Re: [PATCH v7 1/5] locking/qspinlock: relaxes cmpxchg & xchg ops in native code

[Will Deacon]

On Tue, Oct 13, 2015 at 08:02:25PM +0200, Peter Zijlstra wrote:
> On Tue, Sep 22, 2015 at 04:50:40PM -0400, Waiman Long wrote:
> > This patch replaces the cmpxchg() and xchg() calls in the native
> > qspinlock code with more relaxed versions of those calls to enable
> > other architectures to adopt queued spinlocks with less performance
> > overhead.
> 
> > @@ -62,7 +63,7 @@ static __always_inline int queued_spin_is_contended(struct qspinlock *lock)
> >  static __always_inline int queued_spin_trylock(struct qspinlock *lock)
> >  {
> >  	if (!atomic_read(&lock->val) &&
> > -	   (atomic_cmpxchg(&lock->val, 0, _Q_LOCKED_VAL) == 0))
> > +	   (atomic_cmpxchg_acquire(&lock->val, 0, _Q_LOCKED_VAL) == 0))
> >  		return 1;
> >  	return 0;
> >  }
> > @@ -77,7 +78,7 @@ static __always_inline void queued_spin_lock(struct qspinlock *lock)
> >  {
> >  	u32 val;
> >  
> > -	val = atomic_cmpxchg(&lock->val, 0, _Q_LOCKED_VAL);
> > +	val = atomic_cmpxchg_acquire(&lock->val, 0, _Q_LOCKED_VAL);
> >  	if (likely(val == 0))
> >  		return;
> >  	queued_spin_lock_slowpath(lock, val);
> 
> > @@ -319,7 +329,7 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
> >  		if (val == new)
> >  			new |= _Q_PENDING_VAL;
> >  
> > -		old = atomic_cmpxchg(&lock->val, val, new);
> > +		old = atomic_cmpxchg_acquire(&lock->val, val, new);
> >  		if (old == val)
> >  			break;
> >  
> 
> So given recent discussion, all this _release/_acquire stuff is starting
> to worry me.
> 
> So we've not declared if they should be RCsc or RCpc, and given this
> patch (and the previous ones) these lock primitives turn into RCpc if
> the atomic primitives are RCpc.

Our spinlocks are currently RCpc and I think these really should match.

> So far only the proposed PPC implementation is RCpc -- and their current
> spinlock implementation is also RCpc, but that is a point of discussion.
> 
> Just saying..

Well, PPC already made that choice for their spinlocks, so I don't
necessarily think we should worry too much here. They get to choose
between a ~5% performance hit (iirc) or a higher potential for subtle
locking issues. That said, I don't believe the RCpc/RCsc choice actually
affects common locking paradigms (so far, we're only aware of RCU needing
to do something different on PPC).

If we end up getting swamped with horrible bug reports from PPC users,
then we can add a TAINT_RCPC ;) (or just strengthen their implementation).

> Also, I think we should annotate the control dependencies in these
> things.

Yes, please.

Will
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