Hold transaction in scoutfs_quota_mod_rule to prevent alloc corruption.#297
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Hold transaction in scoutfs_quota_mod_rule to prevent alloc corruption.#297
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zabbo
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Apr 13, 2026
scoutfs_quota_mod_rule calls scoutfs_item_create/delete which use the transaction allocator but it never held it. Without the hold, a concurrent transaction commit can call scoutfs_alloc_init to reinitialize the allocator while dirty_alloc_blocks is in the middle of setting up the freed list block. This overwrites alloc->freed with the server's fresh (empty) state, causing a blkno mismatch BUG_ON in list_block_add. Reproduced by stressing concurrent quota add/del operations across mounts. Crashdump analysis confirms dirty_list_block COW'd a freed block (fr_old=9842, new blkno=9852) but by the time list_block_add ran, freed.ref.blkno was 0 with first_nr=0 and total_nr=0: the freed list head had been zeroed by a concurrent alloc_init. Fix by adding scoutfs_hold_trans/scoutfs_release_trans around the item modification in scoutfs_quota_mod_rule, preventing transaction commit from racing with the allocator use. Rename the 'unlock' label to 'release' since 'out' now directly does the unlock. The unlock safely handles a NULL lock. Signed-off-by: Auke Kok <auke.kok@versity.com>
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scoutfs_quota_mod_rule calls scoutfs_item_create/delete which use the transaction allocator but it never held it. Without the hold, a concurrent transaction commit can call scoutfs_alloc_init to reinitialize the allocator while dirty_alloc_blocks is in the middle of setting up the freed list block. This overwrites alloc->freed with the server's fresh (empty) state, causing a blkno mismatch BUG_ON in list_block_add.
Reproduced by stressing concurrent quota add/del operations across mounts. Crashdump analysis confirms dirty_list_block COW'd a freed block (fr_old=9842, new blkno=9852) but by the time list_block_add ran, freed.ref.blkno was 0 with first_nr=0 and total_nr=0: the freed list head had been zeroed by a concurrent alloc_init.
Fix by adding scoutfs_hold_trans/scoutfs_release_trans around the item modification in scoutfs_quota_mod_rule, preventing transaction commit from racing with the allocator use.