Re: handle_exit_race && PF_EXITING

Oleg Nesterov
2019-11-06T12:11:24Z

On 11/06, Thomas Gleixner wrote:
>
> But even if we adapt that patch to the current code it won't solve the
> -ESRCH issue I described above.

Hmm. I must have missed something. Why?

Please see the unfinished/untested patch below.

I think that (with or without this fix) handle_exit_race() logic needs
cleanups, there is no reason for get_futex_value_locked(), we can drop
->pi_lock right after we see PF_EXITPIDONE. Lets discuss this later.

But why we can not rely on handle_exit_race() without PF_EXITING check?

Yes, exit_robust_list() is called before exit_pi_state_list() which (with
this patch) sets PF_EXITPIDONE. But this is fine? handle_futex_death()
doesn't wakeup pi futexes, exit_pi_state_list() does this.

Could you explain?

Oleg.

diff --git a/kernel/exit.c b/kernel/exit.c
index a46a50d..a6c788c 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -761,17 +761,6 @@ void __noreturn do_exit(long code)
}

exit_signals(tsk); /* sets PF_EXITING */
- /*
- * Ensure that all new tsk->pi_lock acquisitions must observe
- * PF_EXITING. Serializes against futex.c:attach_to_pi_owner().
- */
- smp_mb();
- /*
- * Ensure that we must observe the pi_state in exit_mm() ->
- * mm_release() -> exit_pi_state_list().
- */
- raw_spin_lock_irq(&tsk->pi_lock);
- raw_spin_unlock_irq(&tsk->pi_lock);

if (unlikely(in_atomic())) {
pr_info("note: %s[%d] exited with preempt_count %d\n",
@@ -846,12 +835,6 @@ void __noreturn 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);
diff --git a/kernel/fork.c b/kernel/fork.c
index bcdf531..0d8edcf 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1297,8 +1297,7 @@ void mm_release(struct task_struct *tsk, struct mm_struct *mm)
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);
diff --git a/kernel/futex.c b/kernel/futex.c
index bd18f60..c3b5d33 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -905,6 +905,7 @@ void exit_pi_state_list(struct task_struct *curr)
* versus waiters unqueueing themselves:
*/
raw_spin_lock_irq(&curr->pi_lock);
+ curr->flags |= PF_EXITPIDONE;
while (!list_empty(head)) {
next = head->next;
pi_state = list_entry(next, struct futex_pi_state, list);
@@ -1169,18 +1170,11 @@ static int attach_to_pi_state(u32 __user *uaddr, u32 uval,
return ret;
}

-static int handle_exit_race(u32 __user *uaddr, u32 uval,
- struct task_struct *tsk)
+static int handle_exit_race(u32 __user *uaddr, u32 uval)
{
u32 uval2;

/*
- * If PF_EXITPIDONE is not yet set, then try again.
- */
- if (tsk && !(tsk->flags & PF_EXITPIDONE))
- return -EAGAIN;
-
- /*
* Reread the user space value to handle the following situation:
*
* CPU0 CPU1
@@ -1245,7 +1239,7 @@ static int attach_to_pi_owner(u32 __user *uaddr, u32 uval, union futex_key *key,
return -EAGAIN;
p = find_get_task_by_vpid(pid);
if (!p)
- return handle_exit_race(uaddr, uval, NULL);
+ return handle_exit_race(uaddr, uval);

if (unlikely(p->flags & PF_KTHREAD)) {
put_task_struct(p);
@@ -1259,13 +1253,13 @@ static int attach_to_pi_owner(u32 __user *uaddr, u32 uval, union futex_key *key,
* 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 = handle_exit_race(uaddr, uval, p);
+ int ret = handle_exit_race(uaddr, uval);

raw_spin_unlock_irq(&p->pi_lock);
put_task_struct(p);