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[109.80.100.86]) by smtp.gmail.com with ESMTPSA id q21sm1076707wmq.13.2019.06.06.01.39.02 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 06 Jun 2019 01:39:02 -0700 (PDT) Date: Thu, 6 Jun 2019 10:38:56 +0200 From: Andrea Parri To: Herbert Xu Cc: Linus Torvalds , "Paul E. McKenney" , Frederic Weisbecker , Boqun Feng , Fengguang Wu , LKP , LKML , Netdev , "David S. Miller" , Dmitry Vyukov , Alan Stern , Eric Dumazet Subject: Re: rcu_read_lock lost its compiler barrier Message-ID: <20190606083856.GA5337@andrea> 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> <20190603024640.2soysu4rpkwjuash@gondor.apana.org.au> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190603024640.2soysu4rpkwjuash@gondor.apana.org.au> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jun 03, 2019 at 10:46:40AM +0800, Herbert Xu wrote: > The case we were discussing is from net/ipv4/inet_fragment.c from > the net-next tree: BTW, thank you for keeping me and other people who intervened in that discussion in Cc:... Andrea > > void fqdir_exit(struct fqdir *fqdir) > { > ... > fqdir->dead = true; > > /* call_rcu is supposed to provide memory barrier semantics, > * separating the setting of fqdir->dead with the destruction > * work. This implicit barrier is paired with inet_frag_kill(). > */ > > INIT_RCU_WORK(&fqdir->destroy_rwork, fqdir_rwork_fn); > queue_rcu_work(system_wq, &fqdir->destroy_rwork); > } > > and > > void inet_frag_kill(struct inet_frag_queue *fq) > { > ... > rcu_read_lock(); > /* The RCU read lock provides a memory barrier > * guaranteeing that if fqdir->dead is false then > * the hash table destruction will not start until > * after we unlock. Paired with inet_frags_exit_net(). > */ > if (!fqdir->dead) { > rhashtable_remove_fast(&fqdir->rhashtable, &fq->node, > fqdir->f->rhash_params); > ... > } > ... > rcu_read_unlock(); > ... > } > > I simplified this to > > Initial values: > > a = 0 > b = 0 > > CPU1 CPU2 > ---- ---- > a = 1 rcu_read_lock > synchronize_rcu if (a == 0) > b = 2 b = 1 > rcu_read_unlock > > On exit we want this to be true: > b == 2