Re: audit_tree: sleep inside atomic

[Eric Paris]

On Mon, 2010-09-13 at 17:05 -0700, Andrew Morton wrote:
> On Fri, 03 Sep 2010 15:52:30 +0200
> Jiri Slaby <jirislaby@xxxxxxxxx> wrote:
> 
> > Ideas, comments?
> 
> Apparently not.

Sorry, I've been slacking off on vacation the last couple weeks.

> The question is: why is nobody reporting this bug?  Obviously nobody's
> running that code path.  Why not?

The only people who run this code path, that I know of, are govt orgs
who run in certified environments.  I don't know of any upstream kernel
users who really would hit it.

In any case I don't think it would be particularly painful to just
always allocate a chunk between the two locks.  this is not a hot path
by any stretch of the imagination.  I'll see if I can't code something
up today/tomorrow.

-Eric

> > On 06/21/2010 05:15 PM, Jiri Slaby wrote:
> > > Hi,
> > > 
> > > stanse found a sleep inside atomic added by the following commit:
> > > commit fb36de479642bc9bdd3af251ae48b882d8a1ad5d
> > > Author: Eric Paris <eparis@xxxxxxxxxx>
> > > Date:   Thu Dec 17 20:12:05 2009 -0500
> > > 
> > >     audit: reimplement audit_trees using fsnotify rather than inotify
> > > 
> > >     Simply switch audit_trees from using inotify to using fsnotify for it's
> > >     inode pinning and disappearing act information.
> > > 
> > >     Signed-off-by: Eric Paris <eparis@xxxxxxxxxx>
> > > 
> > > 
> > > In untag_chunk, there is
> > >   spin_lock(&entry->lock);
> > >   ...
> > >   new = alloc_chunk(size);
> > >   ...
> > >   spin_unlock(&entry->lock);
> > > 
> > > with
> > > static struct audit_chunk *alloc_chunk(int count)
> > > {
> > >   struct audit_chunk *chunk;
> > >   ...
> > >   chunk = kzalloc(size, GFP_KERNEL);
> > > 
> > > But this can sleep. How big the allocations are? Could it be ATOMIC or
> > > moved outside the spinlock?
> 
> Yes, we could make it GFP_ATOMIC - the code tries to handle allocation
> failures.
> 
> But if we did that we'd be adding a rarely-executed codepath to an
> apparently-never-executed code path.  We'd end up shipping stuff which
> nobody had tested, ever.
> 
> Plus GFP_ATOMIC is unreliable and using it because we screwed up the
> locking is lame.
> 
> untag_chunk() could be converted to use GFP_KERNEL outside hash_lock
> and ->entry_lock.  The usual way is to take a peek, see if it looks
> like we'll probably need to do an allocation and if so, do it outside
> the locks then free it again if it turned out that we didn't need it
> after all.  Or to maintain a one-deep static-local cache and preload
> that cache if it's empty.  Neither are particularly pretty.


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