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 11:50:19 -0700 [thread overview]
Message-ID: <20190608185019.GM28207@linux.ibm.com> (raw)
In-Reply-To: <CAHk-=wiRduKzoLpAwU7iFiOJ6DX7RE+PZ_wFi9Cvq=hDoaNsPA@mail.gmail.com>
On Sat, Jun 08, 2019 at 10:50:51AM -0700, Linus Torvalds wrote:
> On Sat, Jun 8, 2019 at 10:42 AM Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> >
> > There are no atomic rmw sequences that have reasonable performance for
> > the bitfield updates themselves.
>
> Note that this is purely about the writing side. Reads of bitfield
> values can be (and generally _should_ be) atomic, and hopefully C11
> means that you wouldn't see intermediate values.
>
> But I'm not convinced about that either: one natural way to update a
> bitfield is to first do the masking, and then do the insertion of new
> bits, so a bitfield assignment very easily exposes non-real values to
> a concurrent read on another CPU.
Agreed on the "not convinced" part (though perhaps most implementations
would handle concurrent reads and writes involving different fields of
the same bitfield). And the C standard does not guarantee this, because
data races are defined in terms of memory locations. So as far as the
C standard is concerned, if there are two concurrent accesses to fields
within a bitfield that are not separated by ":0", there is a data race
and so the compiler can do whatever it wants.
But do we really care about this case?
> What I think C11 is supposed to protect is from compilers doing
> horribly bad things, and accessing bitfields with bigger types than
> the field itself, ie when you have
>
> struct {
> char c;
> int field1:5;
> };
>
> then a write to "field1" had better not touch "char c" as part of the
> rmw operation, because that would indeed introduce a data-race with a
> completely independent field that might have completely independent
> locking rules.
>
> But
>
> struct {
> int c:8;
> int field1:5;
> };
>
> would not sanely have the same guarantees, even if the layout in
> memory might be identical. Once you have bitfields next to each other,
> and use a base type that means they can be combined together, they
> can't be sanely modified without locking.
>
> (And I don't know if C11 took up the "base type of the bitfield"
> thing. Maybe you still need to use the ":0" thing to force alignment,
> and maybe the C standards people still haven't made the underlying
> type be meaningful other than for sign handling).
The C standard draft (n2310) gives similar examples:
EXAMPLE A structure declared as
struct {
char a;
int b:5, c:11,:0, d:8;
struct { int ee:8; } e;
}
contains four separate memory locations: The member a, and
bit-fields d and e.ee are each separate memory locations,
and can be modified concurrently without interfering with each
other. The bit-fields b and c together constitute the fourth
memory location. The bit-fields b and c cannot be concurrently
modified, but b and a, for example, can be.
So yes, ":0" still forces alignment to the next storage unit. And it
can be used to allow concurrent accesses to fields within a bitfield,
but only when those two fields are separated by ":0".
On the underlying type, according to J.3.9 of the current C working draft,
the following are implementation-specified behavior:
- Whether a "plain" int bit-field is treated as a signed int
bit-field or as an unsigned int bit-field (6.7.2, 6.7.2.1).
- Whether atomic types are permitted for bit-fields (6.7.2.1).
This last is strange because you are not allowed to take the address of
a bit field, and the various operations on atomic types take addresses.
Search me!
Thanx, Paul
next prev parent reply other threads:[~2019-06-08 18:51 UTC|newest]
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 [this message]
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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