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

Ingo Molnar
2015-04-10T17:44:10Z

* 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.

Thanks,

Ingo