linux-ext4 - Re: [PATCH] ext4: fix race conditions in ->d_compare() and ->d

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Date:   Fri, 24 Jan 2020 14:15:31 +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 09:42:56PM -0800, Eric Biggers wrote:
> On Fri, Jan 24, 2020 at 01:34:23PM +0800, Gao Xiang wrote:
> > On Thu, Jan 23, 2020 at 09:16:01PM -0800, Eric Biggers wrote:
> > 
> > []
> > 
> > > So we need READ_ONCE() to ensure that a consistent value is used.
> > 
> > By the way, my understanding is all pointer could be accessed
> > atomicly guaranteed by compiler. In my opinion, we generally
> > use READ_ONCE() on pointers for other uses (such as, avoid
> > accessing a variable twice due to compiler optimization and
> > it will break some logic potentially or need some data
> > dependency barrier...)
> > 
> > Thanks,
> > Gao Xiang
> 
> But that *is* why we need READ_ONCE() here.  Without it, there's no guarantee
> that the compiler doesn't load the variable twice.  Please read:
> https://github.com/google/ktsan/wiki/READ_ONCE-and-WRITE_ONCE

After scanning the patch, it seems the parent variable (dentry->d_parent)
only referenced once as below:

-	struct inode *inode = dentry->d_parent->d_inode;
+	const struct dentry *parent = READ_ONCE(dentry->d_parent);
+	const struct inode *inode = READ_ONCE(parent->d_inode);

So I think it is enough as

	const struct inode *inode = READ_ONCE(dentry->d_parent->d_inode);

to access parent inode once to avoid parent inode being accessed
for more time (and all pointers dereference should be in atomic
by compilers) as one reason on

	if (!inode || !IS_CASEFOLDED(inode) || ...

or etc.

Thanks for your web reference, I will look into it. I think there
is no worry about dentry->d_parent here because of this only one
dereference on dentry->d_parent.

You could ignore my words anyway, just my little thought though.
Other part of the patch is ok.

Thanks,
Gao Xiang

> 
> - Eric