Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus test)

Paul E. McKenney
2023-01-19T11:24:39Z

On Wed, Jan 18, 2023 at 09:19:07PM -0500, Alan Stern wrote:
> On Wed, Jan 18, 2023 at 04:02:14PM -0800, Paul E. McKenney wrote:
> > On Wed, Jan 18, 2023 at 03:54:47PM -0500, Alan Stern wrote:
> > > How does this differ from srcu_read_lock() and srcu_read_unlock()? And
> > > how do the "up" and "down" parts figure into it? -- what is going up or
> > > down?
> >
> > Functionally and from a performance/scalability viewpoint, they
> > are identical to srcu_read_lock() and srcu_read_unlock(). The only
> > difference is that srcu_down_read() and srcu_up_read() lack the lockdep
> > machinery that complains when a matching pair of srcu_read_lock() and
> > srcu_read_unlock() are used from different tasks.
>
> This makes me wonder if there's any need for srcu_down_read and
> srcu_up_read at all. Why not just use srcu_read_lock and
> srcu_read_unlock, and remove the lockdep check?

Because the lockdep check is quite helpful in finding bugs in the
common case.

> > Within the implementation, nothing ever goes down, it is all
> > this_cpu_inc(). The "down" and "up" are by analogy to down() and up(),
> > where "down()" says acquire some rights to a resource and "up()" says
> > release those rights.
>
> Another reason not to use those names. If you insist on making these
> operations distinct from srcu_read_lock and srcu_read_unlock, why not
> borrow the "_get" and "_put" nomenclature used by the device core? I
> suspect more people would associate them with acquiring and releasing
> rights to a resource. (Although in this case it might be so clear
> exactly what that resource is.)
>
> > Wait, I can make "down" work.
> >
> > A call to srcu_down_read() reduces the quantity computed by summing the
> > unlocks then subtracting the sum of the locks. A call to srcu_up_read()
> > increases that same quantity. ;-)
>
> I can't honestly call that a resoundingly convincing argument. :-)

It is exactly the same argument for the name of down() and up(). ;-)

And the analogy between mutex_lock() and down() on the one hand an
srcu_read_lock() and srcu_down_read() should be helpful as well.

Thanx, Paul