* Jason Low <jason.l...@hp.com> wrote:
> On Tue, 2015-03-10 at 09:11 +0100, Ingo Molnar wrote:
> > * Jason Low <jason.l...@hp.com> wrote:
> >
> > > This patch applies on top of tip.
> > >
> > > -------------------------------------------------------------------
> > > Similar to what Linus suggested for rwsem_spin_on_owner(), in
> > > mutex_spin_on_owner(), instead of having while (true) and breaking
> > > out of the spin loop on lock->owner != owner, we can have the loop
> > > directly check for while (lock->owner == owner). This improves the
> > > readability of the code.
> > >
> > > Signed-off-by: Jason Low <jason.l...@hp.com>
> > > ---
> > > kernel/locking/mutex.c | 17 +++++------------
> > > 1 files changed, 5 insertions(+), 12 deletions(-)
> > >
> > > diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
> > > index 16b2d3c..1c3b7c5 100644
> > > --- a/kernel/locking/mutex.c
> > > +++ b/kernel/locking/mutex.c
> > > @@ -224,16 +224,8 @@ ww_mutex_set_context_slowpath(struct ww_mutex *lock,
> > > static noinline
> > > bool mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner)
> > > {
> > > - bool ret;
> > > -
> > > rcu_read_lock();
> > > - while (true) {
> > > - /* Return success when the lock owner changed */
> > > - if (lock->owner != owner) {
> > > - ret = true;
> > > - break;
> > > - }
> > > -
> > > + while (lock->owner == owner) {
> > > /*
> > > * Ensure we emit the owner->on_cpu, dereference _after_
> > > * checking lock->owner still matches owner, if that fails,
> > > @@ -242,16 +234,17 @@ bool mutex_spin_on_owner(struct mutex *lock, struct
> > > task_struct *owner)
> > > */
> > > barrier();
> > >
> > > + /* Stop spinning when need_resched or owner is not running. */
> > > if (!owner->on_cpu || need_resched()) {
> > > - ret = false;
> > > - break;
> > > + rcu_read_unlock();
> > > + return false;
> > > }
> > >
> > > cpu_relax_lowlatency();
> > > }
> > > rcu_read_unlock();
> > >
> > > - return ret;
> > > + return true;
> >
> > A nit: having multiple return statements in a function is not the
> > cleanest approach, especially when we are holding locks.
> >
> > It's better to add an 'out_unlock' label to before the
> > rcu_read_unlock() and use that plus 'ret'.
>
> Okay, I can update this patch. Should we make another similar update
> for the rwsem then?
Yeah, I suppose so.
Thanks,
Ingo
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