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Date: Mon, 8 Aug 2016 19:37:16 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Waiman Long <Waiman.Long@hpe.com>
Cc: Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Ding Tianhong <dingtianhong@huawei.com>,
        Jason Low <jason.low2@hpe.com>, Davidlohr Bueso <dave@stgolabs.net>,
        "Paul E. McKenney" <paulmck@us.ibm.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Will Deacon <Will.Deacon@arm.com>,
        Tim Chen <tim.c.chen@linux.intel.com>, Imre Deak <imre.deak@intel.com>
Subject: Re: [PATCH v4 3/3] locking/mutex: Ensure forward progress of
 waiter-spinner
Message-ID: <20160808173716.GZ6862@twins.programming.kicks-ass.net>
References: <1468874366-56955-1-git-send-email-Waiman.Long@hpe.com>
 <1468874366-56955-4-git-send-email-Waiman.Long@hpe.com>
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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...