* Esben Nielsen <[email protected]> wrote:
> > well, another possibility is that the task got blocked again, and we'll
> > continue boosting _the wrong chain_. I.e. we'll add extra priority to
> > task(s) that might not deserve it at all (it doesnt own the lock we are
> > interested in anymore).
>
> This can't happen. We are always looking at the first waiter on
> task->pi_waiter task->pi_lock held when doing the boosting. If task
> has released the lock the entry task->pi_waiter is gone and no
> boosting will take place!
no, the task got blocked _again_, as part of a _new_ blocking chain, and
there's a _new_ PI waiter! How does the two-lock preemptible boosting
algorithm ensure that if we are in the middle of boosting a
blocking-dependency chain:
T1 -> T2 -> ... -> TI -> TI+1 -> ... TN-1 -> TN
we are at TI, and we [the task doing the boosting] now get preempted.
What prevents TI from being part of a _totally new_ blocking-chain,
where the only similarity between the two chains is that TI is in the
middle of it:
T1' -> T2' -> ... -> TI -> TI+1' -> ... TM-1 -> TM'
the only match between the two chains is 'TI'. Now the algorithm will
happily walk the wrong boosting chain, and will boost the wrong tasks.
Ingo
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