From: Gao Xiang <hsiangkao@aol.com>
To: Eric Biggers <ebiggers@kernel.org>
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 14:15:31 +0800 [thread overview]
Message-ID: <20200124061525.GA2407@hsiangkao-HP-ZHAN-66-Pro-G1> (raw)
In-Reply-To: <20200124054256.GC832@sol.localdomain>
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
next prev parent reply other threads:[~2020-01-24 6:15 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 [this message]
2020-01-24 18:12 ` Eric Biggers
2020-01-24 18:31 ` Al Viro
2020-01-25 3:35 ` Theodore Y. Ts'o
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