linux-kernel - Re: [PATCH -v4 1/8] locking/drm: Kill mutex trickery

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Message-ID: <20161007154351.GL3117@twins.programming.kicks-ass.net>
Date:   Fri, 7 Oct 2016 17:43:51 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Waiman Long <waiman.long@....com>,
        Jason Low <jason.low2@....com>,
        Ding Tianhong <dingtianhong@...wei.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <Will.Deacon@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Imre Deak <imre.deak@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Terry Rudd <terry.rudd@....com>,
        "Paul E. McKenney" <paulmck@...ibm.com>
Cc:     Chris Wilson <chris@...is-wilson.co.uk>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Rob Clark <robdclark@...il.com>
Subject: Re: [PATCH -v4 1/8] locking/drm: Kill mutex trickery

On Fri, Oct 07, 2016 at 04:52:44PM +0200, Peter Zijlstra wrote:
> Poking at lock internals is not cool. Since I'm going to change the
> implementation this will break, take it out.


So something like the below would serve as a replacement for your
previous hacks. Is this API something acceptable to people? Ingo,
Thomas?

---
 include/linux/mutex.h | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/include/linux/mutex.h b/include/linux/mutex.h
index 4d3bccabbea5..afcff2c85957 100644
--- a/include/linux/mutex.h
+++ b/include/linux/mutex.h
@@ -189,4 +189,29 @@ extern void mutex_unlock(struct mutex *lock);
 
 extern int atomic_dec_and_mutex_lock(atomic_t *cnt, struct mutex *lock);
 
+enum mutex_trylock_recursive_enum {
+	mutex_trylock_failed = 0,
+	mutex_trylock_success = 1,
+	mutex_trylock_recursive,
+};
+
+/**
+ * mutex_trylock_recursive - trylock variant that allows recursive locking
+ * @lock: mutex to be locked
+ *
+ *
+ * Returns:
+ *  mutex_trylock_failed    - trylock failed,
+ *  mutex_trylock_success   - lock acquired,
+ *  mutex_trylock_recursive - we already owned the lock.
+ */
+static inline enum mutex_trylock_recursive_enum
+mutex_trylock_recursive(struct mutex *lock)
+{
+	if (unlikely(__mutex_owner(lock) == current))
+		return mutex_trylock_recursive;
+
+	return mutex_trylock(lock);
+}
+
 #endif /* __LINUX_MUTEX_H */