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Subject[tip: locking/core] futex: Remove {get,drop}_futex_key_refs()
The following commit has been merged into the locking/core branch of tip:

Commit-ID: 4b39f99c222a2aff6a52fddfa6d8d4aef1771737
Gitweb: https://git.kernel.org/tip/4b39f99c222a2aff6a52fddfa6d8d4aef1771737
Author: Peter Zijlstra <peterz@infradead.org>
AuthorDate: Wed, 04 Mar 2020 13:24:24 +01:00
Committer: Peter Zijlstra <peterz@infradead.org>
CommitterDate: Fri, 06 Mar 2020 11:06:19 +01:00

futex: Remove {get,drop}_futex_key_refs()

Now that {get,drop}_futex_key_refs() have become a glorified NOP,
remove them entirely.

The only thing get_futex_key_refs() is still doing is an smp_mb(), and
now that we don't need to (ab)use existing atomic ops to obtain them,
we can place it explicitly where we need it.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
kernel/futex.c | 90 +++----------------------------------------------
1 file changed, 6 insertions(+), 84 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index 3463c91..b62cf94 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -135,8 +135,7 @@
*
* Where (A) orders the waiters increment and the futex value read through
* atomic operations (see hb_waiters_inc) and where (B) orders the write
- * to futex and the waiters read -- this is done by the barriers for both
- * shared and private futexes in get_futex_key_refs().
+ * to futex and the waiters read (see hb_waiters_pending()).
*
* This yields the following case (where X:=waiters, Y:=futex):
*
@@ -359,6 +358,10 @@ static inline void hb_waiters_dec(struct futex_hash_bucket *hb)
static inline int hb_waiters_pending(struct futex_hash_bucket *hb)
{
#ifdef CONFIG_SMP
+ /*
+ * Full barrier (B), see the ordering comment above.
+ */
+ smp_mb();
return atomic_read(&hb->waiters);
#else
return 1;
@@ -396,68 +399,6 @@ static inline int match_futex(union futex_key *key1, union futex_key *key2)
&& key1->both.offset == key2->both.offset);
}

-/*
- * Take a reference to the resource addressed by a key.
- * Can be called while holding spinlocks.
- *
- */
-static void get_futex_key_refs(union futex_key *key)
-{
- if (!key->both.ptr)
- return;
-
- /*
- * On MMU less systems futexes are always "private" as there is no per
- * process address space. We need the smp wmb nevertheless - yes,
- * arch/blackfin has MMU less SMP ...
- */
- if (!IS_ENABLED(CONFIG_MMU)) {
- smp_mb(); /* explicit smp_mb(); (B) */
- return;
- }
-
- switch (key->both.offset & (FUT_OFF_INODE|FUT_OFF_MMSHARED)) {
- case FUT_OFF_INODE:
- smp_mb(); /* explicit smp_mb(); (B) */
- break;
- case FUT_OFF_MMSHARED:
- smp_mb(); /* explicit smp_mb(); (B) */
- break;
- default:
- /*
- * Private futexes do not hold reference on an inode or
- * mm, therefore the only purpose of calling get_futex_key_refs
- * is because we need the barrier for the lockless waiter check.
- */
- smp_mb(); /* explicit smp_mb(); (B) */
- }
-}
-
-/*
- * Drop a reference to the resource addressed by a key.
- * The hash bucket spinlock must not be held. This is
- * a no-op for private futexes, see comment in the get
- * counterpart.
- */
-static void drop_futex_key_refs(union futex_key *key)
-{
- if (!key->both.ptr) {
- /* If we're here then we tried to put a key we failed to get */
- WARN_ON_ONCE(1);
- return;
- }
-
- if (!IS_ENABLED(CONFIG_MMU))
- return;
-
- switch (key->both.offset & (FUT_OFF_INODE|FUT_OFF_MMSHARED)) {
- case FUT_OFF_INODE:
- break;
- case FUT_OFF_MMSHARED:
- break;
- }
-}
-
enum futex_access {
FUTEX_READ,
FUTEX_WRITE
@@ -589,7 +530,6 @@ get_futex_key(u32 __user *uaddr, int fshared, union futex_key *key, enum futex_a
if (!fshared) {
key->private.mm = mm;
key->private.address = address;
- get_futex_key_refs(key); /* implies smp_mb(); (B) */
return 0;
}

@@ -729,8 +669,6 @@ again:
rcu_read_unlock();
}

- get_futex_key_refs(key); /* implies smp_mb(); (B) */
-
out:
put_page(page);
return err;
@@ -738,7 +676,6 @@ out:

static inline void put_futex_key(union futex_key *key)
{
- drop_futex_key_refs(key);
}

/**
@@ -1873,7 +1810,6 @@ void requeue_futex(struct futex_q *q, struct futex_hash_bucket *hb1,
plist_add(&q->list, &hb2->chain);
q->lock_ptr = &hb2->lock;
}
- get_futex_key_refs(key2);
q->key = *key2;
}

@@ -1895,7 +1831,6 @@ static inline
void requeue_pi_wake_futex(struct futex_q *q, union futex_key *key,
struct futex_hash_bucket *hb)
{
- get_futex_key_refs(key);
q->key = *key;

__unqueue_futex(q);
@@ -2006,7 +1941,7 @@ static int futex_requeue(u32 __user *uaddr1, unsigned int flags,
u32 *cmpval, int requeue_pi)
{
union futex_key key1 = FUTEX_KEY_INIT, key2 = FUTEX_KEY_INIT;
- int drop_count = 0, task_count = 0, ret;
+ int task_count = 0, ret;
struct futex_pi_state *pi_state = NULL;
struct futex_hash_bucket *hb1, *hb2;
struct futex_q *this, *next;
@@ -2127,7 +2062,6 @@ retry_private:
*/
if (ret > 0) {
WARN_ON(pi_state);
- drop_count++;
task_count++;
/*
* If we acquired the lock, then the user space value
@@ -2247,7 +2181,6 @@ retry_private:
* doing so.
*/
requeue_pi_wake_futex(this, &key2, hb2);
- drop_count++;
continue;
} else if (ret) {
/*
@@ -2268,7 +2201,6 @@ retry_private:
}
}
requeue_futex(this, hb1, hb2, &key2);
- drop_count++;
}

/*
@@ -2283,15 +2215,6 @@ out_unlock:
wake_up_q(&wake_q);
hb_waiters_dec(hb2);

- /*
- * drop_futex_key_refs() must be called outside the spinlocks. During
- * the requeue we moved futex_q's from the hash bucket at key1 to the
- * one at key2 and updated their key pointer. We no longer need to
- * hold the references to key1.
- */
- while (--drop_count >= 0)
- drop_futex_key_refs(&key1);
-
out_put_keys:
put_futex_key(&key2);
out_put_key1:
@@ -2421,7 +2344,6 @@ retry:
ret = 1;
}

- drop_futex_key_refs(&q->key);
return ret;
}

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