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?
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