linux-kernel - [GIT pull] Futex fix for 3.7

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Date:	Mon, 12 Nov 2012 20:16:35 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: [GIT pull] Futex fix for 3.7

Linus,

please pull the latest core-urgent-for-linus git tree from:

   git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git core-urgent-for-linus

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.

Thanks,

	tglx

------------------>
Thomas Gleixner (1):
      futex: Handle futex_pi OWNER_DIED take over correctly


 kernel/futex.c |   41 ++++++++++++++++++++++-------------------
 1 files changed, 22 insertions(+), 19 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index 3717e7b..20ef219 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:
--
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