Re: [PATCH] fix bb_prealloc_list corruption due to wrong group locking

[Eric Sandeen <sandeen@redhat.com>]

Eric Sandeen wrote:
> This is for Red Hat bug 490026,
> EXT4 panic, list corruption in ext4_mb_new_inode_pa
> 
> (this was on backported ext4 from 2.6.29)
> 
> We hit a BUG() in __list_add from  ext4_mb_new_inode_pa()
> because the list head pointed to a removed item:
> 
> list_add corruption. next->prev should be ffff81042f2fe158,
> but was 0000000000200200
> 
> (0000000000200200 is LIST_POISON2, set when the item is deleted)
> 
> ext4_lock_group(sb, group) is supposed to protect this list for
> each group, and a common code flow is 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 its critical that we get the right group number back for
> this pa->pa_pstart block.
> 
> however, ext4_mb_put_pa passes in (pa->pa_pstart - 1) with a 
> comment, "-1 is to protect from crossing allocation group"
> 
> Other list-manipulators do not use the "-1" so we have the 
> potential to lock the wrong group and race.  Given how the 
> ext4_get_group_no_and_offset() function works, it doesn't seem
> to me that the subtraction is correct.

Hm, unless pa_pstart gets advanced to the point where it's in the next
group when it's used up...  might be more reading to do here.

-Eric