linux-kernel - [PATCH-tip v5 05/21] futex: Add helpers to get & cmpxchg futex value without lock

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Date:   Fri,  3 Feb 2017 13:03:38 -0500
From:   Waiman Long <longman@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Jonathan Corbet <corbet@....net>
Cc:     linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        Mike Galbraith <umgwanakikbuti@...il.com>,
        Scott J Norton <scott.norton@....com>,
        Waiman Long <longman@...hat.com>
Subject: [PATCH-tip v5 05/21] futex: Add helpers to get & cmpxchg futex value without lock

Two new helper functions cmpxchg_futex_value() and get_futex_value()
are added to access and change the futex value without the hash
bucket lock.  As a result, page fault is enabled and the page will
be faulted in if not present yet.

Signed-off-by: Waiman Long <longman@...hat.com>
---
 kernel/futex.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/kernel/futex.c b/kernel/futex.c
index b21c6a0..203a388 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -805,6 +805,21 @@ static int get_futex_value_locked(u32 *dest, u32 __user *from)
 	return ret ? -EFAULT : 0;
 }
 
+/*
+ * The equivalents of the above cmpxchg_futex_value_locked() and
+ * get_futex_value_locked which are called without the hash bucket lock
+ * and so can have page fault enabled.
+ */
+static inline int cmpxchg_futex_value(u32 *curval, u32 __user *uaddr,
+				      u32 uval, u32 newval)
+{
+	return futex_atomic_cmpxchg_inatomic(curval, uaddr, uval, newval);
+}
+
+static inline int get_futex_value(u32 *dest, u32 __user *from)
+{
+	return __get_user(*dest, from);
+}
 
 /*
  * PI code:
-- 
1.8.3.1