[Devel] [PATCH hci-8.1 v7 4/8] block/blk-cbt: read a map slot once, publish a map page with release

[no-reply at sashiko.virtuozzo.com]

> CBT_PAGE() loads a map slot twice. A lockless reader racing with the
> cache flush, which marks an empty slot CBT_PAGE_MISSED, can get NULL from
> the first load and the marker from the second, and then use
> (struct page *)0x1 as a page.
> 
> A page is also installed with a plain store under cbt->lock, but readers
> don't take that lock: on a weakly ordered CPU a reader may see the
> pointer before the page's zeroing, and the late zeroing can then clear a
> bit the reader has just set.
> 
> Load the slot once with READ_ONCE(), whose address dependency orders the
> reader's accesses to the page, and install a page with
> smp_store_release().
> 
> https://virtuozzo.atlassian.net/browse/VSTOR-149268
> Feature: cbt: changed block tracking (for backup)
> Signed-off-by: Pavel Tikhomirov <ptikhomirov at virtuozzo.com>

Sashiko has reviewed this patch and found no issues. It looks great!

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Sashiko AI review · https://sashiko.vzint.dev/#/patchset/20261028-ptikhomirov-vstor-149268-cbt-account-at-rq-start-v7-0-5c48672191c2@virtuozzo.com?part=4