Re: Core dumps in redisplay.

[Richard Stallman]

    This sounds like normally only the main thread should ever be touching
    interrupt_input_blocked, unless we have a bug.  Correct?  So we need
    not think about how to synchronize accesses to the variable, but
    rather make sure that no thread except the main thread will ever run
    code touching it.  Correct?

That is one coherent design plan.  It may not be the only one.

Meanwhile, operations carried out in another thread are somewhat like
operations carried out in a signal handler.  So in some cases it may
be necessary for other threads to wait for interrupt_input_blocked to
be 0 before they do their work.

And unless those threads have strictly higher priority than the main
thread, we have to worry about locking: how to prevent the main thread
from continuing and entering code that does BLOCK_INPUT while the
other thread is doing something that's supposed to be blocked by
BLOCK_INPUT.