futex: fixup unlocked requeue pi case
Thomas's testing caught a problem when the requeue target futex is
unowned and multiple tasks are requeued to it. This patch ensures
the FUTEX_WAITERS bit gets set if futex_requeue() will requeue one
or more tasks in addition to the one acquiring the lock.
Signed-off-by: Darren Hart <dvhltc@us.ibm.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
diff --git a/kernel/futex.c b/kernel/futex.c
index 185c981..041bf3a 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -565,12 +565,14 @@
/**
* futex_lock_pi_atomic() - atomic work required to acquire a pi aware futex
- * @uaddr: the pi futex user address
- * @hb: the pi futex hash bucket
- * @key: the futex key associated with uaddr and hb
- * @ps: the pi_state pointer where we store the result of the lookup
- * @task: the task to perform the atomic lock work for. This will be
- * "current" except in the case of requeue pi.
+ * @uaddr: the pi futex user address
+ * @hb: the pi futex hash bucket
+ * @key: the futex key associated with uaddr and hb
+ * @ps: the pi_state pointer where we store the result of the
+ * lookup
+ * @task: the task to perform the atomic lock work for. This will
+ * be "current" except in the case of requeue pi.
+ * @set_waiters: force setting the FUTEX_WAITERS bit (1) or not (0)
*
* Returns:
* 0 - ready to wait
@@ -582,7 +584,7 @@
static int futex_lock_pi_atomic(u32 __user *uaddr, struct futex_hash_bucket *hb,
union futex_key *key,
struct futex_pi_state **ps,
- struct task_struct *task)
+ struct task_struct *task, int set_waiters)
{
int lock_taken, ret, ownerdied = 0;
u32 uval, newval, curval;
@@ -596,6 +598,8 @@
* the locks. It will most likely not succeed.
*/
newval = task_pid_vnr(task);
+ if (set_waiters)
+ newval |= FUTEX_WAITERS;
curval = cmpxchg_futex_value_locked(uaddr, 0, newval);
@@ -1004,14 +1008,18 @@
/**
* futex_proxy_trylock_atomic() - Attempt an atomic lock for the top waiter
- * @pifutex: the user address of the to futex
- * @hb1: the from futex hash bucket, must be locked by the caller
- * @hb2: the to futex hash bucket, must be locked by the caller
- * @key1: the from futex key
- * @key2: the to futex key
+ * @pifutex: the user address of the to futex
+ * @hb1: the from futex hash bucket, must be locked by the caller
+ * @hb2: the to futex hash bucket, must be locked by the caller
+ * @key1: the from futex key
+ * @key2: the to futex key
+ * @ps: address to store the pi_state pointer
+ * @set_waiters: force setting the FUTEX_WAITERS bit (1) or not (0)
*
* Try and get the lock on behalf of the top waiter if we can do it atomically.
- * Wake the top waiter if we succeed. hb1 and hb2 must be held by the caller.
+ * Wake the top waiter if we succeed. If the caller specified set_waiters,
+ * then direct futex_lock_pi_atomic() to force setting the FUTEX_WAITERS bit.
+ * hb1 and hb2 must be held by the caller.
*
* Returns:
* 0 - failed to acquire the lock atomicly
@@ -1022,15 +1030,23 @@
struct futex_hash_bucket *hb1,
struct futex_hash_bucket *hb2,
union futex_key *key1, union futex_key *key2,
- struct futex_pi_state **ps)
+ struct futex_pi_state **ps, int set_waiters)
{
- struct futex_q *top_waiter;
+ struct futex_q *top_waiter = NULL;
u32 curval;
int ret;
if (get_futex_value_locked(&curval, pifutex))
return -EFAULT;
+ /*
+ * Find the top_waiter and determine if there are additional waiters.
+ * If the caller intends to requeue more than 1 waiter to pifutex,
+ * force futex_lock_pi_atomic() to set the FUTEX_WAITERS bit now,
+ * as we have means to handle the possible fault. If not, don't set
+ * the bit unecessarily as it will force the subsequent unlock to enter
+ * the kernel.
+ */
top_waiter = futex_top_waiter(hb1, key1);
/* There are no waiters, nothing for us to do. */
@@ -1038,10 +1054,12 @@
return 0;
/*
- * Either take the lock for top_waiter or set the FUTEX_WAITERS bit.
- * The pi_state is returned in ps in contended cases.
+ * Try to take the lock for top_waiter. Set the FUTEX_WAITERS bit in
+ * the contended case or if set_waiters is 1. The pi_state is returned
+ * in ps in contended cases.
*/
- ret = futex_lock_pi_atomic(pifutex, hb2, key2, ps, top_waiter->task);
+ ret = futex_lock_pi_atomic(pifutex, hb2, key2, ps, top_waiter->task,
+ set_waiters);
if (ret == 1)
requeue_pi_wake_futex(top_waiter, key2);
@@ -1146,9 +1164,14 @@
}
if (requeue_pi && (task_count - nr_wake < nr_requeue)) {
- /* Attempt to acquire uaddr2 and wake the top_waiter. */
+ /*
+ * Attempt to acquire uaddr2 and wake the top waiter. If we
+ * intend to requeue waiters, force setting the FUTEX_WAITERS
+ * bit. We force this here where we are able to easily handle
+ * faults rather in the requeue loop below.
+ */
ret = futex_proxy_trylock_atomic(uaddr2, hb1, hb2, &key1,
- &key2, &pi_state);
+ &key2, &pi_state, nr_requeue);
/*
* At this point the top_waiter has either taken uaddr2 or is
@@ -1810,7 +1833,7 @@
retry_private:
hb = queue_lock(&q);
- ret = futex_lock_pi_atomic(uaddr, hb, &q.key, &q.pi_state, current);
+ ret = futex_lock_pi_atomic(uaddr, hb, &q.key, &q.pi_state, current, 0);
if (unlikely(ret)) {
switch (ret) {
case 1: