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Subject[PATCH 0/1] futex: check PF_KTHREAD rather than !p->mm to filter out kthreads
Thomas, et al, could you please review?

The change looks trivial, but I simply can not understand this logic,
please help.



First of all, why exactly do we need this mm/PF_KTHREAD check added by
f0d71b3dcb8332f7971 ? Of course, it is simply wrong to declare a random
kernel thread to be the owner as the changelog says. But why kthread is
worse than a random user-space task, say, /sbin/init?

IIUC, the fact that we can abuse ->pi_state_list is not that bad, no matter
if this (k)thread will exit or not. AFAICS, the only problem is that we can
boost the prio of this thread. Or I missed another problem?

I am asking because we need to backport some fixes, and I am trying to
convince myself that I actually understand what I am trying to do ;)




And another question. Lets forget about this ->mm check. I simply can not
understand this

ret = (p->flags & PF_EXITPIDONE) ? -ESRCH : -EAGAIN

logic in attach_to_pi_owner(). First of all, why do we need to retry if
PF_EXITING is set but PF_EXITPIDONE is not? Why we can not simply ignore
PF_EXITING and rely on exit_pi_state_list() if PF_EXITPIDONE is not set?

I must have missed something but this looks buggy, I do not see any
preemption point in this "retry" loop. Suppose that max_cpus=1 and rt_task()
preempts the non-rt PF_EXITING owner. Looks like futex_lock_pi() can spin
forever in this case? (OK, ignoring RT throttling).

IOW. Could you explain why the patch below is wrong correctness-wise?
Lets ignore the fact it is obviously ugly and suboptimal...

Thanks,

Oleg.

--- x/kernel/exit.c
+++ x/kernel/exit.c
@@ -684,13 +684,7 @@ void do_exit(long code)
if (unlikely(tsk->flags & PF_EXITING)) {
pr_alert("Fixing recursive fault but reboot is needed!\n");
/*
- * We can do this unlocked here. The futex code uses
- * this flag just to verify whether the pi state
- * cleanup has been done or not. In the worst case it
- * loops once more. We pretend that the cleanup was
- * done as there is no way to return. Either the
- * OWNER_DIED bit is set by now or we push the blocked
- * task into the wait for ever nirwana as well.
+ * XXX: ADD COMMENT
*/
tsk->flags |= PF_EXITPIDONE;
set_current_state(TASK_UNINTERRUPTIBLE);
@@ -699,8 +693,7 @@ void do_exit(long code)

exit_signals(tsk); /* sets PF_EXITING */
/*
- * tsk->flags are checked in the futex code to protect against
- * an exiting task cleaning up the robust pi futexes.
+ * XXX: anobody else needs this ?
*/
smp_mb();
raw_spin_unlock_wait(&tsk->pi_lock);
@@ -779,12 +772,6 @@ void do_exit(long code)
* Make sure we are holding no locks:
*/
debug_check_no_locks_held();
- /*
- * We can do this unlocked here. The futex code uses this flag
- * just to verify whether the pi state cleanup has been done
- * or not. In the worst case it loops once more.
- */
- tsk->flags |= PF_EXITPIDONE;

if (tsk->io_context)
exit_io_context(tsk);
--- x/kernel/fork.c
+++ x/kernel/fork.c
@@ -803,8 +803,7 @@ void mm_release(struct task_struct *tsk,
tsk->compat_robust_list = NULL;
}
#endif
- if (unlikely(!list_empty(&tsk->pi_state_list)))
- exit_pi_state_list(tsk);
+ exit_pi_state_list(tsk);
#endif

uprobe_free_utask(tsk);
--- x/kernel/futex.c
+++ x/kernel/futex.c
@@ -722,6 +722,7 @@ void exit_pi_state_list(struct task_stru
* versus waiters unqueueing themselves:
*/
raw_spin_lock_irq(&curr->pi_lock);
+ curr->flags |= PF_EXITPIDONE;
while (!list_empty(head)) {

next = head->next;
@@ -912,17 +913,15 @@ static int attach_to_pi_owner(u32 uval,
* p->pi_lock:
*/
raw_spin_lock_irq(&p->pi_lock);
- if (unlikely(p->flags & PF_EXITING)) {
+ if (unlikely(p->flags & PF_EXITPIDONE)) {
/*
* The task is on the way out. When PF_EXITPIDONE is
* set, we know that the task has finished the
* cleanup:
*/
- int ret = (p->flags & PF_EXITPIDONE) ? -ESRCH : -EAGAIN;
-
raw_spin_unlock_irq(&p->pi_lock);
put_task_struct(p);
- return ret;
+ return -ESRCH;
}

/*


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