From: "J. Bruce Fields" <[email protected]>
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 <[email protected]>
Signed-off-by: J. Bruce Fields <[email protected]>
---
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
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" <[email protected]>
>
> 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 <[email protected]>
> Signed-off-by: J. Bruce Fields <[email protected]>
> ---
> 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
>
From: "J. Bruce Fields" <[email protected]>
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 <[email protected]>
Signed-off-by: J. Bruce Fields <[email protected]>
---
fs/dcache.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
Resending with minor conflict resolution needed due to b5ae6b15bd73
"merge d_materialise_unique() into d_splice_alias()"
diff --git a/fs/dcache.c b/fs/dcache.c
index af8300b0b03d..95a45c563eed 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -2690,7 +2690,7 @@ static int __d_unalias(struct inode *inode,
struct dentry *dentry, struct dentry *alias)
{
struct mutex *m1 = NULL, *m2 = NULL;
- int ret = -EBUSY;
+ int ret = -ESTALE;
/* If alias and dentry share a parent, then no extra locks required */
if (alias->d_parent == dentry->d_parent)
--
1.9.3