[PATCH] futex: Drop now unnecessary check in exit_pi_state()

[PATCH] futex: Drop now unnecessary check in exit_pi_state()

[Julia Cartwright]

This check was an attempt to protect against a race with put_pi_state()
by ensuring that the pi_state_list was consistent across the
unlock/lock of pi_lock.

However, as of commit 153fbd1226fb3 ("futex: Fix more put_pi_state() vs.
exit_pi_state_list() races"), this check is no longer necessary because
we now hold a reference to the pi_state object across the unlock/lock of
pi_lock.  This reference guarantees that a put_pi_state() on another CPU
won't rip the pi_state object from the list when we drop pi_lock.

Cc: Gratian Crisan <gratian.crisan@ni.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Darren Hart <dvhart@infradead.org>
Signed-off-by: Julia Cartwright <julia@ni.com>
---
I'm not sure my analysis is 100% correct here, so please carefully think
through it, as I'm sure you all always do when futex patches hit your
mailbox :).

   Julia

 kernel/futex.c | 16 ++--------------
 1 file changed, 2 insertions(+), 14 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index ca5bb9cba5cf..e127ec0555b6 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -889,7 +889,7 @@ static struct task_struct *futex_find_get_task(pid_t pid)
  */
 void exit_pi_state_list(struct task_struct *curr)
 {
-	struct list_head *next, *head = &curr->pi_state_list;
+	struct list_head *head = &curr->pi_state_list;
 	struct futex_pi_state *pi_state;
 	struct futex_hash_bucket *hb;
 	union futex_key key = FUTEX_KEY_INIT;
@@ -903,8 +903,7 @@ void exit_pi_state_list(struct task_struct *curr)
 	 */
 	raw_spin_lock_irq(&curr->pi_lock);
 	while (!list_empty(head)) {
-		next = head->next;
-		pi_state = list_entry(next, struct futex_pi_state, list);
+		pi_state = list_first_entry(head, struct futex_pi_state, list);
 		key = pi_state->key;
 		hb = hash_futex(&key);
 
@@ -929,17 +928,6 @@ void exit_pi_state_list(struct task_struct *curr)
 		spin_lock(&hb->lock);
 		raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);
 		raw_spin_lock(&curr->pi_lock);
-		/*
-		 * We dropped the pi-lock, so re-check whether this
-		 * task still owns the PI-state:
-		 */
-		if (head->next != next) {
-			/* retain curr->pi_lock for the loop invariant */
-			raw_spin_unlock(&pi_state->pi_mutex.wait_lock);
-			spin_unlock(&hb->lock);
-			put_pi_state(pi_state);
-			continue;
-		}
 
 		WARN_ON(pi_state->owner != curr);
 		WARN_ON(list_empty(&pi_state->list));
-- 
2.14.2

Re: [PATCH] futex: Drop now unnecessary check in exit_pi_state()

[Peter Zijlstra]

On Fri, Nov 03, 2017 at 05:30:28PM -0500, Julia Cartwright wrote:
> This check was an attempt to protect against a race with put_pi_state()
> by ensuring that the pi_state_list was consistent across the
> unlock/lock of pi_lock.
> 
> However, as of commit 153fbd1226fb3 ("futex: Fix more put_pi_state() vs.
> exit_pi_state_list() races"), this check is no longer necessary because
> we now hold a reference to the pi_state object across the unlock/lock of
> pi_lock.  This reference guarantees that a put_pi_state() on another CPU
> won't rip the pi_state object from the list when we drop pi_lock.

> @@ -929,17 +928,6 @@ void exit_pi_state_list(struct task_struct *curr)
>  		spin_lock(&hb->lock);
>  		raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);
>  		raw_spin_lock(&curr->pi_lock);
> -		/*
> -		 * We dropped the pi-lock, so re-check whether this
> -		 * task still owns the PI-state:
> -		 */
> -		if (head->next != next) {

Quite possibly you're right, but I would sleep a whole lot better if
that were to remain a WARN. You never know with this futex stuff :-)