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Merge branch 'locking-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

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Pull futex fixes from Ingo Molnar:
 "This contains two futex fixes: one fixes a race condition, the other
  clarifies shared/private futex comments"

* 'locking-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  futex: Fix a race condition between REQUEUE_PI and task death
  futex: Mention key referencing differences between shared and private futexes
This commit is contained in:
Linus Torvalds 2014-10-31 13:57:45 -07:00
commit c958f9200f
1 changed files with 21 additions and 15 deletions
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36
kernel/futex.c
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@ -143,9 +143,8 @@
*
* Where (A) orders the waiters increment and the futex value read through
* atomic operations (see hb_waiters_inc) and where (B) orders the write
* to futex and the waiters read -- this is done by the barriers in
* get_futex_key_refs(), through either ihold or atomic_inc, depending on the
* futex type.
* to futex and the waiters read -- this is done by the barriers for both
* shared and private futexes in get_futex_key_refs().
*
* This yields the following case (where X:=waiters, Y:=futex):
*
@ -344,13 +343,20 @@ static void get_futex_key_refs(union futex_key *key)
futex_get_mm(key); /* implies MB (B) */
break;
default:
/*
* Private futexes do not hold reference on an inode or
* mm, therefore the only purpose of calling get_futex_key_refs
* is because we need the barrier for the lockless waiter check.
*/
smp_mb(); /* explicit MB (B) */
}
}
/*
* Drop a reference to the resource addressed by a key.
* The hash bucket spinlock must not be held.
* The hash bucket spinlock must not be held. This is
* a no-op for private futexes, see comment in the get
* counterpart.
*/
static void drop_futex_key_refs(union futex_key *key)
{
@ -641,8 +647,14 @@ static struct futex_pi_state * alloc_pi_state(void)
return pi_state;
}
/*
* Must be called with the hb lock held.
*/
static void free_pi_state(struct futex_pi_state *pi_state)
{
if (!pi_state)
return;
if (!atomic_dec_and_test(&pi_state->refcount))
return;
@ -1521,15 +1533,6 @@ static int futex_requeue(u32 __user *uaddr1, unsigned int flags,
}
retry:
if (pi_state != NULL) {
/*
* We will have to lookup the pi_state again, so free this one
* to keep the accounting correct.
*/
free_pi_state(pi_state);
pi_state = NULL;
}
ret = get_futex_key(uaddr1, flags & FLAGS_SHARED, &key1, VERIFY_READ);
if (unlikely(ret != 0))
goto out;
@ -1619,6 +1622,8 @@ retry_private:
case 0:
break;
case -EFAULT:
free_pi_state(pi_state);
pi_state = NULL;
double_unlock_hb(hb1, hb2);
hb_waiters_dec(hb2);
put_futex_key(&key2);
@ -1634,6 +1639,8 @@ retry_private:
* exit to complete.
* - The user space value changed.
*/
free_pi_state(pi_state);
pi_state = NULL;
double_unlock_hb(hb1, hb2);
hb_waiters_dec(hb2);
put_futex_key(&key2);
@ -1710,6 +1717,7 @@ retry_private:
}
out_unlock:
free_pi_state(pi_state);
double_unlock_hb(hb1, hb2);
hb_waiters_dec(hb2);
@ -1727,8 +1735,6 @@ out_put_keys:
out_put_key1:
put_futex_key(&key1);
out:
if (pi_state != NULL)
free_pi_state(pi_state);
return ret ? ret : task_count;
}