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Message-ID: <20200324162714.GB2518046@kroah.com>
Date: Tue, 24 Mar 2020 17:27:14 +0100
From: Greg KH <gregkh@...uxfoundation.org>
To: Will Deacon <will@...nel.org>
Cc: linux-kernel@...r.kernel.org, Eric Dumazet <edumazet@...gle.com>,
Jann Horn <jannh@...gle.com>, Kees Cook <keescook@...omium.org>,
Maddie Stone <maddiestone@...gle.com>,
Marco Elver <elver@...gle.com>,
"Paul E . McKenney" <paulmck@...nel.org>,
Peter Zijlstra <peterz@...radead.org>,
Thomas Gleixner <tglx@...utronix.de>, kernel-team@...roid.com,
kernel-hardening@...ts.openwall.com
Subject: Re: [RFC PATCH 01/21] list: Remove hlist_unhashed_lockless()
On Tue, Mar 24, 2020 at 03:36:23PM +0000, Will Deacon wrote:
> Commit c54a2744497d ("list: Add hlist_unhashed_lockless()") added a
> "lockless" variant of hlist_unhashed() that uses READ_ONCE() to access
> the 'pprev' pointer of the 'hlist_node', the intention being that this
> could be used by 'timer_pending()' to silence a KCSAN warning. As well
> as forgetting to add the caller, the patch also sprinkles
> {READ,WRITE}_ONCE() invocations over the standard (i.e. non-RCU) hlist
> code, which is undesirable for a number of reasons:
>
> 1. It gives the misleading impression that the non-RCU hlist code is
> in some way lock-free (despite the notable absence of any memory
> barriers!) and silences KCSAN in such cases.
>
> 2. It unnecessarily penalises code generation for non-RCU hlist users
>
> 3. It makes it difficult to introduce list integrity checks because
> of the possibility of concurrent callers.
>
> Retain the {READ,WRITE}_ONCE() invocations for the RCU hlist code, but
> remove them from the non-RCU implementation. Remove the unused
> 'hlist_unhashed_lockless()' function entirely and add the READ_ONCE()
> to hlist_unhashed(), as we do already for hlist_empty() already.
>
> Cc: Eric Dumazet <edumazet@...gle.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Paul E. McKenney <paulmck@...nel.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Signed-off-by: Will Deacon <will@...nel.org>
> ---
> include/linux/list.h | 33 ++++++++++-----------------------
> 1 file changed, 10 insertions(+), 23 deletions(-)
Reviewed-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
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