Al, what's up with this patch?
--b.
On Tue, Feb 10, 2015 at 10:55:53AM -0500, J. Bruce Fields wrote:
> From: "J. Bruce Fields" <bfields@xxxxxxxxxx>
>
> On a distributed filesystem it's possible for lookup to discover that a
> directory it just found is already cached elsewhere in the directory
> heirarchy. The dcache won't let us keep the directory in both places,
> so we have to move the dentry to the new location from the place we
> previously had it cached.
>
> If the parent has changed, then this requires all the same locks as we'd
> need to do a cross-directory rename. But we're already in lookup
> holding one parent's i_mutex, so it's too late to acquire those locks in
> the right order.
>
> The (unreliable) solution in __d_unalias is to trylock() the required
> locks and return -EBUSY if it fails.
>
> I see no particular reason for returning -EBUSY, and -ESTALE is already
> the result of some other lookup races on NFS. I think -ESTALE is the
> more helpful error return. It also allows us to take advantage of the
> logic Jeff Layton added in c6a9428401c0 "vfs: fix renameat to retry on
> ESTALE errors" and ancestors, which hopefully resolves some of these
> errors before they're returned to userspace.
>
> I can reproduce these cases using NFS with:
>
> ssh root@$client '
> mount -olookupcache=pos '$server':'$export' /mnt/
> mkdir /mnt/TO
> mkdir /mnt/DIR
> touch /mnt/DIR/test.txt
> while true; do
> strace -e open cat /mnt/DIR/test.txt 2>&1 | grep EBUSY
> done
> '
> ssh root@$server '
> while true; do
> mv $export/DIR $export/TO/DIR
> mv $export/TO/DIR $export/DIR
> done
> '
>
> It also helps to add some other concurrent use of the directory on the
> client (e.g., "ls /mnt/TO"). And you can replace the server-side mv's
> by client-side mv's that are repeatedly killed. (If the client is
> interrupted while waiting for the RENAME response then it's left with a
> dentry that has to go under one parent or the other, but it doesn't yet
> know which.)
>
> Acked-by: Jeff Layton <jlayton@xxxxxxxxxxxxxxx>
> Signed-off-by: J. Bruce Fields <bfields@xxxxxxxxxx>
> ---
> fs/dcache.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/fs/dcache.c b/fs/dcache.c
> index 5bc72b07fde2..b7de7f1ae38f 100644
> --- a/fs/dcache.c
> +++ b/fs/dcache.c
> @@ -2612,7 +2612,7 @@ static struct dentry *__d_unalias(struct inode *inode,
> struct dentry *dentry, struct dentry *alias)
> {
> struct mutex *m1 = NULL, *m2 = NULL;
> - struct dentry *ret = ERR_PTR(-EBUSY);
> + struct dentry *ret = ERR_PTR(-ESTALE);
>
> /* If alias and dentry share a parent, then no extra locks required */
> if (alias->d_parent == dentry->d_parent)
> --
> 1.9.3
>
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