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
next prev 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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