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SubjectRe: [RFC, PATCH 1/2] qrcu: "quick" srcu implementation
On 11/30, Oleg Nesterov wrote:
>
> On 11/29, Paul E. McKenney wrote:
> >
> > Hmmm... Now I am wondering if the memory barriers inherent in the
> > __wait_event() suffice for this last barrier... :-/ Thoughts?
> >
> > > + smp_mb();
>
> Fastpath skips __wait_event(), and it is possible that the reader does
> lock/unlock between the first 'mb()' and 'if (atomic_read() == 1)'.

In fact, a slow path needs (I think) it too. We can have an unrelated
wakeup, and then the reader does unlock() before we check !atomic_read()
in the __wait_event()'s loop. The reader removes us from ->wq, in that
case finish_wait() does nothing.

Oleg.

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