locking fixes

GIT PULL Ingo Molnar 1 participant 1 message Apr 23, 2016

Subsystems (3)

Generic include/asm header files Locking primitives Futex

lore

Share & tools

lore

Ingo Molnar

author

2016-04-23 11:22 UTC #0 lore raw

Linus,

Please pull the latest locking-urgent-for-linus git tree from:

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

# HEAD: fba7cd681b6155e2d93e7862fcd6f970336b83c3 asm-generic/futex: Re-enable preemption in futex_atomic_cmpxchg_inatomic()

Misc fixes:
- pvqspinlocks: an instrumentation fix
- futexes: a preempt-count vs. pagefault_disable decouple corner case fix
- futexes: futex requeue plist race window fix
- futexes: a futex UNLOCK_PI transaction fix for a corner case

Thanks,

Ingo

------------------>
Davidlohr Bueso (2):
locking/pvqspinlock: Fix division by zero in qstat_read()
futex: Acknowledge a new waiter in counter before plist

Romain Perier (1):
asm-generic/futex: Re-enable preemption in futex_atomic_cmpxchg_inatomic()

Sebastian Andrzej Siewior (1):
futex: Handle unlock_pi race gracefully

 include/asm-generic/futex.h     |  8 ++++++--
 kernel/futex.c                  | 27 +++++++++++++++++++++++----
 kernel/locking/qspinlock_stat.h |  8 +++++---
 3 files changed, 34 insertions(+), 9 deletions(-)

include/asm-generic/futex.h +6 -2

index e56272c919b5..bf2d34c9d804 100644
@@ -108,11 +108,15 @@ futex_atomic_cmpxchg_inatomic(u32 *uval, u32 __user *uaddr,
 	u32 val;
 
 	preempt_disable();
-	if (unlikely(get_user(val, uaddr) != 0))
+	if (unlikely(get_user(val, uaddr) != 0)) {
+		preempt_enable();
 		return -EFAULT;
+	}
 
-	if (val == oldval && unlikely(put_user(newval, uaddr) != 0))
+	if (val == oldval && unlikely(put_user(newval, uaddr) != 0)) {
+		preempt_enable();
 		return -EFAULT;
+	}
 
 	*uval = val;
 	preempt_enable();

kernel/futex.c +23 -4

index a5d2e74c89e0..c20f06f38ef3 100644
@@ -1295,10 +1295,20 @@ static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_q *this,
 	if (unlikely(should_fail_futex(true)))
 		ret = -EFAULT;
 
-	if (cmpxchg_futex_value_locked(&curval, uaddr, uval, newval))
+	if (cmpxchg_futex_value_locked(&curval, uaddr, uval, newval)) {
 		ret = -EFAULT;
-	else if (curval != uval)
-		ret = -EINVAL;
+	} else if (curval != uval) {
+		/*
+		 * If a unconditional UNLOCK_PI operation (user space did not
+		 * try the TID->0 transition) raced with a waiter setting the
+		 * FUTEX_WAITERS flag between get_user() and locking the hash
+		 * bucket lock, retry the operation.
+		 */
+		if ((FUTEX_TID_MASK & curval) == uval)
+			ret = -EAGAIN;
+		else
+			ret = -EINVAL;
+	}
 	if (ret) {
 		raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
 		return ret;

@@ -1525,8 +1535,8 @@ void requeue_futex(struct futex_q *q, struct futex_hash_bucket *hb1,
 	if (likely(&hb1->chain != &hb2->chain)) {
 		plist_del(&q->list, &hb1->chain);
 		hb_waiters_dec(hb1);
-		plist_add(&q->list, &hb2->chain);
 		hb_waiters_inc(hb2);
+		plist_add(&q->list, &hb2->chain);
 		q->lock_ptr = &hb2->lock;
 	}
 	get_futex_key_refs(key2);

@@ -2623,6 +2633,15 @@ static int futex_unlock_pi(u32 __user *uaddr, unsigned int flags)
 		if (ret == -EFAULT)
 			goto pi_faulted;
 		/*
+		 * A unconditional UNLOCK_PI op raced against a waiter
+		 * setting the FUTEX_WAITERS bit. Try again.
+		 */
+		if (ret == -EAGAIN) {
+			spin_unlock(&hb->lock);
+			put_futex_key(&key);
+			goto retry;
+		}
+		/*
 		 * wake_futex_pi has detected invalid state. Tell user
 		 * space.
 		 */

kernel/locking/qspinlock_stat.h +5 -3

index eb2a2c9bc3fc..d734b7502001 100644
@@ -136,10 +136,12 @@ static ssize_t qstat_read(struct file *file, char __user *user_buf,
 	}
 
 	if (counter == qstat_pv_hash_hops) {
-		u64 frac;
+		u64 frac = 0;
 
-		frac = 100ULL * do_div(stat, kicks);
-		frac = DIV_ROUND_CLOSEST_ULL(frac, kicks);
+		if (kicks) {
+			frac = 100ULL * do_div(stat, kicks);
+			frac = DIV_ROUND_CLOSEST_ULL(frac, kicks);
+		}
 
 		/*
 		 * Return a X.XX decimal number