Re: [PATCH 0/2] suck some poison out of cgroups' linked lists

[Christoph Hellwig <hch@infradead.org>]

On Tue, Mar 15, 2011 at 03:08:41PM +0200, Phil Carmody wrote:
> Apparently, whether nodes are on a list is being checked with list_empty(),
> and if they're on a list, they're list_del()ed. According to a subsequent 
> list_empty() check, they're still on a list, as list_del() doesn't turn
> the nodes into singleton lists, it simply poisons both its pointers, and
> merry poison dereferencing may ensue. Oops.
> 
> There are at least 2 to address this matter, I've gone for the latter:
> 
> 1) Do not use list_empty() to check if a node is on a list or not. Have
> an additional new function that checks to see whether the node is either
> a singleton or is poisoned. Something like list_node_{on,off}_list()?
> 
> 2) Ensure that you never leave poison anywhere where you might want
> to use list_empty().

The correct way is to use list_del_init() if you want to do list_empty
checks later on.