linux-kernel - Re: [RFC][PATCH 1/3] locking/mutex: Rework mutex::owner

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Date:   Tue, 23 Aug 2016 23:03:54 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Tim Chen <tim.c.chen@...ux.intel.com>
Cc:     Waiman Long <waiman.long@....com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        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>,
        Terry Rudd <terry.rudd@....com>,
        "Paul E. McKenney" <paulmck@...ibm.com>,
        Jason Low <jason.low2@...com>
Subject: Re: [RFC][PATCH 1/3] locking/mutex: Rework mutex::owner

On Tue, Aug 23, 2016 at 01:52:46PM -0700, Tim Chen wrote:
> On Tue, 2016-08-23 at 15:55 -0400, Waiman Long wrote:
> > On 08/23/2016 08:46 AM, Peter Zijlstra wrote:
> > 
> > I have 2 more comments about the code.
> > 1) There are a couple of places where you only use 0x3 in mutex.c. They 
> > should be replaced by the symbolic name instead.
> 
> May be easier to read if (owner & 0x3) and
> (owner & ~0x3) are changed to something like 
> _owner_flag(owner) and _owner_task(owner).

Yes that would work. Something like the below..

Note the ';' in the last "-" line that currently ensures we always take
the slow path.

----

--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -55,7 +55,17 @@ EXPORT_SYMBOL(__mutex_init);
 #define MUTEX_FLAG_WAITERS	0x01
 #define MUTEX_FLAG_HANDOFF	0x02
 
-#define MUTEX_FLAG_ALL		0x03
+#define MUTEX_FLAGS		0x03
+
+static inline struct task_struct *__owner_task(unsigned long owner)
+{
+	return (struct task_struct *)(owner & ~MUTEX_FLAGS);
+}
+
+static inline unsigned long __owner_flags(unsigned long owner)
+{
+	return owner & MUTEX_FLAGS;
+}
 
 /*
  * Atomically try to take the lock when it is available
@@ -65,7 +75,7 @@ static inline bool __mutex_trylock(struc
 	unsigned long owner, new_owner;
 
 	owner = atomic_long_read(&lock->owner);
-	if (owner & ~0x03)
+	if (__owner_task(owner))
 		return false;
 
 	new_owner = owner | (unsigned long)current;
@@ -466,14 +476,14 @@ void __sched mutex_unlock(struct mutex *
 		if (owner & MUTEX_FLAG_HANDOFF)
 			break;
 
-		old = atomic_long_cmpxchg_release(&lock->owner, owner, owner & 0x03);
+		old = atomic_long_cmpxchg_release(&lock->owner, owner, __owner_flags(owner));
 		if (old == owner)
 			break;
 
 		owner = old;
 	}
 
-	if (owner & 0x03);
+	if (__owner_flags(owner))
 		__mutex_unlock_slowpath(lock, owner);
 }
 EXPORT_SYMBOL(mutex_unlock);