From: "Paul E. McKenney" <paulmck@linux.ibm.com> To: Herbert Xu <herbert@gondor.apana.org.au> Cc: Linus Torvalds <torvalds@linux-foundation.org>, 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> Subject: Re: rcu_read_lock lost its compiler barrier Date: Mon, 3 Jun 2019 02:35:28 -0700 Message-ID: <20190603093528.GJ28207@linux.ibm.com> (raw) In-Reply-To: <20190603052626.nz2qktwmkswxfnsd@gondor.apana.org.au> On Mon, Jun 03, 2019 at 01:26:26PM +0800, Herbert Xu wrote: > On Sun, Jun 02, 2019 at 08:47:07PM -0700, Paul E. McKenney wrote: > > > > 1. These guarantees are of full memory barriers, -not- compiler > > barriers. > > What I'm saying is that wherever they are, they must come with > compiler barriers. I'm not aware of any synchronisation mechanism > in the kernel that gives a memory barrier without a compiler barrier. Yes, if a given synchronization mechanism requires that memory references need to be ordered, both the compiler and the CPU must maintain that ordering. > > 2. These rules don't say exactly where these full memory barriers > > go. SRCU is at one extreme, placing those full barriers in > > srcu_read_lock() and srcu_read_unlock(), and !PREEMPT Tree RCU > > at the other, placing these barriers entirely within the callback > > queueing/invocation, grace-period computation, and the scheduler. > > Preemptible Tree RCU is in the middle, with rcu_read_unlock() > > sometimes including a full memory barrier, but other times with > > the full memory barrier being confined as it is with !PREEMPT > > Tree RCU. > > The rules do say that the (full) memory barrier must precede any > RCU read-side that occur after the synchronize_rcu and after the > end of any RCU read-side that occur before the synchronize_rcu. > > All I'm arguing is that wherever that full mb is, as long as it > also carries with it a barrier() (which it must do if it's done > using an existing kernel mb/locking primitive), then we're fine. Fair enough, and smp_mb() does provide what is needed. > > Interleaving and inserting full memory barriers as per the rules above: > > > > CPU1: WRITE_ONCE(a, 1) > > CPU1: synchronize_rcu > > /* Could put a full memory barrier here, but it wouldn't help. */ > > CPU1: smp_mb(); > CPU2: smp_mb(); What is CPU2's smp_mb() ordering? In other words, what comment would you put on each of the above smp_mb() calls? > Let's put them in because I think they are critical. smp_mb() also > carries with it a barrier(). Again, agreed, smp_mb() implies barrier(). > > CPU2: rcu_read_lock(); > > CPU1: b = 2; > > CPU2: if (READ_ONCE(a) == 0) > > CPU2: if (b != 1) /* Weakly ordered CPU moved this up! */ > > CPU2: b = 1; > > CPU2: rcu_read_unlock > > > > In fact, CPU2's load from b might be moved up to race with CPU1's store, > > which (I believe) is why the model complains in this case. > > Let's put aside my doubt over how we're even allowing a compiler > to turn > > b = 1 > > into > > if (b != 1) > b = 1 > > Since you seem to be assuming that (a == 0) is true in this case > (as the assignment b = 1 is carried out), then because of the > presence of the full memory barrier, the RCU read-side section > must have started prior to the synchronize_rcu. This means that > synchronize_rcu is not allowed to return until at least the end > of the grace period, or at least until the end of rcu_read_unlock. > > So it actually should be: > > CPU1: WRITE_ONCE(a, 1) > CPU1: synchronize_rcu called > /* Could put a full memory barrier here, but it wouldn't help. */ > > CPU1: smp_mb(); > CPU2: smp_mb(); > > CPU2: grace period starts > ...time passes... > CPU2: rcu_read_lock(); > CPU2: if (READ_ONCE(a) == 0) > CPU2: if (b != 1) /* Weakly ordered CPU moved this up! */ > CPU2: b = 1; > CPU2: rcu_read_unlock > ...time passes... > CPU2: grace period ends > > /* This full memory barrier is also guaranteed by RCU. */ > CPU2: smp_mb(); But in this case, given that there are no more statements for CPU2, what is this smp_mb() ordering? Thanx, Paul > CPU1 synchronize_rcu returns > CPU1: b = 2; > > 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 index Thread overview: 62+ 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 10:25 ` Boqun Feng 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 2015-09-12 5:46 ` Boqun Feng 2015-09-21 19:30 ` Frederic Weisbecker 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 20:54 ` Linus Torvalds 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 4:17 ` 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 5:26 ` Herbert Xu 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 16:04 ` Linus Torvalds 2019-06-04 17:00 ` Alan Stern 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 15:26 ` Eric Dumazet 2019-06-07 15:32 ` Herbert Xu 2019-06-07 16:13 ` Eric Dumazet 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 17:50 ` 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 6:05 ` Paul E. McKenney 2019-06-06 6:14 ` Herbert Xu 2019-06-06 9:06 ` Paul E. McKenney 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 13:48 ` Paul E. McKenney 2019-06-06 8:16 ` Andrea Parri 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 16:31 ` Paul E. McKenney 2019-06-03 9:35 ` Paul E. McKenney [this message] 2019-06-06 8:38 ` Andrea Parri 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 9:27 ` Paul E. McKenney 2019-06-03 15:55 ` Linus Torvalds 2019-06-03 16:07 ` Linus Torvalds 2019-06-03 19:53 ` Paul E. McKenney 2019-06-03 20:24 ` Linus Torvalds 2019-06-04 21:14 ` Paul E. McKenney 2019-06-05 2:21 ` Herbert Xu 2019-06-05 3:30 ` Paul E. McKenney 2019-06-06 4:37 ` Herbert Xu
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