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	Wed, 11 Jun 2025 23:33:06 +0000
Date: Thu, 12 Jun 2025 00:33:06 +0100
From: Al Viro <viro@zeniv.linux.org.uk>
To: NeilBrown <neil@brown.name>
Cc: Christian Brauner <brauner@kernel.org>, Jan Kara <jack@suse.cz>,
	David Howells <dhowells@redhat.com>, Tyler Hicks <code@tyhicks.com>,
	Chuck Lever <chuck.lever@oracle.com>,
	Jeff Layton <jlayton@kernel.org>,
	Miklos Szeredi <miklos@szeredi.hu>,
	Amir Goldstein <amir73il@gmail.com>, Kees Cook <kees@kernel.org>,
	Joel Granados <joel.granados@kernel.org>,
	Namjae Jeon <linkinjeon@kernel.org>,
	Steve French <smfrench@gmail.com>,
	Sergey Senozhatsky <senozhatsky@chromium.org>,
	netfs@lists.linux.dev, linux-kernel@vger.kernel.org,
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	linux-nfs@vger.kernel.org, linux-unionfs@vger.kernel.org,
	linux-cifs@vger.kernel.org
Subject: Re: [PATCH 2/2] fs/proc: take rcu_read_lock() in proc_sys_compare()
Message-ID: <20250611233306.GA1647736@ZenIV>
References: <20250611225848.1374929-1-neil@brown.name>
 <20250611225848.1374929-3-neil@brown.name>
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Sender: Al Viro <viro@ftp.linux.org.uk>

On Thu, Jun 12, 2025 at 08:57:03AM +1000, NeilBrown wrote:

> However there is no guarantee that this lock is held by d_same_name()
> (the caller of ->d_compare).  In particularly d_alloc_parallel() calls
> d_same_name() after rcu_read_unlock().

d_alloc_parallel() calls d_same_name() with dentry being pinned;
if it's positive, nothing's going to happen to its inode,
rcu_read_lock() or not.  It can go from negative to positive,
but that's it.

Why is it needed?  We do care about possibly NULL inode (basically,
when RCU dcache lookup runs into a dentry getting evicted right
under it), but that's not relevant here.