On Mon, Jul 04, 2016 at 03:42:59PM +0900, Byungchul Park wrote: [snip] > > > +2. A lock has dependency with all locks in the releasing context, having > > > + been held since the lock was held. > > > > But you cannot tell this. The 'since the lock was held' thing fully > > depends on timing and is not fundamentally correct. > > > > lock(A) > > unlock(A) > > lock(A) > > wait_for(B) > > unlock(A) > > wake(B) > > > > Between the wait_for(B) and wake(B), _nothing_ has been held, yet still > > there's the deadlock potential. > > Crossreleas feature can detect this situation as a deadlock. wait_for() > is not an actual lock, but we can make it detectable by using acquring and > releasing semantics on wait_for() and wake(). > > > And note that if the timing was 'right', you would never get to wake(B) > > because deadlock, so you'd never establish that there would be a > > deadlock. > > If a deadlock actually happens, then we cannot establish it as you said. > Remind that current lockdep does nothing for this situation. But at least > crossrelease feature can detect this deadlock possibility at the time the > dependency tree(graph) is built, which is better than doing nothing. >
Confused, how?
Say the sequence of events is as follow:
(two tasks are initially with no lock held)
Task 1 Task 2
============= ====================
lock(A)
unlock(A)
lock(A)
wait_for(B) // acquire
wake(B) // commit + release
unlock(A)
by the time, the commit are called, the dependency tree will be built,
and we will find there is _no_ lock held before wake(B). Therefore at
the release stage, you will end up only adding dependency chain A->B in
the lockdep, right? And it looks like neither Task1 or Task2 will break
the dependency chain A->B. So how can crossrelease detect the potential
deadlock?
It will be better, that you could provide some samples that crossrelease
can detect after your confirmation.
Regards,
Boqun
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