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

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Date:   Fri, 24 Jan 2020 18:31:03 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     Gao Xiang <hsiangkao@....com>, linux-ext4@...r.kernel.org,
        linux-fsdevel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net,
        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 Fri, Jan 24, 2020 at 10:12:54AM -0800, Eric Biggers wrote:

> > 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.

*nod*

Note that on everything except alpha it'll generate the same code, unless
the compiler screws up an generates multiple loads.  On alpha it adds
a barrier and I really don't want to sit down and check if its absense
could lead to anything unpleasant.