Re: Why does test_bit() take a volatile addr?

[Oliver Neukum]

On Mon, 2013-09-16 at 13:38 +0930, Rusty Russell wrote:
> 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()?

It seems to me that if you do

b = *ptr & 0xf;
c = b << 2;
if (test_bit(1, ptr))

the compiler could optimize away the memory access on ptr without
the volatile. We'd have to add a lot of mb().

	Regards
		Oliver


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