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Date:   Mon, 9 Sep 2019 15:59:10 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     "zhengbin (A)" <zhengbin13@huawei.com>
Cc:     jack@suse.cz, akpm@linux-foundation.org,
        linux-fsdevel@vger.kernel.org, "zhangyi (F)" <yi.zhang@huawei.com>,
        renxudong1@huawei.com, Hou Tao <houtao1@huawei.com>
Subject: Re: Possible FS race condition between iterate_dir and
 d_alloc_parallel
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On Mon, Sep 09, 2019 at 10:10:00PM +0800, zhengbin (A) wrote:

Hmm...  So your theory is that what you are seeing is the insertion
into the list done by list_add() exposing an earlier ->next pointer
to those who might be doing lockless walk through the list.
Potentially up to the last barrier done before the list_add()...

> We can solute it in 2 ways:
> 
> 1. add a smp_wmb between __d_alloc and list_add(&dentry->d_child, &parent->d_subdirs)
> 2. revert commit ebaaa80e8f20 ("lockless next_positive()")

I want to take another look at the ->d_subdirs/->d_child readers...
I agree that the above sounds plausible, but I really want to be
sure about the exclusion we have for those accesses.

I'm not sure that smp_wmb() alone would suffice, BTW - the reader side
loop would need to be careful as well.

Which architecture it was, again?  arm64?