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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: Tue, 4 Jun 2019 20:30:39 -0700
Message-ID: <20190605033039.GY28207@linux.ibm.com> (raw)
In-Reply-To: <20190605022117.kw6tldcwhdkyqd6u@gondor.apana.org.au>

On Wed, Jun 05, 2019 at 10:21:17AM +0800, Herbert Xu wrote:
> On Tue, Jun 04, 2019 at 02:14:49PM -0700, Paul E. McKenney wrote:
> >
> > Yeah, I know, even with the "volatile" keyword, it is not entirely clear
> > how much reordering the compiler is allowed to do.  I was relying on
> > https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html, which says:
> 
> The volatile keyword doesn't give any guarantees of this kind.
> The key to ensuring ordering between unrelated variable/register
> reads/writes is the memory clobber:
> 
> 	6.47.2.6 Clobbers and Scratch Registers
> 
> 	...
> 
> 	"memory" The "memory" clobber tells the compiler that the assembly
> 	code performs memory reads or writes to items other than those
> 	listed in the input and output operands (for example, accessing
> 	the memory pointed to by one of the input parameters). To ensure
> 	memory contains correct values, GCC may need to flush specific
> 	register values to memory before executing the asm. Further,
> 	the compiler does not assume that any values read from memory
> 	before an asm remain unchanged after that asm; it reloads them as
> 	needed. Using the "memory" clobber effectively forms a read/write
> 	memory barrier for the compiler.
> 
> 	Note that this clobber does not prevent the processor from
> 	doing speculative reads past the asm statement. To prevent that,
> 	you need processor-specific fence instructions.
> 
> IOW you need a barrier().

Understood.  Does the patch I sent out a few hours ago cover it?  Or is
something else needed?

Other than updates to the RCU requirements documentation, which is
forthcoming.

							Thanx, Paul


  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
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 [this message]
2019-06-06  4:37                               ` Herbert Xu

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