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

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Date:   Thu, 23 Jan 2020 21:42:56 -0800
From:   Eric Biggers <ebiggers@...nel.org>
To:     Gao Xiang <hsiangkao@....com>
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 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

- Eric