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Add comment on metaslab_class_throttle_reserve() locking. #12419

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Merged
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Jul 26, 2021
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Add comment on metaslab_class_throttle_reserve() locking. #12419

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7 changes: 7 additions & 0 deletions module/zfs/metaslab.c
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Original file line number Diff line number Diff line change
Expand Up @@ -5617,6 +5617,13 @@ metaslab_class_throttle_reserve(metaslab_class_t *mc, int slots, int allocator,
if (GANG_ALLOCATION(flags) || (flags & METASLAB_MUST_RESERVE) ||
zfs_refcount_count(&mca->mca_alloc_slots) + slots <= max) {
/*
* The potential race between _count() and _add() is covered
* by the allocator lock in most cases, or irrelevant due to
* GANG_ALLOCATION() or METASLAB_MUST_RESERVE set in others.
* But even if we assume some other non-existing scenario, the
* worst that can happen is few more I/Os get to allocation
* earlier, that is not a problem.
*
* We reserve the slots individually so that we can unreserve
* them individually when an I/O completes.
*/
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