linux-kernel - Re: [PATCH v3 05/14] locking/qspinlock: Remove unbounded cmpxchg loop from locking slowpath

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Date:   Sat, 28 Apr 2018 14:45:37 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Will Deacon <will.deacon@....com>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        mingo@...nel.org, boqun.feng@...il.com, paulmck@...ux.vnet.ibm.com,
        longman@...hat.com
Subject: Re: [PATCH v3 05/14] locking/qspinlock: Remove unbounded cmpxchg
 loop from locking slowpath

On Thu, Apr 26, 2018 at 05:55:19PM +0100, Will Deacon wrote:
> Hi Peter,
> 
> On Thu, Apr 26, 2018 at 05:53:35PM +0200, Peter Zijlstra wrote:
> > On Thu, Apr 26, 2018 at 11:34:19AM +0100, Will Deacon wrote:
> > > @@ -290,58 +312,50 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
> > >  	}
> > >  
> > >  	/*
> > > +	 * If we observe any contention; queue.
> > > +	 */
> > > +	if (val & ~_Q_LOCKED_MASK)
> > > +		goto queue;
> > > +
> > > +	/*
> > >  	 * trylock || pending
> > >  	 *
> > >  	 * 0,0,0 -> 0,0,1 ; trylock
> > >  	 * 0,0,1 -> 0,1,1 ; pending
> > >  	 */
> > > +	val = atomic_fetch_or_acquire(_Q_PENDING_VAL, &lock->val);
> > > +	if (!(val & ~_Q_LOCKED_MASK)) {
> > >  		/*
> > > +		 * we're pending, wait for the owner to go away.
> > > +		 *
> > > +		 * *,1,1 -> *,1,0
> > 
> > Tail must be 0 here, right?
> 
> Not necessarily. If we're concurrently setting pending with another slowpath
> locker, they could queue in the tail behind us, so we can't mess with those
> upper bits.

Could be my brain just entirely stopped working; but I read that as:

	!(val & ~0xFF) := !(val & 0xFFFFFF00)

which then pretty much mandates the top bits are empty, no?