Currently, `GClosure` does atomic writes to the flags in its initial
bitfield, but it never does atomic reads from them. This is not safe —
even if the memory read for an int is atomic, the compiler might
duplicate or rearrange a non-atomic read and give an unexpected result.
Using an atomic fetch inserts the necessary compiler and hardware
barriers to ensure a single result is fetched.
(See !4575 of an example of where this kind of bug has caused
misbehaviour.)
So, introduce an atomic read helper for the `GClosure` bitfield. Rather
than reading a single member of the bitfield, it returns the entire
bitfield, as several `GClosure` methods need access to multiple members
of the bitfield, and atomically fetching them all separately would not
be atomic overall.
Fix various `GClosure` methods to use the new atomic read function.
There are more parts of the code still to fix — these were just the
straightforward ones.
Signed-off-by: Philip Withnall <pwithnall@gnome.org>
d1beecf1a3