linux-kernel - Re: [PATCH] fuse: make fuse

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Date:	Thu, 13 Jan 2011 12:09:07 +0100
From:	Miklos Szeredi <miklos@...redi.hu>
To:	Nick Piggin <npiggin@...il.com>
CC:	miklos@...redi.hu, linux-fsdevel@...r.kernel.org,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fuse: make fuse_permission() RCU aware

On Thu, 13 Jan 2011, Nick Piggin wrote:
> >        inode = entry->d_inode;
> >        if (inode && is_bad_inode(inode))
> >                return 0;
> 
> Now it can be the case that entry->d_inode is not stable -- it can
> go away or even flip between inodes in the case of concurrent
> unlink/creat activity. And you may be using a different inode than
> the namei path walk is using!
> This isn't as scary as it sounds actually, because any such changes
> do get detected and the path walk restarted in that case.
> 
> You might be OK becuse you do test for NULL (although it really
> wants an ACCESS_ONCE() so it doesn't load a NULL or different
> inodes, but that's quite theoretical).
> 
> But this is a little unfriendly for filesystems, and I do want to impress
> the rule to not touch ->d_parent or ->d_inode in rcu walk mode
> (just to avoid any surprises).
> 
> So what I have done in such cases is to update the API to provide
> what the callers want. In this case, we could consider adding an
> inode parameter to .d_revalidate, which callers can be sure matches
> the inode used by vfs, and will not change.

Yes, I think supplying an inode to ->d_revalidate() would be the right
thing.

I see that there's nd->inode, and it's documented in vfs.txt, but IMO
we shouln'd be encouraging new uses of 'nd' inside filesystem
callbacks, rather the opposite.  A 'flags' parameter too might make
sense which, for the moment, would only have LOOKUP_RCU instead of all
the lookup flags.

Thanks,
Miklos
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