linux-kernel - Re: [PATCH v4 3/3] locking/mutex: Ensure forward progress of waiter-spinner

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Message-ID: <20160810092905.GN6879@twins.programming.kicks-ass.net>
Date:	Wed, 10 Aug 2016 11:29:05 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Waiman Long <waiman.long@....com>
Cc:	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ding Tianhong <dingtianhong@...wei.com>,
	Jason Low <jason.low2@....com>,
	Davidlohr Bueso <dave@...olabs.net>,
	"Paul E. McKenney" <paulmck@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Will Deacon <Will.Deacon@....com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Imre Deak <imre.deak@...el.com>
Subject: Re: [PATCH v4 3/3] locking/mutex: Ensure forward progress of
 waiter-spinner

On Tue, Aug 09, 2016 at 02:00:00PM -0400, Waiman Long wrote:
> >Alternative might be to use the LSB of mutex::owner, but that's going to
> >be somewhat icky too.
> 
> I was thinking about doing that. However, the owner field is used in quite a
> number of places. It may be a bit risky to change all of them.

Agreed.

> >I'm not sure the 32bit platforms are going to be excited about growing
> >struct mutex...
> 
> Or we can make this a 64-bit architecture specific change if the increase in
> mutex size is a real concern. Actually, we don't need to use a list_head
> structure for wait_list. It can be just a pointer to mutex_waiter that has
> the list_head structure. This can save a pointer from the structure.

Just grow the thing, we can poke at it later if we get complaints.