Kaushik Iska d8390d3f38 Simplify task queues locking mechanism (flutter/engine#16477) ...

We now have one mutex guarding all accesses to
the underlying task heaps. This simplifies the more granular
but bug prone mechanism of having striped locks.

This also re-enables GPUThreadMerger tests that are currently
disabled due to their flaky nature. The scenario that gets fixed by this
change is as follows:

1. Thread-1: We lock `queue_meta_mutex_` and grab locks on `queue_1` and release the meta mutex.
2. Thread-1: We add an Observer on `queues` object.
3. Thread-2: We lock `queue_meta_mutex_` and grab locks on `queue_2`.
4. Thread-2: We try to dispose all the pending tasks on `queue_2` which calls `erase` on `queues`.

The above situation is not thread safe without having 1 lock.

Note: This increases the contention on one lock and could potentially be bad for perf. We are
explicitly making this trade-off towards reducing the complexity.

Fixes: https://github.com/flutter/flutter/issues/49007

2020-02-07 10:24:05 -08:00

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src/flutter

Simplify task queues locking mechanism (flutter/engine#16477)

2020-02-07 10:24:05 -08:00