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

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Date:	Mon, 8 Aug 2016 19:37:16 +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 Mon, Jul 18, 2016 at 04:39:26PM -0400, Waiman Long wrote:
> As both an optimistic spinner and a waiter-spinner (a woken task from
> the wait queue spinning) can be spinning on the lock at the same time,
> we cannot ensure forward progress for the waiter-spinner. Therefore,
> it is possible for the waiter-spinner to be starved of getting the
> lock, though not likely.

Right; yet your previous two changelogs/comments implied otherwise.

> This patch adds a flag to indicate that a waiter-spinner is
> spinning and hence has priority over the acquisition of the lock. A
> waiter-spinner sets this flag while spinning. An optimistic spinner
> will check this flag and yield if set. This essentially makes the
> waiter-spinner jump to the head of the optimistic spinning queue to
> acquire the lock.
> 
> There will be no increase in size for the mutex structure for 64-bit
> architectures. For 32-bit architectures, there will be a size increase
> of 4 bytes.

Alternative might be to use the LSB of mutex::owner, but that's going to
be somewhat icky too.

I'm not sure the 32bit platforms are going to be excited about growing
struct mutex...