... > > raw_spin_lock_irq(&p->pi_lock); > > .... > > pi_state = alloc_pi_state(); > > .... > > do_exit->mm_release > > if (unlikely(tsk->robust_list)) { > > exit_robust_list(tsk); > > tsk->robust_list = NULL; > > } > > if (unlikely(!list_empty(&tsk->pi_state_list))) > > exit_pi_state_list(tsk); /*WILL MISS*/ > > list_add(&pi_state->list, &p->pi_state_list); > > WILL BLOCKED, NEVER WAKEUP! > > Did you forget/overlook the pi_lock fiddling in do_exit() ? I'm thinking > that would make the above impossible.
Right. I was trying to construct a case which allows the above, but failed to do so.
Let's look at the exiting task:
exit() exit_signals() tsk->flags |= PF_EXITING;
smp_mb();
raw_spin_lock_irq(&tsk->pi_lock); (1) raw_spin_unlock_irq(&tsk->pi_lock);
exit_mm() mm_release() exit_robust_list()
if (!list_empty(&tsk->pi_state_list))) exit_pi_state_list(tsk);
And now at the attaching task:
attach_to_pi_owner() raw_spin_lock_irq(tsk->pi_lock); if (tsk->flags & PF_EXITING) return;
pi_state = alloc_pi_state()
list_add(pi_state, tsk->pi_state_list);
See (1) above. That's the crucial point.
Once the exiting task has set PF_EXITING and acquired tsk->pi_lock, it is impossible for the attaching task to queue itself as it _must_ observe PF_EXITING after it acquired tsk->pi_lock.
If it manages to acquire tsk->pi_lock _before_ the existing task does that, then it either observes PF_EXITING or not.
If it does, it goes out. If it does not, it queues itself on tsk->pi_state_list and will be cleaned up by the exiting task.
Simplified concurrency picture:
Case 1: Attacher does not see PF_EXITING
CPU 0 CPU 1
lock(&tsk->pi_lock); tsk->flags |= PF_EXITING; if (!(tsk->flags & PF_EXITING)) queue(pi_state, tsk); smp_mb(); unlock(&tsk->pi_lock); lock(&tsk->pi_lock); (1) unlock(&tsk->pi_lock);
if (!list_empty(&tsk->pi_state_list))) exit_pi_state_list(tsk);
Case 2: Attacher does see PF_EXITING before (1)
CPU 0 CPU 1
lock(&tsk->pi_lock); tsk->flags |= PF_EXITING; if (tsk->flags & PF_EXITING) { unlock(&tsk->pi_lock); smp_mb(); return; lock(&tsk->pi_lock); } (1) unlock(&tsk->pi_lock);
The attacher CANNOT be queued in tsk->pi_state_list
Case 2: Attacher does see PF_EXITING after (1)
CPU 0 CPU 1
tsk->flags |= PF_EXITING; smp_mb(); lock(&tsk->pi_lock); lock(&tsk->pi_lock); (1) unlock(&tsk->pi_lock); if (tsk->flags & PF_EXITING) { unlock(&tsk->pi_lock); return; }
There are no other cases possible. If the attacher can observe
!(tsk->flags & PF_EXITING)
_after_ (1) then there is something seriously wrong, but not in the futex code. That would be a massive memory ordering issue.
Thanks,
tglx
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