Re: [PATCH 002 of 6] md: Fix use-after-free bug when dropping an rdev from an md array.

[Al Viro <viro@ZenIV.linux.org.uk>]

On Mon, Jan 14, 2008 at 02:21:45PM +1100, Neil Brown wrote:

> Maybe it isn't there any more....
> 
> Once upon a time, when I 
>    echo remove > /sys/block/mdX/md/dev-YYY/state

Egads.  And just what will protect you from parallel callers
of state_store()?  buffer->mutex does *not* do that - it only
gives you exclusion on given struct file.  Run the command
above from several shells and you've got independent open
from each redirect => different struct file *and* different
buffer for each => no exclusion whatsoever.

And _that_ is present right in the mainline tree - it's unrelated
to -mm kobject changes.

BTW, yes, you do have a deadlock there - kobject_del() will try to evict
children, which will include waiting for currently running ->store()
to finish, which will include the caller since .../state *is* a child of
that sucker.

The real problem is the lack of any kind of exclusion considerations in
md.c itself, AFAICS.  Fun with ordering is secondary (BTW, yes, it is
a problem - will sysfs ->store() to attribute between export_rdev() and
kobject_del() work correctly?)