[08/12] locks: break delegations on unlink

Commit Message

J. Bruce Fields July 9, 2013, 3:58 p.m. UTC

On Tue, Jul 09, 2013 at 09:05:06AM -0400, Jeff Layton wrote:
> On Wed,  3 Jul 2013 16:12:32 -0400
> "J. Bruce Fields" <[e-mail]> wrote:
> 
> > From: "J. Bruce Fields" <[e-mail]>
> > 
> > We need to break delegations on any operation that changes the set of
> > links pointing to an inode.  Start with unlink.
> > 
> > Such operations also hold the i_mutex on a parent directory.  Breaking a
> > delegation may require waiting for a timeout (by default 90 seconds) in
> > the case of a unresponsive NFS client.  To avoid blocking all directory
> > operations, we therefore drop locks before waiting for the delegation.
> > The logic then looks like:
> > 
> > 	acquire locks
> > 	...
> > 	test for delegation; if found:
> > 		take reference on inode
> > 		release locks
> > 		wait for delegation break
> > 		drop reference on inode
> > 		retry
> > 
> > It is possible this could never terminate.  (Even if we take precautions
> > to prevent another delegation being acquired on the same inode, we could
> > get a different inode on each retry.)  But this seems very unlikely.
> > 
> > The initial test for a delegation happens after the lock on the target
> > inode is acquired, but the directory inode may have been acquired
> > further up the call stack.  We therefore add a "struct inode **"
> > argument to any intervening functions, which we use to pass the inode
> > back up to the caller in the case it needs a delegation synchronously
> > broken.
...
> > -int vfs_unlink(struct inode *dir, struct dentry *dentry)
> > +int vfs_unlink(struct inode *dir, struct dentry *dentry, struct inode **delegated_inode)
> 
> nit: this might be a good time to add a kerneldoc header on this
> function. The delegated_inode thing might not be clear to the
> uninitiated.

Something like this?

--b.

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Comments

Jeff Layton July 9, 2013, 4:02 p.m. UTC | #1

On Tue, 9 Jul 2013 11:58:43 -0400
"J. Bruce Fields" <[e-mail]> wrote:

> On Tue, Jul 09, 2013 at 09:05:06AM -0400, Jeff Layton wrote:
> > On Wed,  3 Jul 2013 16:12:32 -0400
> > "J. Bruce Fields" <[e-mail]> wrote:
> > 
> > > From: "J. Bruce Fields" <[e-mail]>
> > > 
> > > We need to break delegations on any operation that changes the set of
> > > links pointing to an inode.  Start with unlink.
> > > 
> > > Such operations also hold the i_mutex on a parent directory.  Breaking a
> > > delegation may require waiting for a timeout (by default 90 seconds) in
> > > the case of a unresponsive NFS client.  To avoid blocking all directory
> > > operations, we therefore drop locks before waiting for the delegation.
> > > The logic then looks like:
> > > 
> > > 	acquire locks
> > > 	...
> > > 	test for delegation; if found:
> > > 		take reference on inode
> > > 		release locks
> > > 		wait for delegation break
> > > 		drop reference on inode
> > > 		retry
> > > 
> > > It is possible this could never terminate.  (Even if we take precautions
> > > to prevent another delegation being acquired on the same inode, we could
> > > get a different inode on each retry.)  But this seems very unlikely.
> > > 
> > > The initial test for a delegation happens after the lock on the target
> > > inode is acquired, but the directory inode may have been acquired
> > > further up the call stack.  We therefore add a "struct inode **"
> > > argument to any intervening functions, which we use to pass the inode
> > > back up to the caller in the case it needs a delegation synchronously
> > > broken.
> ...
> > > -int vfs_unlink(struct inode *dir, struct dentry *dentry)
> > > +int vfs_unlink(struct inode *dir, struct dentry *dentry, struct inode **delegated_inode)
> > 
> > nit: this might be a good time to add a kerneldoc header on this
> > function. The delegated_inode thing might not be clear to the
> > uninitiated.
> 
> Something like this?
> 
> --b.
> 
> diff --git a/fs/namei.c b/fs/namei.c
> index cba3db1..7c6e244 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -3384,6 +3384,24 @@ SYSCALL_DEFINE1(rmdir, const char __user *, pathname)
>  	return do_rmdir(AT_FDCWD, pathname);
>  }
>  
> +/**
> + * vfs_unlink - unlink a filesystem object
> + * @dir:	parent directory
> + * @dentry:	victim
> + * @delegated_inode: returns victim inode, if the inode is delegated.
> + *
> + * The caller must hold dir->i_mutex.
> + *
> + * If vfs_unlink discovers a delegation, it will return -EWOULDBLOCK and
> + * return a reference to the inode in delegated_inode.  The caller
> + * should then break the delegation on that inode and retry.  Because
> + * breaking a delegation may take a long time, the caller should drop
> + * dir->i_mutex before doing so.
> + *
> + * Alternatively, a caller may pass NULL for delegated_inode.  This may
> + * be appropriate for callers that expect the underlying filesystem not
> + * to be NFS exported.
> + */
>  int vfs_unlink(struct inode *dir, struct dentry *dentry, struct inode **delegated_inode)
>  {
>  	struct inode *target = dentry->d_inode;

ACK -- looks good to me.

Patch

diff --git a/fs/namei.c b/fs/namei.c
index cba3db1..7c6e244 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -3384,6 +3384,24 @@  SYSCALL_DEFINE1(rmdir, const char __user *, pathname)
 	return do_rmdir(AT_FDCWD, pathname);
 }
 
+/**
+ * vfs_unlink - unlink a filesystem object
+ * @dir:	parent directory
+ * @dentry:	victim
+ * @delegated_inode: returns victim inode, if the inode is delegated.
+ *
+ * The caller must hold dir->i_mutex.
+ *
+ * If vfs_unlink discovers a delegation, it will return -EWOULDBLOCK and
+ * return a reference to the inode in delegated_inode.  The caller
+ * should then break the delegation on that inode and retry.  Because
+ * breaking a delegation may take a long time, the caller should drop
+ * dir->i_mutex before doing so.
+ *
+ * Alternatively, a caller may pass NULL for delegated_inode.  This may
+ * be appropriate for callers that expect the underlying filesystem not
+ * to be NFS exported.
+ */
 int vfs_unlink(struct inode *dir, struct dentry *dentry, struct inode **delegated_inode)
 {
 	struct inode *target = dentry->d_inode;