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From: Herbert Xu <herbert@gondor.apana.org.au>
To: "Paul E. McKenney" <paulmck@linux.ibm.com>
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 13:26:26 +0800
Message-ID: <20190603052626.nz2qktwmkswxfnsd@gondor.apana.org.au> (raw)
In-Reply-To: <20190603034707.GG28207@linux.ibm.com>

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.

> 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.

> 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();

Let's put them in because I think they are critical.  smp_mb() also
carries with it a 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();

	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

  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 [this message]
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
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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