Re: [PATCH v2 00/35] bitops: add atomic find_bit() operations

Jan Kara
2023-12-04T18:51:09Z

Hello Yury!

On Sun 03-12-23 11:23:47, Yury Norov wrote:
> Add helpers around test_and_{set,clear}_bit() that allow to search for
> clear or set bits and flip them atomically.
>
> The target patterns may look like this:
>
> for (idx = 0; idx < nbits; idx++)
> if (test_and_clear_bit(idx, bitmap))
> do_something(idx);
>
> Or like this:
>
> do {
> bit = find_first_bit(bitmap, nbits);
> if (bit >= nbits)
> return nbits;
> } while (!test_and_clear_bit(bit, bitmap));
> return bit;
>
> In both cases, the opencoded loop may be converted to a single function
> or iterator call. Correspondingly:
>
> for_each_test_and_clear_bit(idx, bitmap, nbits)
> do_something(idx);
>
> Or:
> return find_and_clear_bit(bitmap, nbits);

These are fine cleanups but they actually don't address the case that has
triggered all these changes - namely the xarray use of find_next_bit() in
xas_find_chunk().

...
> This series is a result of discussion [1]. All find_bit() functions imply
> exclusive access to the bitmaps. However, KCSAN reports quite a number
> of warnings related to find_bit() API. Some of them are not pointing
> to real bugs because in many situations people intentionally allow
> concurrent bitmap operations.
>
> If so, find_bit() can be annotated such that KCSAN will ignore it:
>
> bit = data_race(find_first_bit(bitmap, nbits));

No, this is not a correct thing to do. If concurrent bitmap changes can
happen, find_first_bit() as it is currently implemented isn't ever a safe
choice because it can call __ffs(0) which is dangerous as you properly note
above. I proposed adding READ_ONCE() into find_first_bit() / find_next_bit()
implementation to fix this issue but you disliked that. So other option we
have is adding find_first_bit() and find_next_bit() variants that take
volatile 'addr' and we have to use these in code like xas_find_chunk()
which cannot be converted to your new helpers.

> This series addresses the other important case where people really need
> atomic find ops. As the following patches show, the resulting code
> looks safer and more verbose comparing to opencoded loops followed by
> atomic bit flips.
>
> In [1] Mirsad reported 2% slowdown in a single-thread search test when
> switching find_bit() function to treat bitmaps as volatile arrays. On
> the other hand, kernel robot in the same thread reported +3.7% to the
> performance of will-it-scale.per_thread_ops test.

It was actually me who reported the regression here [2] but whatever :)

[2] https://lore.kernel.org/all/20231011150252.32737-1-jack@suse.cz

> Assuming that our compilers are sane and generate better code against
> properly annotated data, the above discrepancy doesn't look weird. When
> running on non-volatile bitmaps, plain find_bit() outperforms atomic
> find_and_bit(), and vice-versa.
>
> So, all users of find_bit() API, where heavy concurrency is expected,
> are encouraged to switch to atomic find_and_bit() as appropriate.

Well, all users where any concurrency can happen should switch. Otherwise
they are prone to the (admittedly mostly theoretical) data race issue.

Honza
--
Jan Kara <jack@suse.com>
SUSE Labs, CR