linux-kernel - Re: [PATCH] proc_sysctl: remove RCU annotations for accessing ->sysctl

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Message-ID: <20250613030417.GG1647736@ZenIV>
Date: Fri, 13 Jun 2025 04:04:17 +0100
From: Al Viro <viro@...iv.linux.org.uk>
To: NeilBrown <neil@...wn.name>
Cc: Kees Cook <kees@...nel.org>, Joel Granados <joel.granados@...nel.org>,
	linux-fsdevel@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] proc_sysctl: remove RCU annotations for accessing
 ->sysctl

On Fri, Jun 13, 2025 at 12:52:32PM +1000, NeilBrown wrote:
> 
> The ->sysctl field of a procfs inode is only set when the inode is
> created, and when it is being evicted.  In both these cases there cannot
> be concurrent accesses and so using RCU_INIT_POINTER() and
> rcu_dereference() is misleading.

Wait a minute.  Why can't there be concurrent accesses?  ->evict_inode()
is *before* RCU delay, not after it.

Sure, you can't hit it from d_alloc_parallel(), but you very much can
in normal dcache lookup...