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