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Date: Fri, 24 Jan 2020 13:27:50 +0800 From: Gao Xiang <hsiangkao@....com> To: Eric Biggers <ebiggers@...nel.org> Cc: linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org, linux-f2fs-devel@...ts.sourceforge.net, Alexander Viro <viro@...iv.linux.org.uk>, Daniel Rosenberg <drosen@...gle.com>, Gabriel Krisman Bertazi <krisman@...labora.com> Subject: Re: [PATCH] ext4: fix race conditions in ->d_compare() and ->d_hash() On Thu, Jan 23, 2020 at 09:16:01PM -0800, Eric Biggers wrote: > On Fri, Jan 24, 2020 at 01:04:25PM +0800, Gao Xiang wrote: > > > diff --git a/fs/ext4/dir.c b/fs/ext4/dir.c > > > index 8964778aabefb..0129d14629881 100644 > > > --- a/fs/ext4/dir.c > > > +++ b/fs/ext4/dir.c > > > @@ -671,9 +671,11 @@ static int ext4_d_compare(const struct dentry *dentry, unsigned int len, > > > const char *str, const struct qstr *name) > > > { > > > struct qstr qstr = {.name = str, .len = len }; > > > - struct inode *inode = dentry->d_parent->d_inode; > > > + const struct dentry *parent = READ_ONCE(dentry->d_parent); > > > > I'm not sure if we really need READ_ONCE d_parent here (p.s. d_parent > > won't be NULL anyway), and d_seq will guard all its validity. If I'm > > wrong, correct me kindly... > > > > Otherwise, it looks good to me... > > Reviewed-by: Gao Xiang <gaoxiang25@...wei.com> > > > > While d_parent can't be set to NULL, it can still be changed concurrently. > So we need READ_ONCE() to ensure that a consistent value is used. If I understand correctly, unlazy RCU->ref-walk will be guarded by seqlock, and for ref-walk we have d_lock (and even parent lock) in relative paths. So I prematurely think no race of renaming or unlinking evenually. I'm curious about that if experts could correct me about this. Thanks, Gao Xiang > > - Eric
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