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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 12:17:31 +0800
Message-ID: <20190603041731.m6rvcaivgeh4iw4g@gondor.apana.org.au> (raw)
In-Reply-To: <20190603040114.st646bujtgyu7adn@gondor.apana.org.au>

On Mon, Jun 03, 2019 at 12:01:14PM +0800, Herbert Xu wrote:
> On Sun, Jun 02, 2019 at 08:47:07PM -0700, Paul E. McKenney wrote:
> >
> > 	CPU2:         if (b != 1)
> > 	CPU2:                 b = 1;
> 
> Stop right there.  The kernel is full of code that assumes that
> assignment to an int/long is atomic.  If your compiler breaks this
> assumption that we can kiss the kernel good-bye.

The slippery slope apparently started here:

: commit ea435467500612636f8f4fb639ff6e76b2496e4b
: Author: Matthew Wilcox <matthew@wil.cx>
: Date:   Tue Jan 6 14:40:39 2009 -0800
: 
:     atomic_t: unify all arch definitions
:
: diff --git a/arch/x86/include/asm/atomic_32.h b/arch/x86/include/asm/atomic_32.h
: index ad5b9f6ecddf..85b46fba4229 100644
: --- a/arch/x86/include/asm/atomic_32.h
: +++ b/arch/x86/include/asm/atomic_32.h
: ...
: @@ -10,15 +11,6 @@
:   * resource counting etc..
:   */
:
: -/*
: - * Make sure gcc doesn't try to be clever and move things around
: - * on us. We need to use _exactly_ the address the user gave us,
: - * not some alias that contains the same information.
: - */
: -typedef struct {
: -       int counter;
: -} atomic_t;
:
: diff --git a/include/linux/types.h b/include/linux/types.h
: index 121f349cb7ec..3b864f2d9560 100644
: --- a/include/linux/types.h
: +++ b/include/linux/types.h
: @@ -195,6 +195,16 @@ typedef u32 phys_addr_t;
:  
:  typedef phys_addr_t resource_size_t;
:
: +typedef struct {
: +       volatile int counter;
: +} atomic_t;
: +

Before evolving into the READ_ONCE/WRITE_ONCE that we have now.

Linus, are we now really supporting a compiler where an assignment
(or a read) from an int/long/pointer can be non-atomic without the
volatile marker? Because if that's the case then we have a lot of
code to audit.

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

  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 [this message]
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
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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