linux-kernel - Re: [PATCH] dcache: ensure d_flags & d_inode are consistent in lookup

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Message-ID: <20190422155520.GJ2217@ZenIV.linux.org.uk>
Date:   Mon, 22 Apr 2019 16:55:20 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Hou Tao <houtao1@...wei.com>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        yi.zhang@...wei.com
Subject: Re: [PATCH] dcache: ensure d_flags & d_inode are consistent in
 lookup_fast()

On Fri, Apr 19, 2019 at 04:48:10PM +0800, Hou Tao wrote:

> The root cause is the inconsistency between d_flags & d_inode
> during the REF-walk in lookup_fast(): d_is_negative(dentry)
> returns false, but d_backing_inode() still returns a NULL pointer.
> 
> The RCU-walk path in lookup_fast() uses d_seq to ensure d_flags & d_inode
> are consistent, and lookup_slow() use inode lock to ensure that, so only
> the REF-walk path in lookup_fast() is problematic.
> 
> Fixing it by adding a paired smp_rmb/smp_wmb between the reading/writing
> of d_inode & d_flags to ensure the consistency.

The problem is real, but I'm not sure I like the proposed fix ;-/
We could simply use d_really_is_negative() there, avoiding all that
mess.  If and when we get around to whiteouts-in-dcache (i.e. if
unionfs series gets resurrected), we can revisit that...