RE: [PATCH v2 2/6] bitops: always define asm-generic non-atomic bitops

Luck, Tony
2022-06-10T16:02:31Z

> > +/**
> > + * generic_test_bit - Determine whether a bit is set
> > + * @nr: bit number to test
> > + * @addr: Address to start counting from
> > + */
>
> Shouldn't we add in this or in separate patch a big NOTE to explain that this
> is actually atomic and must be kept as a such?

"atomic" isn't really the right word. The volatile access makes sure that the
compiler does the test at the point that the source code asked, and doesn't
move it before/after other operations.

But there is no such thing as an atomic test_bit() operation:

if (test_bit(5, addr)) {
/* some other CPU nukes bit 5 */

/* I know it was set when I looked, but now, could be anything */

...
}

-Tony