Re: [rfc] "fair" rw spinlocks

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Sat, Nov 28, 2009 at 12:15:07PM +0100, Andi Kleen wrote:
> "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> writes:
> >
> > The usual trick would be to keep per-fair-rwlock state in per-CPU
> > variables.  If it is forbidden to read-acquire one nestable fair rwlock
> > while read-holding another, then this per-CPU state can be a single
> > pointer and a nesting count.  On the other hand, if it is permitted to
> > read-acquire one nestable fair rwlock while holding another, then one
> > can use a small per-CPU array of pointer/count pairs.
> 
> The problem is that in preemptible kernels kernel threads can switch
> CPUs all the time.  How would you sync the per CPU state then?

In preemptible kernels, put the state into the task structure.  Perhaps do
this in non-preemptible kernels too, just to save a bit of source code.

							Thanx, Paul