On Wed, 15 Jun 2022 at 04:47, Yury Norov [off-list ref] wrote:

On Mon, Jun 13, 2022 at 04:33:17PM +0200, Marco Elver wrote:

quoted

On Mon, 13 Jun 2022 at 16:21, Alexander Lobakin
[off-list ref] wrote:

quoted

From: Marco Elver <elver@google.com>
Date: Fri, 10 Jun 2022 18:32:36 +0200

quoted

On Fri, 10 Jun 2022 at 18:02, Luck, Tony [off-list ref] wrote:

quoted

quoted

quoted

+/**
+ * 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/