From: Eric Biggers <ebiggers@kernel.org>
To: Gao Xiang <hsiangkao@aol.com>
Cc: linux-ext4@vger.kernel.org, linux-fsdevel@vger.kernel.org,
linux-f2fs-devel@lists.sourceforge.net,
Alexander Viro <viro@zeniv.linux.org.uk>,
Daniel Rosenberg <drosen@google.com>,
Gabriel Krisman Bertazi <krisman@collabora.com>
Subject: Re: [PATCH] ext4: fix race conditions in ->d_compare() and ->d_hash()
Date: Fri, 24 Jan 2020 10:12:54 -0800 [thread overview]
Message-ID: <20200124181253.GA41762@gmail.com> (raw)
In-Reply-To: <20200124061525.GA2407@hsiangkao-HP-ZHAN-66-Pro-G1>
On Fri, Jan 24, 2020 at 02:15:31PM +0800, Gao Xiang wrote:
> 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.
>
While that does make it really unlikely to cause a real-world problem, it's
still undefined behavior to not properly annotate a data race, it would make the
code harder to understand as there would be no indication that there's a data
race, and it would confuse tools that try to automatically detect data races.
So let's keep the READ_ONCE() on d_parent.
- Eric
next prev parent reply other threads:[~2020-01-24 18:12 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-24 4:12 [PATCH] ext4: fix race conditions in ->d_compare() and ->d_hash() Eric Biggers
2020-01-24 5:04 ` Gao Xiang
2020-01-24 5:16 ` Eric Biggers
2020-01-24 5:27 ` Gao Xiang
2020-01-24 5:53 ` Eric Biggers
2020-01-24 5:34 ` Gao Xiang
2020-01-24 5:42 ` Eric Biggers
2020-01-24 6:15 ` Gao Xiang
2020-01-24 18:12 ` Eric Biggers [this message]
2020-01-24 18:31 ` Al Viro
2020-01-25 3:35 ` Theodore Y. Ts'o
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