linux-kernel - Re: [RFC] Cancellable MCS spinlock rework

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Date:	Wed, 02 Jul 2014 21:39:18 -0700
From:	Jason Low <jason.low2@...com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	torvalds@...ux-foundation.org, paulmck@...ux.vnet.ibm.com,
	mingo@...nel.org, Waiman.Long@...com, davidlohr@...com,
	linux-kernel@...r.kernel.org, riel@...hat.com,
	akpm@...ux-foundation.org, hpa@...or.com, andi@...stfloor.org,
	James.Bottomley@...senpartnership.com, rostedt@...dmis.org,
	tim.c.chen@...ux.intel.com, aswin@...com, scott.norton@...com,
	chegu_vinod@...com
Subject: Re: [RFC] Cancellable MCS spinlock rework

On Wed, 2014-07-02 at 10:30 -0700, Jason Low wrote:
> On Wed, 2014-07-02 at 19:23 +0200, Peter Zijlstra wrote:
> > On Wed, Jul 02, 2014 at 09:59:16AM -0700, Jason Low wrote:
> > > 
> > > Why I converted pointers to atomic_t?
> > > 
> > > This would avoid the potentially racy ACCESS_ONCE stores + cmpxchg while
> > > also using less overhead, since atomic_t is often only 32 bits while
> > > pointers could be 64 bits.
> > 
> > So no real good reason.. The ACCESS_ONCE stores + cmpxchg stuff is
> > likely broken all over the place, and 'fixing' this one place doesn't
> > cure the problem.
> 
> Right, fixing the ACCESS_ONCE + cmpxchg and avoiding the architecture
> workarounds for optimistic spinning was just a nice side effect.
> 
> Would potentially reducing the size of the rw semaphore structure by 32
> bits (for all architectures using optimistic spinning) be a nice
> benefit?

And due to padding, the additional modification below reduces the
size of struct rw_semaphore by 64 bits on my machine  :)


 struct rw_semaphore {
 	long count;
-	raw_spinlock_t wait_lock;
 	struct list_head wait_list;
+	raw_spinlock_t wait_lock;
 #ifdef CONFIG_SMP
+	struct optimistic_spin_tail osq; /* spinner MCS lock */
 	/*
 	 * Write owner. Used as a speculative check to see
 	 * if the owner is running on the cpu.
 	 */
 	struct task_struct *owner;
-	struct optimistic_spin_tail osq; /* spinner MCS lock */
 #endif
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 	struct lockdep_map	dep_map;


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