Re: [patch] fs: fix superblock iteration race

[Nick Piggin <npiggin@suse.de>]

On Fri, Jun 11, 2010 at 09:06:01AM -0700, Linus Torvalds wrote:
> On Fri, Jun 11, 2010 at 7:50 AM, Nick Piggin <npiggin@suse.de> wrote:
> > Not sure if this is really the _cleanest_ way to fix it. But open coding
> > the list walking is a bit annoying too. And I couldn't see any real way to
> > make the list macro safe. Better ideas?
> 
> I really think we should open-code the list walking instead. You
> basically already are doing that, and in a very non-obvious way too
> (ie you are mixing the non-open-coded list walker with also explicitly
> playing with the internal variable for that magic walker.
> 
> So I would get rid of the 'list_for_each_entry_safe' entirely, and
> replace it with something like
> 
>    struct list_head *list;
> 
>    spin_lock(&sb_lock);
>    list = super_blocks->next;
>    while (list != &super_blocks) {
>       struct super_block *sb = list_entry(next, struct super_block, s_list);
>       list = list->next;
> 
>       if (list_empty(&sb->s_instances))
>          continue;
> 
>       if (!sb->s_nr_dentry_unused)
>          continue;
> 
>       sb->s_count++;
>       spin_unlock(&sb_lock);
> 
>       .... whatever ...
> 
>       spin_lock(&sb_lock);
>       /* We dropped the lock, need to re-load the next list entry */
>       list = sb->s_list.next;
>       __put_super(sb);
>    }

Yeah I do agree really. I guess the bug came about in the first place
because it's easy to overlook where the memory accesses happen.

 
> which isn't that much more complicated, now is it? Sure, it's
> open-coded, but at least it doesn't play games. And being open-coded,
> it's a lot more honest about the issue. Maybe even add a comment
> saying "we can't use the list_for_each[_safe]() macro, because we
> don't hold the lock and we're not the only ones that may delete
> things" explaining _why_ it's open-coded.
> 
> I dunno. Maybe Al disagrees. I just don't like using the "simple
> helpers" and then changing subtly how they work by knowing their
> internals.

I'll respin the patch and we'll see.