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Date:   Tue, 30 Jun 2020 13:48:32 +0200
From:   Jan Kara <>
To:     Costa Sapuntzakis <>
Subject: Re: [BUG] invalid superblock checksum possibly due to race


On Wed 24-06-20 16:56:18, Costa Sapuntzakis wrote:
> 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?

Thanks for report and the analysis. What you describe indeed seems

> If not, a spinlock around the calculation and assignment should fix it.

Yes, probably ext4_superblock_csum_set() should use


to synchronize updating of superblock checksum. Will you send a patch?

> 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.

Yes, jbd2 layer will make sure that inconsistent block contents will not
make it to disk in this case.


Jan Kara <>

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