[tip PATCH v6 2/8] RFC: futex: futex_top_waiter()

[Darren Hart <dvhltc@us.ibm.com>]

Improve legibility by wrapping finding the top waiter in a function.  This
will be used by the follow-on patches for enabling requeue pi.

Changelog:
V6: -Shortened function length after review from tglx
    -Corrected kernel-doc formatting
V4: -Simplified logic in the hb loop
    -Return the futex_q as the task can be had from there if needed
    -Updated comments and dropped the -safe iterator per review from tglx
V3: -Comment linewidth cleanup
V1: -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 |   19 +++++++++++++++++++
 1 files changed, 19 insertions(+), 0 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index b2ed762..8ebed88 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -276,6 +276,25 @@ void put_futex_key(int fshared, union futex_key *key)
 	drop_futex_key_refs(key);
 }
 
+/**
+ * futex_top_waiter() - return the highest priority waiter on a futex
+ * @hb:     the hash bucket the futex_q's reside in
+ * @key:    the futex key (to distinguish it from other futex futex_q's)
+ *
+ * Must be called with the hb lock held.
+ */
+static struct futex_q *futex_top_waiter(struct futex_hash_bucket *hb,
+					union futex_key *key)
+{
+	struct futex_q *this;
+
+	plist_for_each_entry(this, &hb->chain, list) {
+		if (match_futex(&this->key, key))
+			return this;
+	}
+	return NULL;
+}
+
 static u32 cmpxchg_futex_value_locked(u32 __user *uaddr, u32 uval, u32 newval)
 {
 	u32 curval;