nfs_instantiate(): prevent multiple aliases for directory inode
Since NFS allows open-by-fhandle, we have to cope with the possibility of mkdir vs. open-by-guessed-handle races. A local filesystem could decide what the inumber of the new object will be and insert a locked inode with that inumber into icache _before_ the on-disk data structures begin to look good and unlock it only once it has a dentry alias, so that open-by-handle coming first would quietly fail and mkdir coming first would have open-by-handle grab its dentry. For NFS it's a non-starter - the icache key is server-supplied fhandle and we do not get that until the object has been fully created on server. We really have to deal with the possibility that open-by-handle gets the in-core inode and attaches a dentry to it before mkdir does. Solution: let nfs_mkdir() use d_splice_alias() to catch those. We can * get an error. Just return it to our caller. * get NULL - no preexisting dentry aliases, we'd just done what d_add() would've done. Success. * get a reference to preexisting alias. In that case the alias had been moved in place of nfs_mkdir() argument (and hashed there), while nfs_mkdir() argument is left unhashed negative. Which is just fine for ->mkdir() callers, all we need is to release the reference we'd got from d_splice_alias() and report success. Cc: Trond Myklebust <trond.myklebust@primarydata.com> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
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@ -1641,6 +1641,7 @@ int nfs_instantiate(struct dentry *dentry, struct nfs_fh *fhandle,
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struct dentry *parent = dget_parent(dentry);
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struct inode *dir = d_inode(parent);
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struct inode *inode;
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struct dentry *d;
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int error = -EACCES;
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d_drop(dentry);
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@ -1662,10 +1663,12 @@ int nfs_instantiate(struct dentry *dentry, struct nfs_fh *fhandle,
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goto out_error;
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}
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inode = nfs_fhget(dentry->d_sb, fhandle, fattr, label);
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error = PTR_ERR(inode);
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if (IS_ERR(inode))
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d = d_splice_alias(inode, dentry);
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if (IS_ERR(d)) {
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error = PTR_ERR(d);
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goto out_error;
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d_add(dentry, inode);
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}
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dput(d);
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out:
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dput(parent);
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return 0;
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