CODEWITHSUNDEEP
javaconcurrencyreentrantlock
goodGuy
goodGuy

Reputation: 51

Java Reentrant lock dead lock

Why java reentrant lock does not cause dead lock?

I was studying Reentrant lock, and did realize that a thread failed to try acquire will be placed in a sync queue, and before it parks, it set its prev node's waitStatus to SIGNAL, and recheck if the prev node has set its stat to 0, if not, it might be sure that the prev node has not unparked it, because it has not cleared the SIGNAL state, so it parks. But what if the interval between shoudParkAFterFailedAcquire and parkAndCheckInterrrupt takes, say 1 second or 1 minute long, so unpark arrives first before parking. Would this cause a deadlock? enter image description here

Upvotes: 0

Views: 309

Answers (1)

Matt Timmermans
Matt Timmermans

Reputation: 59253

park and unpark don't work like you seem to think they do. They are based on a permit-like system:

  • When a thread is unparked, it is granted a "permit" if it doesn't already have one.
  • When a thread calls park, it blocks until it has a permit and the permit is revoked. If it already has a permit when park is called, then it doesn't block at all.

So it doesn't matter if it takes a minute or more for acquireQueued to get to the park, because the preceding call to unpark has already provided the thread with a permit. The permit will be revoked without blocking.

Upvotes: 4

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