The Work-stealing dequeue indices could wrap around after ~2 billion
calls to steal(). This could probably be achieved in a few hours.
By using 64-bits indices, we avoid the problem entirely.
* Fix a reuse after free in the job system
Jobs were destroyed and recycled while still in use
by wait() or run(). To fix this we introduce reference-counting of
jobs.
Jobs start with a ref-count of 1, which is decremented when a job
naturally finishes. Additionally, all user-facing methods acquire
a reference for the duration of the call.
* Fix an API inconsistency with JobSystem
JobSystem's API lets the user create jobs but not destroy them.
Jobs are destroyed automatically, without a way for the caller to
know when that happens.
We now explicitly enforce that jobs are no longer valid when
wait() returns. Multiple concurrent wait() are allowed however.
This is enforced by clearing the job pointer upon returning
from JobSystem::wait(Job* job).
* Rename linked-list put/get to push/pop
* Better fix for Job use after free
There was still a race condition where a run()'ed
job could be destroyed before wait() was called,
wait would then use a destroyed object.
The available APIs now are:
run() - runs and destroys a job
runAndWait() - run, then waits for and destroys a job
runAndRetain() - runs and keep a reference to the job
wait() - waits and destroys a job
wait() can only be used with a job obtained with runAndRetain().
* Get rid of unused code
This version of parallel_for has use-after-free issues anyways,
since we changed the semantics of run/wait/etc...
* Fix decRef() memory order
decRef() must ensure that all access to the
object have happened before destroying it.
* Fix memory order in atomic linked list's pop()
It needs acquire semantic, since we want to make
sure that no read/write are reordered before the
pop() -- which returns an object to the caller.
* Fix memory order on runningJobCount
we needed acquire semantic when about to destroy
the last job -- it's similar to decRef.
* Comment usages of std::memory_order_*
* Fix AtomicFreeList A-B-A bug
Turns out AtomicFreeList was not immune to the ABA bug. W're fixing
it here by using a 64-bits CAS, which is available on aarch64 and armv7.
