linux-kernel - Re: [git pull] vfs and fs fixes

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Date:	Tue, 17 Apr 2012 15:08:26 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"J. Bruce Fields" <bfields@...ldses.org>
Cc:	Al Viro <viro@...iv.linux.org.uk>, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [git pull] vfs and fs fixes

On Tue, Apr 17, 2012 at 2:14 PM, J. Bruce Fields <bfields@...ldses.org> wrote:
> On Tue, Apr 17, 2012 at 07:28:26PM +0100, Al Viro wrote:>
> Maybe instead I could continue using the i_mutex but handle rename some
> other way; e.g. in delegation code:
>
>        if (!mutex_trylock(inode->i_mutex))
>                return -EAGAIN;
>        if (atomic_read(inode->i_renames_in_progress))
>                return -EAGAIN;
>
> and add an
>
>        atomic_inc(inode->i_renames_in_progress);
>        atomic_dec(inode->i_renames_in_progress);
>
> pair around rename.

Please don't make up your own locking. Plus it's broken anyway, since
a rename could come in directly after your atomic_read (and this is
*why* people shouldn't make up their own locks - they are invariably
broken).

> Or I could increment that counter for all the conflicting operations and
> rely on it instead of the i_mutex.  I was trying to avoid adding
> something like that (an inc, a dec, another error path) to every
> operation.  And hoping to avoid adding another field to struct inode.
> Oh well.

We could just say that we can do a double inode lock, but then
standardize on the order. And the only sane order is comparing inode
pointers, not inode numbers like ext4 apparently does.

With a standard order, I don't think it would be at all wrong to just
take the inode lock on rename.

                        Linus
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