linux-kernel - Re: [RFC PATCH 08/21] Revert "list: Use WRITE_ONCE() when initializing list

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Message-ID: <20200331134725.GL19865@paulmck-ThinkPad-P72>
Date:   Tue, 31 Mar 2020 06:47:25 -0700
From:   "Paul E. McKenney" <paulmck@...nel.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>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>, kernel-team@...roid.com,
        kernel-hardening@...ts.openwall.com
Subject: Re: [RFC PATCH 08/21] Revert "list: Use WRITE_ONCE() when
 initializing list_head structures"

On Tue, Mar 31, 2020 at 02:11:54PM +0100, Will Deacon wrote:
> On Mon, Mar 30, 2020 at 04:25:05PM -0700, Paul E. McKenney wrote:
> > On Tue, Mar 24, 2020 at 03:36:30PM +0000, Will Deacon wrote:
> > > This reverts commit 2f073848c3cc8aff2655ab7c46d8c0de90cf4e50.
> > > 
> > > There is no need to use WRITE_ONCE() to initialise a non-RCU 'list_head'.
> > > 
> > > Cc: Paul E. McKenney <paulmck@...nel.org>
> > > Cc: Peter Zijlstra <peterz@...radead.org>
> > > Signed-off-by: Will Deacon <will@...nel.org>
> > 
> > And attention to lockless uses of list_empty() here, correct?
> > 
> > Depending on the outcome of discussions on 3/21, I should have added in
> > all three cases.
> 
> Yes, patch 3 is where this will get sorted. It looks like we'll have to
> disable KCSAN around the READ_ONCE() over there, but I also need to finish
> wrapping my head around list_empty_careful() because I'm deeply suspicious!

At the very least, it does have the disadvantage of touching an additional
cache line, and up to two additional cache lines in the non-empty case.  :-(

							Thanx, Paul