[tip PATCH v7 4/9] RFC: futex: fixup_owner()

Darren Hart
2009-04-03T20:41:44Z

Refactor the post lock acquisition logic from futex_lock_pi(). This code will
be reused in futex_wait_requeue_pi().

Changelog:
V7: -Rename finish_futex_lock_pi() to fixup_owner() (per Dinakar G.)
V6: -Minor comment updates
V4: -Corrected string paranoia check
-Move the spinlock(q->lock_ptr) out of finish_futex_lock_pi to retain
some semblance of lock/unlock in the same function.
V3: -Initial version

Signed-off-by: Darren Hart <dvhltc@us.ibm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Sripathi Kodi <sripathik@in.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: John Stultz <johnstul@us.ibm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Dinakar Guniguntala <dino@in.ibm.com>
Cc: Ulrich Drepper <drepper@redhat.com>
Cc: Eric Dumazet <dada1@cosmosbay.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Jakub Jelinek <jakub@redhat.com>
---

kernel/futex.c | 158 ++++++++++++++++++++++++++++++++------------------------
1 files changed, 89 insertions(+), 69 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index 36b5367..c2b58d8 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1256,6 +1256,79 @@ handle_fault:
static long futex_wait_restart(struct restart_block *restart);

/**
+ * fixup_owner() - Post lock pi_state and corner case management
+ * @uaddr: user address of the futex
+ * @fshared: whether the futex is shared (1) or not (0)
+ * @q: futex_q (contains pi_state and access to the rt_mutex)
+ * @locked: if the attempt to take the rt_mutex succeeded (1) or not (0)
+ *
+ * After attempting to lock an rt_mutex, this function is called to cleanup
+ * the pi_state owner as well as handle race conditions that may allow us to
+ * acquire the lock. Must be called with the hb lock held.
+ *
+ * Returns:
+ * 1 - success, lock taken
+ * 0 - success, lock not taken
+ * <0 - on error (-EFAULT)
+ */
+static int fixup_owner(u32 __user *uaddr, int fshared, struct futex_q *q,
+ int locked)
+{
+ struct task_struct *owner;
+ int ret = 0;
+
+ if (locked) {
+ /*
+ * Got the lock. We might not be the anticipated owner if we
+ * did a lock-steal - fix up the PI-state in that case:
+ */
+ if (q->pi_state->owner != current)
+ ret = fixup_pi_state_owner(uaddr, q, current, fshared);
+ goto out;
+ }
+
+ /*
+ * Catch the rare case, where the lock was released when we were on the
+ * way back before we locked the hash bucket.
+ */
+ if (q->pi_state->owner == current) {
+ /*
+ * Try to get the rt_mutex now. This might fail as some other
+ * task acquired the rt_mutex after we removed ourself from the
+ * rt_mutex waiters list.
+ */
+ if (rt_mutex_trylock(&q->pi_state->pi_mutex)) {
+ locked = 1;
+ goto out;
+ }
+
+ /*
+ * pi_state is incorrect, some other task did a lock steal and
+ * we returned due to timeout or signal without taking the
+ * rt_mutex. Too late. We can access the rt_mutex_owner without
+ * locking, as the other task is now blocked on the hash bucket
+ * lock. Fix the state up.
+ */
+ owner = rt_mutex_owner(&q->pi_state->pi_mutex);
+ ret = fixup_pi_state_owner(uaddr, q, owner, fshared);
+ goto out;
+ }
+
+ /*
+ * Paranoia check. If we did not take the lock, then we should not be
+ * the owner, nor the pending owner, of the rt_mutex.
+ */
+ if (rt_mutex_owner(&q->pi_state->pi_mutex) == current)
+ printk(KERN_ERR "fixup_owner: ret = %d pi-mutex: %p "
+ "pi-state %p\n", ret,
+ q->pi_state->pi_mutex.owner,
+ q->pi_state->owner);
+
+out:
+ return ret ? ret : locked;
+}
+
+/**
* futex_wait_queue_me() - queue_me() and wait for wakeup, timeout, or signal
* @hb: the futex hash bucket, must be locked by the caller
* @q: the futex_q to queue up on
@@ -1459,11 +1532,10 @@ static int futex_lock_pi(u32 __user *uaddr, int fshared,
int detect, ktime_t *time, int trylock)
{
struct hrtimer_sleeper timeout, *to = NULL;
- struct task_struct *curr = current;
struct futex_hash_bucket *hb;
u32 uval;
struct futex_q q;
- int ret;
+ int res, ret;

if (refill_pi_state_cache())
return -ENOMEM;
@@ -1527,71 +1599,21 @@ retry_private:
}

spin_lock(q.lock_ptr);
-
- if (!ret) {
- /*
- * Got the lock. We might not be the anticipated owner
- * if we did a lock-steal - fix up the PI-state in
- * that case:
- */
- if (q.pi_state->owner != curr)
- ret = fixup_pi_state_owner(uaddr, &q, curr, fshared);
- } else {
- /*
- * Catch the rare case, where the lock was released
- * when we were on the way back before we locked the
- * hash bucket.
- */
- if (q.pi_state->owner == curr) {
- /*
- * Try to get the rt_mutex now. This might
- * fail as some other task acquired the
- * rt_mutex after we removed ourself from the
- * rt_mutex waiters list.
- */
- if (rt_mutex_trylock(&q.pi_state->pi_mutex))
- ret = 0;
- else {
- /*
- * pi_state is incorrect, some other
- * task did a lock steal and we
- * returned due to timeout or signal
- * without taking the rt_mutex. Too
- * late. We can access the
- * rt_mutex_owner without locking, as
- * the other task is now blocked on
- * the hash bucket lock. Fix the state
- * up.
- */
- struct task_struct *owner;
- int res;
-
- owner = rt_mutex_owner(&q.pi_state->pi_mutex);
- res = fixup_pi_state_owner(uaddr, &q, owner,
- fshared);
-
- /* propagate -EFAULT, if the fixup failed */
- if (res)
- ret = res;
- }
- } else {
- /*
- * Paranoia check. If we did not take the lock
- * in the trylock above, then we should not be
- * the owner of the rtmutex, neither the real
- * nor the pending one:
- */
- if (rt_mutex_owner(&q.pi_state->pi_mutex) == curr)
- printk(KERN_ERR "futex_lock_pi: ret = %d "
- "pi-mutex: %p pi-state %p\n", ret,
- q.pi_state->pi_mutex.owner,
- q.pi_state->owner);
- }
- }
+ /*
+ * Fixup the pi_state owner and possibly acquire the lock if we
+ * haven't already.
+ */
+ res = fixup_owner(uaddr, fshared, &q, !ret);
+ /*
+ * If fixup_owner() returned an error, proprogate that. If it acquired
+ * the lock, clear our -ETIMEDOUT or -EINTR.
+ */
+ if (res)
+ ret = (res < 0) ? res : 0;

/*
- * If fixup_pi_state_owner() faulted and was unable to handle the
- * fault, unlock it and return the fault to userspace.
+ * If fixup_owner() faulted and was unable to handle the fault, unlock
+ * it and return the fault to userspace.
*/
if (ret && (rt_mutex_owner(&q.pi_state->pi_mutex) == current))
rt_mutex_unlock(&q.pi_state->pi_mutex);
@@ -1599,9 +1621,7 @@ retry_private:
/* Unqueue and drop the lock */
unqueue_me_pi(&q);

- if (to)
- destroy_hrtimer_on_stack(&to->timer);
- return ret != -EINTR ? ret : -ERESTARTNOINTR;
+ goto out;

out_unlock_put_key:
queue_unlock(&q, hb);
@@ -1611,7 +1631,7 @@ out_put_key:
out:
if (to)
destroy_hrtimer_on_stack(&to->timer);
- return ret;
+ return ret != -EINTR ? ret : -ERESTARTNOINTR;

uaddr_faulted:
/*