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authorLinus Torvalds <torvalds@linux-foundation.org>2012-11-12 17:02:21 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2012-11-12 17:02:21 -0800
commitb0db954c0491ce846bc49c0746ca6e1f868a1881 (patch)
tree1d239bdf2f0fe62f278faf895f662f2f5b43c3e6 /kernel
parent77b67063bb6bce6d475e910d3b886a606d0d91f7 (diff)
parent59fa6245192159ab5e1e17b8e31f15afa9cff4bf (diff)
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Merge branch 'core-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip
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
Diffstat (limited to 'kernel')
-rw-r--r--kernel/futex.c41
1 files changed, 22 insertions, 19 deletions
diff --git a/kernel/futex.c b/kernel/futex.c
index 3717e7b306e0..20ef219bbe9b 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -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: