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Comment by KaeseEs | Hacker News Reader

I see they made the same mistake as POSIX threads in implementing recursive mutexes. Disheartening.

Of all the ways to shoot yourself in the foot with C, recursive mutexes seem relatively benign. The implementation is a handful of lines of code, and they're not always a symptom of bad design, either; imagine needing to lock each node in a path through a (possibly cyclic) graph.

If two different threads were trying to lock different paths that entered the same cycle at different nodes, there can be a deadlock.

  X -> B
  B -> C
  C -> B
  Y -> C

Thread 1 tries to lock X B C; thread 2 tries to lock Y C B; deadlock with 1 holding X and B, 2 holding Y and C.

It's possible that your paths may be more restricted than this, but I reckon keeping the paths and cycles clean would be a bigger problem than recursive mutexes.

Recursive mutexes are useful when you need to implement multithreading a posteriori and cannot evaluate all the locking paths.

When adding locking without evaluating all locking paths, how can you be sure none of those paths cause a deadlock ?

You cannot self deadlock with a a single recursive mutex.

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