Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
MIME-Version: 1.0
References: <20150910005708.GA23369@wfg-t540p.sh.intel.com>
 <20150910102513.GA1677@fixme-laptop.cn.ibm.com> <20150910171649.GE4029@linux.vnet.ibm.com>
 <20150911021933.GA1521@fixme-laptop.cn.ibm.com> <20150921193045.GA13674@lerouge>
 <20150921204327.GH4029@linux.vnet.ibm.com> <20190602055607.bk5vgmwjvvt4wejd@gondor.apana.org.au>
In-Reply-To: <20190602055607.bk5vgmwjvvt4wejd@gondor.apana.org.au>
From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Sun, 2 Jun 2019 13:54:12 -0700
Message-ID: <CAHk-=whLGKOmM++OQi5GX08_dfh8Xfz9Wq7khPo+MVtPYh_8hw@mail.gmail.com>
Subject: Re: rcu_read_lock lost its compiler barrier
To:     Herbert Xu <herbert@gondor.apana.org.au>
Cc:     "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Fengguang Wu <fengguang.wu@intel.com>, LKP <lkp@01.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Netdev <netdev@vger.kernel.org>,
        "David S. Miller" <davem@davemloft.net>
Content-Type: text/plain; charset="UTF-8"
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Sat, Jun 1, 2019 at 10:56 PM Herbert Xu <herbert@gondor.apana.org.au> wrote:
>
> You can't then go and decide to remove the compiler barrier!  To do
> that you'd need to audit every single use of rcu_read_lock in the
> kernel to ensure that they're not depending on the compiler barrier.

What's the possible case where it would matter when there is no preemption?

          Linus