linux-kernel - Re: [PATCH v3 2/5] locking/qspinlock: Refactor the qspinlock slow path

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Date:   Tue, 16 Jul 2019 17:58:39 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alex Kogan <alex.kogan@...cle.com>
Cc:     linux@...linux.org.uk, mingo@...hat.com, will.deacon@....com,
        arnd@...db.de, longman@...hat.com, linux-arch@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        tglx@...utronix.de, bp@...en8.de, hpa@...or.com, x86@...nel.org,
        guohanjun@...wei.com, jglauber@...vell.com,
        steven.sistare@...cle.com, daniel.m.jordan@...cle.com,
        dave.dice@...cle.com, rahul.x.yadav@...cle.com
Subject: Re: [PATCH v3 2/5] locking/qspinlock: Refactor the qspinlock slow
 path

On Tue, Jul 16, 2019 at 10:53:02AM -0400, Alex Kogan wrote:
> On Jul 16, 2019, at 6:20 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> > 
> > On Mon, Jul 15, 2019 at 03:25:33PM -0400, Alex Kogan wrote:
> > 
> >> +/*
> >> + * set_locked_empty_mcs - Try to set the spinlock value to _Q_LOCKED_VAL,
> >> + * and by doing that unlock the MCS lock when its waiting queue is empty
> >> + * @lock: Pointer to queued spinlock structure
> >> + * @val: Current value of the lock
> >> + * @node: Pointer to the MCS node of the lock holder
> >> + *
> >> + * *,*,* -> 0,0,1
> >> + */
> >> +static __always_inline bool __set_locked_empty_mcs(struct qspinlock *lock,
> >> +						   u32 val,
> >> +						   struct mcs_spinlock *node)
> >> +{
> >> +	return atomic_try_cmpxchg_relaxed(&lock->val, &val, _Q_LOCKED_VAL);
> >> +}
> > 
> > That name is nonsense. It should be something like:
> > 
> > static __always_inline bool __try_clear_tail(…)
> 
> We already have set_locked(), so I was trying to convey the fact that we are
> doing the same here, but only when the MCS chain is empty.
> 
> I can use __try_clear_tail() instead.

Thing is, we go into this function with: *,0,1 and are trying to obtain
0,0,1. IOW, we're trying to clear the tail, while preserving pending and
locked.