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    SubjectRe: general protection fault: from release_blocks_on_commit
    Theodore Tso wrote:

    > On Tue, Oct 21, 2008 at 02:03:01PM -0400, Eric Paris wrote:
    >
    >> I can consistently get the below backtrace any time I try to shutdown my
    >> machine. This machine has ext4 as it's root FS. This is 100%
    >> reproducible. I backed out commit
    >> 3e624fc72fba09b6f999a9fbb87b64efccd38036 and it fixed the problem.
    >>
    >> This is a regression.
    >>
    >
    > Can you send me your .config, please? I'm trying to duplicate it on
    > my end.
    >
    > - Ted
    >
    Ted, you probably need some slab debugging on to hit it.

    I think the problem is that jbd2_journal_commit_transaction may call
    __jbd2_journal_drop_transaction(journal, commit_transaction) if the
    checkpoint lists are NULL, and this frees the commit_transaction.

    However, the call to ->j_commit_callback() tries to use it after that.

    I'm out of time for now to be sure this is the right fix, but something
    like this perhaps?

    Index: linux-2.6/fs/jbd2/commit.c
    ===================================================================
    --- linux-2.6.orig/fs/jbd2/commit.c 2008-10-27 11:24:42.000000000 -0500
    +++ linux-2.6/fs/jbd2/commit.c 2008-10-27 17:19:22.771063324 -0500
    @@ -992,15 +992,15 @@ restart_loop:
    commit_transaction->t_cpprev->t_cpnext =
    commit_transaction;
    }
    + if (journal->j_commit_callback)
    + journal->j_commit_callback(journal, commit_transaction);
    +
    + trace_mark(jbd2_end_commit, "dev %s transaction %d head %d",
    + journal->j_devname, commit_transaction->t_tid,
    + journal->j_tail_sequence);
    }
    spin_unlock(&journal->j_list_lock);

    - if (journal->j_commit_callback)
    - journal->j_commit_callback(journal, commit_transaction);
    -
    - trace_mark(jbd2_end_commit, "dev %s transaction %d head %d",
    - journal->j_devname, commit_transaction->t_tid,
    - journal->j_tail_sequence);
    jbd_debug(1, "JBD: commit %d complete, head %d\n",
    journal->j_commit_sequence, journal->j_tail_sequence);

    Also, I'm not certain that it matters, but the loop in
    release_blocks_on_commit() is kfreeing list entries w/o taking
    them off the list; I suppose maybe this is safe if the whole thing
    is getting discarded when we're done, but just to keep things sane,
    would this make sense (also, I think we need to double-check use of
    s_md_lock; it's taken when adding things to the list, but not when
    freeing/removing ... if it's needed, isn't it needed on both ends...):


    Index: linux-2.6/fs/ext4/mballoc.c
    ===================================================================
    --- linux-2.6.orig/fs/ext4/mballoc.c 2008-10-27 11:24:41.000000000 -0500
    +++ linux-2.6/fs/ext4/mballoc.c 2008-10-27 17:19:43.401064490 -0500
    @@ -2644,6 +2644,7 @@ static void release_blocks_on_commit(jou
    struct super_block *sb = journal->j_private;
    struct ext4_buddy e4b;
    struct ext4_group_info *db;
    + struct ext4_sb_info *sbi = EXT4_SB(sb);
    int err, count = 0, count2 = 0;
    struct ext4_free_data *entry;
    ext4_fsblk_t discard_block;
    @@ -2683,6 +2684,9 @@ static void release_blocks_on_commit(jou
    (unsigned long long) discard_block, entry->count);
    sb_issue_discard(sb, discard_block, entry->count);

    + spin_lock(&sbi->s_md_lock);
    + list_del(&entry->list);
    + spin_unlock(&sbi->s_md_lock);
    kmem_cache_free(ext4_free_ext_cachep, entry);
    ext4_mb_release_desc(&e4b);
    }
    -Eric



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