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From: "Paul E. McKenney" <paulmck@linux.ibm.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
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 11:14:31 -0700
Message-ID: <20190608181431.GL28207@linux.ibm.com> (raw)
In-Reply-To: <CAHk-=wj1G9nXMzAu=Ldbd4_bbzVtWgNORDKMD4bKTO6dRrMPmQ@mail.gmail.com>

On Sat, Jun 08, 2019 at 10:42:41AM -0700, Linus Torvalds wrote:
> 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.

Apologies, I should have explicitly stated that I was talking about char
stores, not bitfield stores.  And last I checked, the C11 standard's
prohibition against data races did not extend to individual fields within
a bitfield.  So, yes, for bitfields, the programmer must use a lock or
similar if it is necessary for updates to fields within a bitfield to
be atomic.

> 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.

Yes, and again the C standard doesn't make any atomicity guarantees
regarding storing to different fields within a bitfield.  The compiler is
free to assume that nothing else is happening anywhere in the bitfield
when storing to a field within that bitfield.  Which gets back to your
"bitfields obviously do need locks", and it is of course the developer
(not the compiler) who must supply those locks.  Plus a given lock must
cover the entire bitfield -- having one lock for half the fields within
a given bitfield and another lock for the other half will break.

Switching from bitfields to char, the C standard -does- require that
storing to one char must avoid even momentary corruption of adjacent
char, so given an old Alpha the compiler would need to use something
like an LL/SC loop.  If it fails to do so, that compiler is failing to
comply with the standard.

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

Agreed, in the general case.  In a few specific special cases, we do
sometimes hand-craft bitfields using shifts and masks, and sometimes
we use atomic RMW operations to update them.  I suppose we could use
unions as an alternative, but it is not clear to me that this would
help anything.

> 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".

And the C standard does require the compiler to make that guarantee, so
for once the standard is even on our side.  ;-)

							Thanx, Paul


  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 [this message]
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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