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

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Pull futex fix from Thomas Gleixner:
 "Single fix for a long standing futex race when taking over a futex
  whose owner died.  You can end up with two owners, which violates
  quite some rules."

* 'core-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  futex: Handle futex_pi OWNER_DIED take over correctly
This commit is contained in:
Linus Torvalds 2012-11-12 17:02:21 -08:00
commit b0db954c04
1 changed files with 22 additions and 19 deletions
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41
kernel/futex.c
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@ -716,7 +716,7 @@ static int futex_lock_pi_atomic(u32 __user *uaddr, struct futex_hash_bucket *hb,
struct futex_pi_state **ps,
struct task_struct *task, int set_waiters)
{
int lock_taken, ret, ownerdied = 0;
int lock_taken, ret, force_take = 0;
u32 uval, newval, curval, vpid = task_pid_vnr(task);
retry:
@ -755,17 +755,15 @@ retry:
newval = curval | FUTEX_WAITERS;
/*
* There are two cases, where a futex might have no owner (the
* owner TID is 0): OWNER_DIED. We take over the futex in this
* case. We also do an unconditional take over, when the owner
* of the futex died.
*
* This is safe as we are protected by the hash bucket lock !
* Should we force take the futex? See below.
*/
if (unlikely(ownerdied || !(curval & FUTEX_TID_MASK))) {
/* Keep the OWNER_DIED bit */
if (unlikely(force_take)) {
/*
* Keep the OWNER_DIED and the WAITERS bit and set the
* new TID value.
*/
newval = (curval & ~FUTEX_TID_MASK) | vpid;
ownerdied = 0;
force_take = 0;
lock_taken = 1;
}
@ -775,7 +773,7 @@ retry:
goto retry;
/*
* We took the lock due to owner died take over.
* We took the lock due to forced take over.
*/
if (unlikely(lock_taken))
return 1;
@ -790,20 +788,25 @@ retry:
switch (ret) {
case -ESRCH:
/*
* No owner found for this futex. Check if the
* OWNER_DIED bit is set to figure out whether
* this is a robust futex or not.
* We failed to find an owner for this
* futex. So we have no pi_state to block
* on. This can happen in two cases:
*
* 1) The owner died
* 2) A stale FUTEX_WAITERS bit
*
* Re-read the futex value.
*/
if (get_futex_value_locked(&curval, uaddr))
return -EFAULT;
/*
* We simply start over in case of a robust
* futex. The code above will take the futex
* and return happy.
* If the owner died or we have a stale
* WAITERS bit the owner TID in the user space
* futex is 0.
*/
if (curval & FUTEX_OWNER_DIED) {
ownerdied = 1;
if (!(curval & FUTEX_TID_MASK)) {
force_take = 1;
goto retry;
}
default: