From: Herbert Xu <herbert@gondor.apana.org.au> To: "Paul E. McKenney" <paulmck@linux.ibm.com> Cc: Alan Stern <stern@rowland.harvard.edu>, Boqun Feng <boqun.feng@gmail.com>, Linus Torvalds <torvalds@linux-foundation.org>, Frederic Weisbecker <fweisbec@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>, Andrea Parri <andrea.parri@amarulasolutions.com>, Luc Maranget <luc.maranget@inria.fr>, Jade Alglave <j.alglave@ucl.ac.uk> Subject: Re: rcu_read_lock lost its compiler barrier Date: Thu, 6 Jun 2019 17:28:55 +0800 [thread overview] Message-ID: <20190606092855.dfeuvyk5lbvm4zbf@gondor.apana.org.au> (raw) In-Reply-To: <20190606090619.GC28207@linux.ibm.com> On Thu, Jun 06, 2019 at 02:06:19AM -0700, Paul E. McKenney wrote: > > Or is your point instead that given the initial value of "a" being > zero and the value stored to "a" being one, there is no way that > any possible load and store tearing (your slicing and dicing) could > possibly mess up the test of the value loaded from "a"? Exactly. If you can dream up of a scenario where the compiler can get this wrong I'm all ears. > > But I do concede that in the general RCU case you must have the > > READ_ONCE/WRITE_ONCE calls for rcu_dereference/rcu_assign_pointer. > > OK, good that we are in agreement on this part, at least! ;-) Well only because we're allowing crazy compilers that can turn a simple word-aligned word assignment (a = b) into two stores. Cheers, -- Email: Herbert Xu <herbert@gondor.apana.org.au> Home Page: http://gondor.apana.org.au/~herbert/ PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
WARNING: multiple messages have this Message-ID (diff)
From: Herbert Xu <herbert@gondor.apana.org.au> To: lkp@lists.01.org Subject: Re: rcu_read_lock lost its compiler barrier Date: Thu, 06 Jun 2019 17:28:55 +0800 [thread overview] Message-ID: <20190606092855.dfeuvyk5lbvm4zbf@gondor.apana.org.au> (raw) In-Reply-To: <20190606090619.GC28207@linux.ibm.com> [-- Attachment #1: Type: text/plain, Size: 952 bytes --] On Thu, Jun 06, 2019 at 02:06:19AM -0700, Paul E. McKenney wrote: > > Or is your point instead that given the initial value of "a" being > zero and the value stored to "a" being one, there is no way that > any possible load and store tearing (your slicing and dicing) could > possibly mess up the test of the value loaded from "a"? Exactly. If you can dream up of a scenario where the compiler can get this wrong I'm all ears. > > But I do concede that in the general RCU case you must have the > > READ_ONCE/WRITE_ONCE calls for rcu_dereference/rcu_assign_pointer. > > OK, good that we are in agreement on this part, at least! ;-) Well only because we're allowing crazy compilers that can turn a simple word-aligned word assignment (a = b) into two stores. Cheers, -- Email: Herbert Xu <herbert@gondor.apana.org.au> Home Page: http://gondor.apana.org.au/~herbert/ PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
next prev parent reply other threads:[~2019-06-06 9:29 UTC|newest] Thread overview: 99+ messages / expand[flat|nested] mbox.gz Atom feed top 2015-09-10 0:57 [rcu] kernel BUG at include/linux/pagemap.h:149! Fengguang Wu 2015-09-10 0:57 ` Fengguang Wu 2015-09-10 10:25 ` Boqun Feng 2015-09-10 17:16 ` Paul E. McKenney 2015-09-10 17:16 ` Paul E. McKenney 2015-09-11 2:19 ` Boqun Feng [not found] ` <CAJzB8QG=1iZW3dQEie6ZSTLv8GZ3YSut0aL1VU7LLmiHQ1B1DQ@mail.gmail.com> 2015-09-11 21:59 ` Paul E. McKenney 2015-09-11 21:59 ` Paul E. McKenney 2015-09-12 5:46 ` Boqun Feng 2015-09-21 19:30 ` Frederic Weisbecker 2015-09-21 19:30 ` Frederic Weisbecker 2015-09-21 20:43 ` Paul E. McKenney 2015-09-21 20:43 ` Paul E. McKenney 2019-06-02 5:56 ` rcu_read_lock lost its compiler barrier Herbert Xu 2019-06-02 5:56 ` Herbert Xu 2019-06-02 20:54 ` Linus Torvalds 2019-06-02 20:54 ` Linus Torvalds 2019-06-03 2:46 ` Herbert Xu 2019-06-03 2:46 ` Herbert Xu 2019-06-03 3:47 ` Paul E. McKenney 2019-06-03 4:01 ` Herbert Xu 2019-06-03 4:01 ` Herbert Xu 2019-06-03 4:17 ` Herbert Xu 2019-06-03 4:17 ` Herbert Xu 2019-06-03 7:23 ` Paul E. McKenney 2019-06-03 8:42 ` Paul E. McKenney 2019-06-03 15:26 ` David Laight 2019-06-03 15:40 ` Linus Torvalds 2019-06-03 15:40 ` Linus Torvalds 2019-06-03 5:26 ` Herbert Xu 2019-06-03 5:26 ` Herbert Xu 2019-06-03 6:42 ` Boqun Feng 2019-06-03 6:42 ` Boqun Feng 2019-06-03 20:03 ` Paul E. McKenney 2019-06-04 14:44 ` Alan Stern 2019-06-04 14:44 ` Alan Stern 2019-06-04 16:04 ` Linus Torvalds 2019-06-04 16:04 ` Linus Torvalds 2019-06-04 17:00 ` Alan Stern 2019-06-04 17:00 ` Alan Stern 2019-06-04 17:29 ` Linus Torvalds 2019-06-04 17:29 ` Linus Torvalds 2019-06-07 14:09 ` inet: frags: Turn fqdir->dead into an int for old Alphas Herbert Xu 2019-06-07 14:09 ` Herbert Xu 2019-06-07 15:26 ` Eric Dumazet 2019-06-07 15:26 ` Eric Dumazet 2019-06-07 15:32 ` Herbert Xu 2019-06-07 15:32 ` Herbert Xu 2019-06-07 16:13 ` Eric Dumazet 2019-06-07 16:13 ` Eric Dumazet 2019-06-07 16:19 ` Linus Torvalds 2019-06-07 16:19 ` Linus Torvalds 2019-06-08 15:27 ` Paul E. McKenney 2019-06-08 17:42 ` Linus Torvalds 2019-06-08 17:42 ` Linus Torvalds 2019-06-08 17:50 ` Linus Torvalds 2019-06-08 17:50 ` Linus Torvalds 2019-06-08 18:50 ` Paul E. McKenney 2019-06-08 18:14 ` Paul E. McKenney 2019-06-06 4:51 ` rcu_read_lock lost its compiler barrier Herbert Xu 2019-06-06 4:51 ` Herbert Xu 2019-06-06 6:05 ` Paul E. McKenney 2019-06-06 6:14 ` Herbert Xu 2019-06-06 6:14 ` Herbert Xu 2019-06-06 9:06 ` Paul E. McKenney 2019-06-06 9:28 ` Herbert Xu [this message] 2019-06-06 9:28 ` Herbert Xu 2019-06-06 10:58 ` Paul E. McKenney 2019-06-06 13:38 ` Herbert Xu 2019-06-06 13:38 ` Herbert Xu 2019-06-06 13:48 ` Paul E. McKenney 2019-06-06 8:16 ` Andrea Parri 2019-06-06 14:19 ` Alan Stern 2019-06-06 14:19 ` Alan Stern 2019-06-08 15:19 ` Paul E. McKenney 2019-06-08 15:56 ` Alan Stern 2019-06-08 15:56 ` Alan Stern 2019-06-08 16:31 ` Paul E. McKenney 2019-06-03 9:35 ` Paul E. McKenney 2019-06-06 8:38 ` Andrea Parri 2019-06-06 9:32 ` Herbert Xu 2019-06-06 9:32 ` Herbert Xu 2019-06-03 0:06 ` Paul E. McKenney 2019-06-03 3:03 ` Herbert Xu 2019-06-03 3:03 ` Herbert Xu 2019-06-03 9:27 ` Paul E. McKenney 2019-06-03 15:55 ` Linus Torvalds 2019-06-03 15:55 ` Linus Torvalds 2019-06-03 16:07 ` Linus Torvalds 2019-06-03 16:07 ` Linus Torvalds 2019-06-03 19:53 ` Paul E. McKenney 2019-06-03 20:24 ` Linus Torvalds 2019-06-03 20:24 ` Linus Torvalds 2019-06-04 21:14 ` Paul E. McKenney 2019-06-05 2:21 ` Herbert Xu 2019-06-05 2:21 ` Herbert Xu 2019-06-05 3:30 ` Paul E. McKenney 2019-06-06 4:37 ` Herbert Xu 2019-06-06 4:37 ` Herbert Xu
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