Why does test_bit() take a volatile addr?

[Rusty Russell]

Predates git, does anyone remember the rationale?

ie:
        int test_bit(int nr, const volatile unsigned long *addr)

I noticed because gcc failed to elimiate some code in a patch I was
playing with.

I'm nervous about subtle bugs involved in ripping it out, even if noone
knows why.  Should I add __test_bit()?

Thanks,
Rusty.
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