[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.cri...@ni.com>
Cc: Thomas Gleixner <t...@linutronix.de>
Cc: Ingo Molnar <mi...@redhat.com>
Cc: Peter Zijlstra <pet...@infradead.org>
Cc: Darren Hart <dvh...@infradead.org>
Signed-off-by: Julia Cartwright <ju...@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