tree e6c74f004cb23a3e1d6073fd91112685b35c5957
parent d0638817a349df0e72bb97e14be8b22332d5b346
author Hans Boehm <hboehm@google.com> 1474349489 -0700
committer Hans Boehm <hboehm@google.com> 1474652003 -0700

Make reference counted assignment check for races.

If SkRefCnt_SafeAssign is erroneously invoked simultaneously by two
threads on the same destination, a likely outcome is that the
reference count of an object other than the replaced dst will
be decremented. This results in an extra reference count decrement,
which usually means that the final reference count decrement will
be applied to a random location in an object that has already been
reallocated.  This is exceedingly hard to debug.

We add a hack to detect such data races with sufficiently high
probability, so that such a data race bug should sometimes actually
generate bug reports that lend themselves to diagnosis.

We detect changes within a relatively larger range that normally
includes several (typically slow) memory fences.  Not all such changes
would provoke a crash. Even if we decrement the wrong reference count,
there's a chance we would decrement a dead location. Thus, to
avoid potentially adding instability, we currently only log.

This change tries to minimize additional runtime overhead.
The macro is only expanded a few dozen times, so we do not worry too
much about code size.

Bug: 31227650

Change-Id: Ieb7150cbe75654d312667bac27837e489591ee0e