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    ext4: fix bb_prealloc_list corruption due to wrong group locking · d33a1976
    Eric Sandeen 提交于
    This is for Red Hat bug 490026: EXT4 panic, list corruption in
    ext4_mb_new_inode_pa
    
    ext4_lock_group(sb, group) is supposed to protect this list for
    each group, and a common code flow to remove an album is like
    this:
    
        ext4_get_group_no_and_offset(sb, pa->pa_pstart, &grp, NULL);
        ext4_lock_group(sb, grp);
        list_del(&pa->pa_group_list);
        ext4_unlock_group(sb, grp);
    
    so it's critical that we get the right group number back for
    this prealloc context, to lock the right group (the one 
    associated with this pa) and prevent concurrent list manipulation.
    
    however, ext4_mb_put_pa() passes in (pa->pa_pstart - 1) with a 
    comment, "-1 is to protect from crossing allocation group".
    
    This makes sense for the group_pa, where pa_pstart is advanced
    by the length which has been used (in ext4_mb_release_context()),
    and when the entire length has been used, pa_pstart has been
    advanced to the first block of the next group.
    
    However, for inode_pa, pa_pstart is never advanced; it's just
    set once to the first block in the group and not moved after
    that.  So in this case, if we subtract one in ext4_mb_put_pa(),
    we are actually locking the *previous* group, and opening the
    race with the other threads which do not subtract off the extra
    block.
    Signed-off-by: NEric Sandeen <sandeen@redhat.com>
    Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>
    d33a1976
mballoc.c 135.2 KB