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Date: Wed, 24 Jun 2020 16:56:18 -0700 From: Costa Sapuntzakis <costa@...estorage.com> To: linux-ext4@...r.kernel.org Subject: [BUG] invalid superblock checksum possibly due to race Our workload: taking snapshots repeatedly of an active ext4 filesystem (vdbench fwiw). e2fsck discovered a snapshot that had a corrupted superblock after journal replay. Diffing the corrupted superblock to the superblock before journal replay revealed that only s_last_orphan and the checksum had changed. The following race could explain it: Thread 1 (T1): ext4_orphan_del -> update s_last_orphan to value A -> ext4_handle_dirty_super -> ext4_superblock_csum_set -- PAUSE right before setting es->s_checksum T2: ext4_orphan_del -> update s_last_orphan to value B -> ext4_handle_dirty_super -> ext4_superblock_csum_set runs to completion T1: Resume and assign es->s_checksum Is there higher level synchronization going on that makes this race benign? If not, a spinlock around the calculation and assignment should fix it. The spinlock still has the race where s_last_orphan is being updated while the checksum is calculated. But the last thread to set s_last_orphan will also eventually try to recalculate the checksum and set it right (though it's possible some other thread will do it for it). And I'm guessing/hoping jbd2 won't flush the superblock to the journal and close a transaction until the references from journal_get_write_access drain. The checksum is recalculated before the get_write_access reference is dropped. -Costa
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