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From: Linus Torvalds <torvalds@linux-foundation.org>
To: "Paul E. McKenney" <paulmck@linux.ibm.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>,
	Herbert Xu <herbert@gondor.apana.org.au>,
	Alan Stern <stern@rowland.harvard.edu>,
	Boqun Feng <boqun.feng@gmail.com>,
	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: inet: frags: Turn fqdir->dead into an int for old Alphas
Date: Sat, 8 Jun 2019 10:42:41 -0700
Message-ID: <CAHk-=wj1G9nXMzAu=Ldbd4_bbzVtWgNORDKMD4bKTO6dRrMPmQ@mail.gmail.com> (raw)
In-Reply-To: <20190608152707.GF28207@linux.ibm.com>

On Sat, Jun 8, 2019 at 8:32 AM Paul E. McKenney <paulmck@linux.ibm.com> wrote:
>
> On Fri, Jun 07, 2019 at 09:19:42AM -0700, Linus Torvalds wrote:
> >
> >  - bitfields obviously do need locks. 'char' does not.
> >
> > If there's somebody who really notices the alpha issue in PRACTICE, we
> > can then bother to fix it. But there is approximately one user, and
> > it's not a heavy-duty one.
>
> C11 and later compilers are supposed to use read-modify-write atomic
> operations in this sort of situation anyway because they are not supposed
> to introduce data races.

I don't think that's possible on any common architecture. The
bitfields themselves will need locking, to serialize writes of
different fields against each other.

There are no atomic rmw sequences that have reasonable performance for
the bitfield updates themselves.

The fields *around* the bitfields had better be safe, but that's
something we already depend on, and which falls under the heading of
"we don't accept garbage compilers".

              Linus

  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 [this message]
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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