This passes my review, but it needs an ACK from Al or someone for the
addition of the new lookup_one_len_unlocked (which is the same as
lookup_one_len except that it takes the i_mutex itself when required
instead of requiring the caller to).
--b.
On Fri, May 08, 2015 at 04:01:33PM -0400, J. Bruce Fields wrote:
> From: NeilBrown <neilb@xxxxxxx>
>
> We need information about exports when crossing mountpoints during
> lookup or NFSv4 readdir. If we don't already have that information
> cached, we may have to ask (and wait for) rpc.mountd.
>
> In both cases we currently hold the i_mutex on the parent of the
> directory we're asking rpc.mountd about. We've seen situations where
> rpc.mountd performs some operation on that directory that tries to take
> the i_mutex again, resulting in deadlock.
>
> With some care, we may be able to avoid that in rpc.mountd. But it
> seems better just to avoid holding a mutex while waiting on userspace.
>
> It appears that lookup_one_len is pretty much the only operation that
> needs the i_mutex. So we could just drop the i_mutex elsewhere and do
> something like
>
> mutex_lock()
> lookup_one_len()
> mutex_unlock()
>
> In many cases though the lookup would have been cached and not required
> the i_mutex, so it's more efficient to create a lookup_one_len() variant
> that only takes the i_mutex when necessary.
>
> Signed-off-by: J. Bruce Fields <bfields@xxxxxxxxxx>
> ---
> fs/namei.c | 74 +++++++++++++++++++++++++++++++++++++++++++++++++++
> fs/nfsd/nfs3xdr.c | 2 +-
> fs/nfsd/nfs4xdr.c | 8 +++---
> fs/nfsd/vfs.c | 23 +++++++---------
> include/linux/namei.h | 1 +
> 5 files changed, 89 insertions(+), 19 deletions(-)
>
> Here's an updated patch.
>
> diff --git a/fs/namei.c b/fs/namei.c
> index 4a8d998b7274..8b866d79c5b7 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -2139,6 +2139,8 @@ static struct dentry *lookup_hash(struct nameidata *nd)
> *
> * Note that this routine is purely a helper for filesystem usage and should
> * not be called by generic code.
> + *
> + * The caller must hold base->i_mutex.
> */
> struct dentry *lookup_one_len(const char *name, struct dentry *base, int len)
> {
> @@ -2182,6 +2184,78 @@ struct dentry *lookup_one_len(const char *name, struct dentry *base, int len)
> }
> EXPORT_SYMBOL(lookup_one_len);
>
> +/**
> + * lookup_one_len_unlocked - filesystem helper to lookup single pathname component
> + * @name: pathname component to lookup
> + * @base: base directory to lookup from
> + * @len: maximum length @len should be interpreted to
> + *
> + * Note that this routine is purely a helper for filesystem usage and should
> + * not be called by generic code.
> + *
> + * Unlike lookup_one_len, it should be called without the parent
> + * i_mutex held, and will take the i_mutex itself if necessary.
> + */
> +struct dentry *lookup_one_len_unlocked(const char *name,
> + struct dentry *base, int len)
> +{
> + struct qstr this;
> + unsigned int c;
> + int err;
> + struct dentry *ret;
> +
> + this.name = name;
> + this.len = len;
> + this.hash = full_name_hash(name, len);
> + if (!len)
> + return ERR_PTR(-EACCES);
> +
> + if (unlikely(name[0] == '.')) {
> + if (len < 2 || (len == 2 && name[1] == '.'))
> + return ERR_PTR(-EACCES);
> + }
> +
> + while (len--) {
> + c = *(const unsigned char *)name++;
> + if (c == '/' || c == '\0')
> + return ERR_PTR(-EACCES);
> + }
> + /*
> + * See if the low-level filesystem might want
> + * to use its own hash..
> + */
> + if (base->d_flags & DCACHE_OP_HASH) {
> + int err = base->d_op->d_hash(base, &this);
> + if (err < 0)
> + return ERR_PTR(err);
> + }
> +
> + err = inode_permission(base->d_inode, MAY_EXEC);
> + if (err)
> + return ERR_PTR(err);
> +
> + ret = __d_lookup(base, &this);
> + if (ret)
> + return ret;
> + /*
> + * __d_lookup() is used to try to get a quick answer and avoid the
> + * mutex. A false-negative does no harm.
> + */
> + ret = __d_lookup(base, &this);
> + if (ret && ret->d_flags & DCACHE_OP_REVALIDATE) {
> + dput(ret);
> + ret = NULL;
> + }
> + if (ret)
> + return ret;
> +
> + mutex_lock(&base->d_inode->i_mutex);
> + ret = __lookup_hash(&this, base, 0);
> + mutex_unlock(&base->d_inode->i_mutex);
> + return ret;
> +}
> +EXPORT_SYMBOL(lookup_one_len_unlocked);
> +
> int user_path_at_empty(int dfd, const char __user *name, unsigned flags,
> struct path *path, int *empty)
> {
> diff --git a/fs/nfsd/nfs3xdr.c b/fs/nfsd/nfs3xdr.c
> index f6e7cbabac5a..01dcd494f781 100644
> --- a/fs/nfsd/nfs3xdr.c
> +++ b/fs/nfsd/nfs3xdr.c
> @@ -823,7 +823,7 @@ compose_entry_fh(struct nfsd3_readdirres *cd, struct svc_fh *fhp,
> } else
> dchild = dget(dparent);
> } else
> - dchild = lookup_one_len(name, dparent, namlen);
> + dchild = lookup_one_len_unlocked(name, dparent, namlen);
> if (IS_ERR(dchild))
> return rv;
> if (d_mountpoint(dchild))
> diff --git a/fs/nfsd/nfs4xdr.c b/fs/nfsd/nfs4xdr.c
> index 158badf945df..2c1adaa0bd2f 100644
> --- a/fs/nfsd/nfs4xdr.c
> +++ b/fs/nfsd/nfs4xdr.c
> @@ -2804,14 +2804,14 @@ nfsd4_encode_dirent_fattr(struct xdr_stream *xdr, struct nfsd4_readdir *cd,
> __be32 nfserr;
> int ignore_crossmnt = 0;
>
> - dentry = lookup_one_len(name, cd->rd_fhp->fh_dentry, namlen);
> + dentry = lookup_one_len_unlocked(name, cd->rd_fhp->fh_dentry, namlen);
> if (IS_ERR(dentry))
> return nfserrno(PTR_ERR(dentry));
> if (d_really_is_negative(dentry)) {
> /*
> - * nfsd_buffered_readdir drops the i_mutex between
> - * readdir and calling this callback, leaving a window
> - * where this directory entry could have gone away.
> + * we're not holding the i_mutex here, so there's
> + * a window where this directory entry could have gone
> + * away.
> */
> dput(dentry);
> return nfserr_noent;
> diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
> index a30e79900086..6d5b33458e91 100644
> --- a/fs/nfsd/vfs.c
> +++ b/fs/nfsd/vfs.c
> @@ -217,10 +217,16 @@ nfsd_lookup_dentry(struct svc_rqst *rqstp, struct svc_fh *fhp,
> host_err = PTR_ERR(dentry);
> if (IS_ERR(dentry))
> goto out_nfserr;
> - /*
> - * check if we have crossed a mount point ...
> - */
> if (nfsd_mountpoint(dentry, exp)) {
> + /*
> + * We don't need the i_mutex after all. It's
> + * still possible we could open this (regular
> + * files can be mountpoints too), but the
> + * i_mutex is just there to prevent renames of
> + * something that we might be about to delegate,
> + * and a mountpoint won't be renamed:
> + */
> + fh_unlock(fhp);
> if ((host_err = nfsd_cross_mnt(rqstp, &dentry, &exp))) {
> dput(dentry);
> goto out_nfserr;
> @@ -1876,7 +1882,6 @@ static __be32 nfsd_buffered_readdir(struct file *file, nfsd_filldir_t func,
> offset = *offsetp;
>
> while (1) {
> - struct inode *dir_inode = file_inode(file);
> unsigned int reclen;
>
> cdp->err = nfserr_eof; /* will be cleared on successful read */
> @@ -1895,15 +1900,6 @@ static __be32 nfsd_buffered_readdir(struct file *file, nfsd_filldir_t func,
> if (!size)
> break;
>
> - /*
> - * Various filldir functions may end up calling back into
> - * lookup_one_len() and the file system's ->lookup() method.
> - * These expect i_mutex to be held, as it would within readdir.
> - */
> - host_err = mutex_lock_killable(&dir_inode->i_mutex);
> - if (host_err)
> - break;
> -
> de = (struct buffered_dirent *)buf.dirent;
> while (size > 0) {
> offset = de->offset;
> @@ -1920,7 +1916,6 @@ static __be32 nfsd_buffered_readdir(struct file *file, nfsd_filldir_t func,
> size -= reclen;
> de = (struct buffered_dirent *)((char *)de + reclen);
> }
> - mutex_unlock(&dir_inode->i_mutex);
> if (size > 0) /* We bailed out early */
> break;
>
> diff --git a/include/linux/namei.h b/include/linux/namei.h
> index c8990779f0c3..bb3a2f7cca67 100644
> --- a/include/linux/namei.h
> +++ b/include/linux/namei.h
> @@ -62,6 +62,7 @@ extern struct dentry *kern_path_locked(const char *, struct path *);
> extern int kern_path_mountpoint(int, const char *, struct path *, unsigned int);
>
> extern struct dentry *lookup_one_len(const char *, struct dentry *, int);
> +extern struct dentry *lookup_one_len_unlocked(const char *, struct dentry *, int);
>
> extern int follow_down_one(struct path *);
> extern int follow_down(struct path *);
> --
> 1.9.3
>
--
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