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SubjectRe: [PATCH v3 05/14] locking/qspinlock: Remove unbounded cmpxchg loop from locking slowpath
On Thu, Apr 26, 2018 at 05:55:19PM +0100, Will Deacon wrote:
> Hi Peter,
>
> On Thu, Apr 26, 2018 at 05:53:35PM +0200, Peter Zijlstra wrote:
> > On Thu, Apr 26, 2018 at 11:34:19AM +0100, Will Deacon wrote:
> > > @@ -290,58 +312,50 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
> > > }
> > >
> > > /*
> > > + * If we observe any contention; queue.
> > > + */
> > > + if (val & ~_Q_LOCKED_MASK)
> > > + goto queue;
> > > +
> > > + /*
> > > * trylock || pending
> > > *
> > > * 0,0,0 -> 0,0,1 ; trylock
> > > * 0,0,1 -> 0,1,1 ; pending
> > > */
> > > + val = atomic_fetch_or_acquire(_Q_PENDING_VAL, &lock->val);
> > > + if (!(val & ~_Q_LOCKED_MASK)) {
> > > /*
> > > + * we're pending, wait for the owner to go away.
> > > + *
> > > + * *,1,1 -> *,1,0
> >
> > Tail must be 0 here, right?
>
> Not necessarily. If we're concurrently setting pending with another slowpath
> locker, they could queue in the tail behind us, so we can't mess with those
> upper bits.

Could be my brain just entirely stopped working; but I read that as:

!(val & ~0xFF) := !(val & 0xFFFFFF00)

which then pretty much mandates the top bits are empty, no?

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