Re: [PATCH v4] lib/dlock-list: Scale dlock_lists_empty()

[James Bottomley <James.Bottomley@HansenPartnership.com>]

On Tue, 2017-11-07 at 13:57 -0500, Waiman Long wrote:
> On 11/07/2017 12:59 PM, Andreas Dilger wrote:
> > 
> > On Nov 7, 2017, at 4:59 AM, Jan Kara <jack@suse.cz> wrote:
> > > 
> > > On Mon 06-11-17 10:47:08, Davidlohr Bueso wrote:
> > > > 
> > > > +	/*
> > > > +	 * Serialize dlist->used_lists such that a 0->1
> > > > transition is not
> > > > +	 * missed by another thread checking if any of the
> > > > dlock lists are
> > > > +	 * used.
> > > > +	 *
> > > > +	 * CPU0				    CPU1
> > > > +	 *
> > > > dlock_list_add()                 dlock_lists_empty()
> > > > +	 *   [S] atomic_inc(used_lists);
> > > > +	 *       smp_mb__after_atomic();
> > > > +	 *					  smp_mb__be
> > > > fore_atomic();
> > > > +	 *				      [L]
> > > > atomic_read(used_lists)
> > > > +	 *       list_add()
> > > > +	 */
> > > > +	smp_mb__before_atomic();
> > > > +	return !atomic_read(&dlist->used_lists);
> > Just a general kernel programming question here - I thought the
> > whole point of atomics is that they are, well, atomic across all
> > CPUs so there is no need for a memory barrier?  If there is a need
> > for a memory barrier for each atomic access (assuming it isn't
> > accessed under another lock, which would make the use of atomic
> > types pointless, IMHO) then I'd think there is a lot of code in the
> > kernel that isn't doing this properly.
> > 
> > What am I missing here?
> 
> Atomic update and memory barrier are 2 different things. Atomic
> update means other CPUs see either the value before or after the
> update. They won't see anything in between. For a counter, it means
> we won't miss any counts. However, not all atomic operations give an
> ordering guarantee. The atomic_read() and atomic_inc() are examples
> that do not provide memory ordering guarantee. See
> Documentation/memory-barriers.txt for more information about it.
> 
> A CPU can perform atomic operations 1 & 2 in program order, but other
> CPUs may see operation 2 first before operation 1. Here memory
> barrier can be used to guarantee that other CPUs see the memory
> updates in certain order.

There's an omission here which I think Andreas may have been referring
to:  atomic_inc/dec operations *are* strongly ordered with respect to
each other, so if two CPUs are simultaneously executing atomic_inc, the
order in which they execute isn't guaranteed, but it is guaranteed that
the losing atomic_inc will not begin until the winning one is
completed, so after both are done the value will have +2.  So although
atomic_read and atomic_inc have no ordering guarantee at all (the point
of the barrier above), if you're looking at the return values of
atomic_inc/dec you don't need a barrier because regardless of which
order the CPUs go in, they'll see distinct values (we use this for
reference counting).

James