Re: handle_exit_race && PF_EXITING

[Oleg Nesterov]

On 11/05, Thomas Gleixner wrote:
>
>  sys_futex()
>     loop infinite because
>     	 PF_EXITING is set,
> 	 but PF_EXITPIDONE not

Yes.

IOW, the problem is very simple. RT task preempts the exiting lock owner
after it sets PF_EXITING but before it sets PF_EXITPIDONE, if they run on
the same CPU futex_lock_pi() will spin forever.

> So the obvious question is why PF_EXITPIDONE is set way after the futex
> exit cleanup has run,

Another obvious question is why this code checks PF_EXITING. I still think
it should not.

> The way we can deal with that is:
>
>     do_exit()
>     tsk->flags |= PF_EXITING;
>     ...
>     mutex_lock(&tsk->futex_exit_mutex);
>     futex_exit();
>     tsk->flags |= PF_EXITPIDONE;
>     mutex_unlock(&tsk->futex_exit_mutex);
>
> and on the futex lock_pi side:
>
>     if (!(tsk->flags & PF_EXITING))
>     	return 0;		<- All good
>
>     if (tsk->flags & PF_EXITPIDONE)
>         return -EOWNERDEAD;	<- Locker can take over
>
>     mutex_lock(&tsk->futex_exit_mutex);
>     if (tsk->flags & PF_EXITPIDONE) {
>         mutex_unlock(&tsk->futex_exit_mutex);
>         return -EOWNERDEAD;	<- Locker can take over
>     }
>
>     queue_futex();
>     mutex_unlock(&tsk->futex_exit_mutex);
>
> Not that I think it's pretty, but it plugs all holes AFAICT.

I have found the fix I sent in 2015, attached below. I forgot everything
I knew about futex.c, so I need some time to adapt it to the current code.

But I think it is clear what this patch tries to do, do you see any hole?

Oleg.

[PATCH] futex: don't spin waiting for PF_EXITING -> PF_EXITPIDONE transition

It is absolutely not clear why attach_to_pi_owner() returns -EAGAIN which
triggers "retry" if the lock owner is PF_EXITING but not PF_EXITPIDONE.
This burns CPU for no reason and this can even livelock if the rt_task()
caller preempts a PF_EXITING owner.

Remove the PF_EXITING check altogether. We do not care if it is exiting,
all we need to know is can we rely on exit_pi_state_list() or not. So we
also need to set PF_EXITPIDONE before we flush ->pi_state_list and call
exit_pi_state_list() unconditionally.

Perhaps we can add the fast-path list_empty() check in mm_release() back,
but lets fix the problem first. Besides, in theory this check is already
not correct, at least it should be list_empty_careful() to avoid the race
with free_pi_state() in progress.

Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
---
 kernel/exit.c  |   22 +---------------------
 kernel/fork.c  |    3 +--
 kernel/futex.c |   40 ++++++++++------------------------------
 3 files changed, 12 insertions(+), 53 deletions(-)

diff --git a/kernel/exit.c b/kernel/exit.c
index 6806c55..bc969ed 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -683,27 +683,13 @@ 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.
-		 */
+		/* Avoid the new additions to ->pi_state_list at least */
 		tsk->flags |= PF_EXITPIDONE;
 		set_current_state(TASK_UNINTERRUPTIBLE);
 		schedule();
 	}
 
 	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.
-	 */
-	smp_mb();
-	raw_spin_unlock_wait(&tsk->pi_lock);
 
 	if (unlikely(in_atomic()))
 		pr_info("note: %s[%d] exited with preempt_count %d\n",
@@ -779,12 +765,6 @@ 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);
diff --git a/kernel/fork.c b/kernel/fork.c
index 4dc2dda..ec3208e 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -803,8 +803,7 @@ 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 b101381..c1104a8 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -716,11 +716,13 @@ void exit_pi_state_list(struct task_struct *curr)
 
 	if (!futex_cmpxchg_enabled)
 		return;
+
 	/*
-	 * We are a ZOMBIE and nobody can enqueue itself on
-	 * pi_state_list anymore, but we have to be careful
-	 * versus waiters unqueueing themselves:
+	 * attach_to_pi_owner() can no longer add the new entry. But
+	 * we have to be careful versus waiters unqueueing themselves.
 	 */
+	curr->flags |= PF_EXITPIDONE;
+
 	raw_spin_lock_irq(&curr->pi_lock);
 	while (!list_empty(head)) {
 
@@ -905,24 +907,12 @@ static int attach_to_pi_owner(u32 uval, union futex_key *key,
 		return -EPERM;
 	}
 
-	/*
-	 * We need to look at the task state flags to figure out,
-	 * whether the task is exiting. To protect against the do_exit
-	 * change of the task flags, we do this protected by
-	 * p->pi_lock:
-	 */
 	raw_spin_lock_irq(&p->pi_lock);
-	if (unlikely(p->flags & PF_EXITING)) {
-		/*
-		 * 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;
-
+	if (unlikely(p->flags & PF_EXITPIDONE)) {
+		/* exit_pi_state_list() was already called */
 		raw_spin_unlock_irq(&p->pi_lock);
 		put_task_struct(p);
-		return ret;
+		return -ESRCH;
 	}
 
 	/*
@@ -1633,12 +1623,7 @@ retry_private:
 				goto retry;
 			goto out;
 		case -EAGAIN:
-			/*
-			 * Two reasons for this:
-			 * - Owner is exiting and we just wait for the
-			 *   exit to complete.
-			 * - The user space value changed.
-			 */
+			/* The user space value changed. */
 			free_pi_state(pi_state);
 			pi_state = NULL;
 			double_unlock_hb(hb1, hb2);
@@ -2295,12 +2280,7 @@ retry_private:
 		case -EFAULT:
 			goto uaddr_faulted;
 		case -EAGAIN:
-			/*
-			 * Two reasons for this:
-			 * - Task is exiting and we just wait for the
-			 *   exit to complete.
-			 * - The user space value changed.
-			 */
+			/* The user space value changed. */
 			queue_unlock(hb);
 			put_futex_key(&q.key);
 			cond_resched();
-- 
1.5.5.1