futex: Move drop_futex_key_refs out of spinlock'ed region

commit 89061d3d58e1f0742139605dc6a7950aa1ecc019 upstream.

When requeuing tasks from one futex to another, the reference held
by the requeued task to the original futex location needs to be
dropped eventually.

Dropping the reference may ultimately lead to a call to
"iput_final" and subsequently call into filesystem- specific code -
which may be non-atomic.

It is therefore safer to defer this drop operation until after the
futex_hash_bucket spinlock has been dropped.

Originally-From: Helge Bahmann <hcb@chaoticmind.net>
Signed-off-by: Darren Hart <dvhltc@us.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Dinakar Guniguntala <dino@in.ibm.com>
Cc: John Stultz <johnstul@linux.vnet.ibm.com>
Cc: Sven-Thorsten Dietrich <sdietrich@novell.com>
Cc: John Kacur <jkacur@redhat.com>
LKML-Reference: <4AD7A298.5040802@us.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

diff --git a/kernel/futex.c b/kernel/futex.c
index dcd98ec595fa25c04d7299c3b95cf5741c1950af..f435af120a67409f5170d374d2c792d524fd935c 100644 (file)
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1028,7 +1028,6 @@ static inline
 void requeue_pi_wake_futex(struct futex_q *q, union futex_key *key,
                           struct futex_hash_bucket *hb)
 {
-       drop_futex_key_refs(&q->key);
        get_futex_key_refs(key);
        q->key = *key;
 
@@ -1226,6 +1225,7 @@ retry_private:
                 */
                if (ret == 1) {
                        WARN_ON(pi_state);
+                       drop_count++;
                        task_count++;
                        ret = get_futex_value_locked(&curval2, uaddr2);
                        if (!ret)
@@ -1304,6 +1304,7 @@ retry_private:
                        if (ret == 1) {
                                /* We got the lock. */
                                requeue_pi_wake_futex(this, &key2, hb2);
+                               drop_count++;
                                continue;
                        } else if (ret) {
                                /* -EDEADLK */