On Wed, 15 Jun 2022 at 04:47, Yury Norov <yury.norov@xxxxxxxxx> wrote: > > On Mon, Jun 13, 2022 at 04:33:17PM +0200, Marco Elver wrote: > > On Mon, 13 Jun 2022 at 16:21, Alexander Lobakin > > <alexandr.lobakin@xxxxxxxxx> wrote: > > > > > > From: Marco Elver <elver@xxxxxxxxxx> > > > Date: Fri, 10 Jun 2022 18:32:36 +0200 > > > > > > > On Fri, 10 Jun 2022 at 18:02, Luck, Tony <tony.luck@xxxxxxxxx> wrote: > > > > > > > > > > > > +/** > > > > > > > + * 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. > > > > > > > > It's listed in Documentation/atomic_bitops.txt. > > > > > > Oh, so my memory was actually correct that I saw it in the docs > > > somewhere. > > > WDYT, should I mention this here in the code (block comment) as well > > > that it's atomic and must not lose `volatile` as Andy suggested or > > > it's sufficient to have it in the docs (+ it's not underscored)? > > > > Perhaps a quick comment in the code (not kerneldoc above) will be > > sufficient, with reference to Documentation/atomic_bitops.txt. > > If it may help, we can do: > > /* > * Bit testing is a naturally atomic operation because bit is > * a minimal quantum of information. > */ > #define __test_bit test_bit That's redundant and we'll end up with a random mix of both. What'd be more interesting is having a __test_bit without the volatile that allows compilers to optimize things more. But I think that also becomes mostly redundant with the optimizations that this series seeks out to do. The distinction is ever so subtle, and clever compilers *will* break concurrent code in ways that are rather hard to imagine: https://lwn.net/Articles/793253/
