Re: [PATCH] mutex: Speed up mutex_spin_on_owner() by not taking the RCU lock

Paul E. McKenney
2015-04-10T18:05:30Z

On Fri, Apr 10, 2015 at 07:44:00PM +0200, Ingo Molnar wrote:
>
> * Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:
>
> > > No RCU overhead, and this is the access to owner->on_cpu:
> > >
> > > 69: 49 8b 81 10 c0 ff ff mov -0x3ff0(%r9),%rax
> > >
> > > Totally untested and all that, I only built the mutex.o.
> > >
> > > What do you think? Am I missing anything?
> >
> > I suspect it is good, but let's take a look at Linus' summary of the code:
> >
> > rcu_read_lock();
> > while (sem->owner == owner) {
> > if (!owner->on_cpu || need_resched())
> > break;
> > cpu_relax_lowlatency();
> > }
> > rcu_read_unlock();
>
> Note that I patched the mutex case as a prototype, which is more
> commonly used than rwsem-xadd. But the rwsem case is similar as well.
>
> > The cpu_relax_lowlatency() looks to have barrier() semantics, so the
> > sem->owner should get reloaded every time through the loop. This is
> > needed, because otherwise the task structure could get freed and
> > reallocated as something else that happened to have the field at the
> > ->on_cpu offset always zero, resulting in an infinite loop.
>
> So at least with the get_kernel(..., &owner->on_cpu) approach, the
> get_kernel() copy has barrier semantics as well (it's in assembly), so
> it will be reloaded in every iteration in a natural fashion.

Good point, even better!

Thanx, Paul