From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Peter Sewell <Peter.Sewell@cl.cam.ac.uk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
Torvald Riegel <triegel@redhat.com>,
Alec Teal <a.teal@warwick.ac.uk>,
Will Deacon <will.deacon@arm.com>,
Peter Zijlstra <peterz@infradead.org>,
Ramana Radhakrishnan <Ramana.Radhakrishnan@arm.com>,
David Howells <dhowells@redhat.com>,
"linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"akpm@linux-foundation.org" <akpm@linux-foundation.org>,
"mingo@kernel.org" <mingo@kernel.org>,
"gcc@gcc.gnu.org" <gcc@gcc.gnu.org>
Subject: Re: [RFC][PATCH 0/5] arch: atomic rework
Date: Tue, 18 Feb 2014 11:42:18 -0800 [thread overview]
Message-ID: <20140218194218.GU4250@linux.vnet.ibm.com> (raw)
In-Reply-To: <CAHWkzRSXxAPShfFTAQj=f5M1L4K1E5D6yLmQS72-b1jNGTPpEA@mail.gmail.com>
On Tue, Feb 18, 2014 at 06:23:47PM +0000, Peter Sewell wrote:
> On 18 February 2014 17:16, Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:
> > On Tue, Feb 18, 2014 at 08:49:13AM -0800, Linus Torvalds wrote:
> >> On Tue, Feb 18, 2014 at 7:31 AM, Torvald Riegel <triegel@redhat.com> wrote:
> >> > On Mon, 2014-02-17 at 16:05 -0800, Linus Torvalds wrote:
> >> >> And exactly because I know enough, I would *really* like atomics to be
> >> >> well-defined, and have very clear - and *local* - rules about how they
> >> >> can be combined and optimized.
> >> >
> >> > "Local"?
> >>
> >> Yes.
> >>
> >> So I think that one of the big advantages of atomics over volatile is
> >> that they *can* be optimized, and as such I'm not at all against
> >> trying to generate much better code than for volatile accesses.
> >>
> >> But at the same time, that can go too far. For example, one of the
> >> things we'd want to use atomics for is page table accesses, where it
> >> is very important that we don't generate multiple accesses to the
> >> values, because parts of the values can be change *by*hardware* (ie
> >> accessed and dirty bits).
> >>
> >> So imagine that you have some clever global optimizer that sees that
> >> the program never ever actually sets the dirty bit at all in any
> >> thread, and then uses that kind of non-local knowledge to make
> >> optimization decisions. THAT WOULD BE BAD.
> >
> > Might as well list other reasons why value proofs via whole-program
> > analysis are unreliable for the Linux kernel:
> >
> > 1. As Linus said, changes from hardware.
> >
> > 2. Assembly code that is not visible to the compiler.
> > Inline asms will -normally- let the compiler know what
> > memory they change, but some just use the "memory" tag.
> > Worse yet, I suspect that most compilers don't look all
> > that carefully at .S files.
> >
> > Any number of other programs contain assembly files.
> >
> > 3. Kernel modules that have not yet been written. Now, the
> > compiler could refrain from trying to prove anything about
> > an EXPORT_SYMBOL() or EXPORT_SYMBOL_GPL() variable, but there
> > is currently no way to communicate this information to the
> > compiler other than marking the variable "volatile".
> >
> > Other programs have similar issues, e.g., via dlopen().
> >
> > 4. Some drivers allow user-mode code to mmap() some of their
> > state. Any changes undertaken by the user-mode code would
> > be invisible to the compiler.
> >
> > 5. JITed code produced based on BPF: https://lwn.net/Articles/437981/
> >
> > And probably other stuff as well.
>
> interesting list. So are you saying that value-range-analysis and
> such-like (I say glibly, without really knowing what "such-like"
> refers to here) are fundamentally incompatible with
> the kernel code, or can you think of some way to tell the compiler a
> bound on the footprint of the "unseen" changes in each of those cases?
Other than the "volatile" keyword, no.
Well, I suppose you could also create a function that changed the
variables in question, then arrange to never call it, but in such a way
that the compiler could not prove that it was never called. But ouch!
Thanx, Paul
next prev parent reply other threads:[~2014-02-18 19:42 UTC|newest]
Thread overview: 299+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-02-06 13:48 [RFC][PATCH 0/5] arch: atomic rework Peter Zijlstra
2014-02-06 13:48 ` [RFC][PATCH 1/5] ia64: Fix up smp_mb__{before,after}_clear_bit Peter Zijlstra
2014-02-06 13:48 ` [RFC][PATCH 2/5] arc,hexagon: Delete asm/barrier.h Peter Zijlstra
2014-02-06 13:48 ` [RFC][PATCH 3/5] arch: s/smp_mb__(before|after)_(atomic|clear)_(dec,inc,bit)/smp_mb__\1/g Peter Zijlstra
2014-02-06 19:12 ` Paul E. McKenney
2014-02-07 9:52 ` Will Deacon
2014-02-06 13:48 ` [RFC][PATCH 4/5] arch: Generic atomic.h cleanup Peter Zijlstra
2014-02-06 17:49 ` Will Deacon
2014-02-06 13:48 ` [RFC][PATCH 5/5] arch: Sanitize atomic_t bitwise ops Peter Zijlstra
2014-02-06 14:43 ` Geert Uytterhoeven
2014-02-06 16:14 ` Peter Zijlstra
2014-02-06 16:53 ` Linus Torvalds
2014-02-06 17:52 ` Peter Zijlstra
2014-02-06 17:56 ` Linus Torvalds
2014-02-06 18:09 ` Peter Zijlstra
2014-02-06 18:25 ` [RFC][PATCH 0/5] arch: atomic rework David Howells
2014-02-06 18:30 ` Peter Zijlstra
2014-02-06 18:42 ` Paul E. McKenney
2014-02-06 18:55 ` Ramana Radhakrishnan
2014-02-06 18:59 ` Will Deacon
2014-02-06 19:27 ` Paul E. McKenney
2014-02-06 21:17 ` Torvald Riegel
2014-02-06 22:11 ` Paul E. McKenney
2014-02-06 23:44 ` Torvald Riegel
2014-02-07 4:20 ` Paul E. McKenney
2014-02-07 7:44 ` Peter Zijlstra
2014-02-07 16:50 ` Paul E. McKenney
2014-02-07 16:55 ` Will Deacon
2014-02-07 17:06 ` Peter Zijlstra
2014-02-07 17:13 ` Will Deacon
2014-02-07 17:20 ` Peter Zijlstra
2014-02-07 18:03 ` Paul E. McKenney
2014-02-07 17:46 ` Joseph S. Myers
2014-02-07 18:43 ` Torvald Riegel
2014-02-07 18:02 ` Paul E. McKenney
2014-02-10 0:27 ` Torvald Riegel
2014-02-10 0:56 ` Linus Torvalds
2014-02-10 1:16 ` Torvald Riegel
2014-02-10 1:24 ` Linus Torvalds
2014-02-10 1:46 ` Torvald Riegel
2014-02-10 2:04 ` Linus Torvalds
2014-02-10 3:21 ` Paul E. McKenney
2014-02-10 3:45 ` Paul E. McKenney
2014-02-10 11:46 ` Peter Zijlstra
2014-02-10 19:09 ` Linus Torvalds
2014-02-11 15:59 ` Paul E. McKenney
2014-02-12 6:06 ` Torvald Riegel
2014-02-12 9:19 ` Peter Zijlstra
2014-02-12 17:42 ` Paul E. McKenney
2014-02-12 18:12 ` Peter Zijlstra
2014-02-17 18:18 ` Paul E. McKenney
2014-02-17 20:39 ` Richard Biener
2014-02-17 22:14 ` Paul E. McKenney
2014-02-17 22:27 ` Torvald Riegel
2014-02-14 5:07 ` Torvald Riegel
2014-02-14 9:50 ` Peter Zijlstra
2014-02-14 19:19 ` Torvald Riegel
2014-02-12 17:39 ` Paul E. McKenney
2014-02-12 5:39 ` Torvald Riegel
2014-02-12 18:07 ` Paul E. McKenney
2014-02-12 20:22 ` Linus Torvalds
2014-02-13 0:23 ` Paul E. McKenney
2014-02-13 20:03 ` Torvald Riegel
2014-02-14 2:01 ` Paul E. McKenney
2014-02-14 4:43 ` Torvald Riegel
2014-02-14 17:29 ` Paul E. McKenney
2014-02-14 19:21 ` Torvald Riegel
2014-02-14 19:50 ` Linus Torvalds
2014-02-14 20:02 ` Linus Torvalds
2014-02-15 2:08 ` Paul E. McKenney
2014-02-15 2:44 ` Linus Torvalds
2014-02-15 2:48 ` Linus Torvalds
2014-02-15 6:35 ` Paul E. McKenney
2014-02-15 6:58 ` Paul E. McKenney
2014-02-15 18:07 ` Torvald Riegel
2014-02-17 18:59 ` Joseph S. Myers
2014-02-17 19:19 ` Will Deacon
2014-02-17 19:41 ` Torvald Riegel
2014-02-17 23:12 ` Joseph S. Myers
2014-02-15 17:45 ` Torvald Riegel
2014-02-15 18:49 ` Linus Torvalds
2014-02-17 19:55 ` Torvald Riegel
2014-02-17 20:18 ` Linus Torvalds
2014-02-17 21:21 ` Torvald Riegel
2014-02-17 22:02 ` Linus Torvalds
2014-02-17 22:25 ` Torvald Riegel
2014-02-17 22:47 ` Linus Torvalds
2014-02-17 23:41 ` Torvald Riegel
2014-02-18 0:18 ` Linus Torvalds
2014-02-18 1:26 ` Paul E. McKenney
2014-02-18 15:38 ` Torvald Riegel
2014-02-18 16:55 ` Paul E. McKenney
2014-02-18 19:57 ` Torvald Riegel
2014-02-17 23:10 ` Alec Teal
2014-02-18 0:05 ` Linus Torvalds
2014-02-18 15:31 ` Torvald Riegel
2014-02-18 16:49 ` Linus Torvalds
2014-02-18 17:16 ` Paul E. McKenney
2014-02-18 18:23 ` Peter Sewell
2014-02-18 19:00 ` Linus Torvalds
2014-02-18 19:42 ` Paul E. McKenney [this message]
2014-02-18 21:40 ` Torvald Riegel
2014-02-18 21:52 ` Peter Zijlstra
2014-02-19 9:52 ` Torvald Riegel
2014-02-18 22:58 ` Paul E. McKenney
2014-02-19 10:59 ` Torvald Riegel
2014-02-19 15:14 ` Paul E. McKenney
2014-02-19 17:55 ` Torvald Riegel
2014-02-19 22:12 ` Paul E. McKenney
2014-02-18 21:21 ` Torvald Riegel
2014-02-18 21:40 ` Peter Zijlstra
2014-02-18 21:47 ` Torvald Riegel
2014-02-19 15:23 ` David Lang
2014-02-19 18:11 ` Torvald Riegel
2014-02-18 21:47 ` Peter Zijlstra
2014-02-19 11:07 ` Torvald Riegel
2014-02-19 11:42 ` Peter Zijlstra
2014-02-18 22:14 ` Linus Torvalds
2014-02-19 14:40 ` Torvald Riegel
2014-02-19 19:49 ` Linus Torvalds
2014-02-18 3:00 ` Paul E. McKenney
2014-02-18 3:24 ` Linus Torvalds
2014-02-18 3:42 ` Linus Torvalds
2014-02-18 5:22 ` Paul E. McKenney
2014-02-18 16:17 ` Torvald Riegel
2014-02-18 17:44 ` Linus Torvalds
2014-02-18 19:40 ` Paul E. McKenney
2014-02-18 19:47 ` Torvald Riegel
2014-02-20 0:53 ` Linus Torvalds
2014-02-20 4:01 ` Paul E. McKenney
2014-02-20 4:43 ` Linus Torvalds
2014-02-20 8:30 ` Paul E. McKenney
2014-02-20 9:20 ` Paul E. McKenney
2014-02-20 17:01 ` Linus Torvalds
2014-02-20 18:11 ` Paul E. McKenney
2014-02-20 18:32 ` Linus Torvalds
2014-02-20 18:53 ` Torvald Riegel
2014-02-20 19:09 ` Linus Torvalds
2014-02-22 18:53 ` Torvald Riegel
2014-02-22 21:53 ` Linus Torvalds
2014-02-23 0:39 ` Paul E. McKenney
2014-02-23 3:50 ` Linus Torvalds
2014-02-23 6:34 ` Paul E. McKenney
2014-02-23 19:31 ` Linus Torvalds
2014-02-24 1:16 ` Paul E. McKenney
2014-02-24 1:35 ` Linus Torvalds
2014-02-24 4:59 ` Paul E. McKenney
2014-02-24 5:25 ` Linus Torvalds
2014-02-24 15:57 ` Linus Torvalds
2014-02-24 16:27 ` Richard Biener
2014-02-24 16:37 ` Linus Torvalds
2014-02-24 16:40 ` Linus Torvalds
2014-02-24 16:55 ` Michael Matz
2014-02-24 17:28 ` Paul E. McKenney
2014-02-24 17:57 ` Paul E. McKenney
2014-02-26 17:39 ` Torvald Riegel
2014-02-24 17:38 ` Linus Torvalds
2014-02-24 18:12 ` Paul E. McKenney
2014-02-26 17:34 ` Torvald Riegel
2014-02-24 17:21 ` Paul E. McKenney
2014-02-24 18:14 ` Linus Torvalds
2014-02-24 18:53 ` Paul E. McKenney
2014-02-24 19:54 ` Linus Torvalds
2014-02-24 22:37 ` Paul E. McKenney
2014-02-24 23:35 ` Linus Torvalds
2014-02-25 6:00 ` Paul E. McKenney
2014-02-26 1:47 ` Linus Torvalds
2014-02-26 5:12 ` Paul E. McKenney
2014-02-25 6:05 ` Linus Torvalds
2014-02-26 0:15 ` Paul E. McKenney
2014-02-26 3:32 ` Jeff Law
2014-02-26 5:23 ` Paul E. McKenney
2014-02-27 15:37 ` Torvald Riegel
2014-02-27 17:01 ` Linus Torvalds
2014-02-27 19:06 ` Paul E. McKenney
2014-02-27 19:47 ` Linus Torvalds
2014-02-27 20:53 ` Paul E. McKenney
2014-03-01 0:50 ` Paul E. McKenney
2014-03-01 10:06 ` Peter Sewell
2014-03-01 14:03 ` Paul E. McKenney
2014-03-02 10:05 ` Peter Sewell
2014-03-02 23:20 ` Paul E. McKenney
2014-03-02 23:44 ` Peter Sewell
2014-03-03 4:25 ` Paul E. McKenney
2014-03-03 20:44 ` Torvald Riegel
2014-03-04 22:11 ` Peter Sewell
2014-03-05 17:15 ` Torvald Riegel
2014-03-05 18:37 ` Peter Sewell
2014-03-03 18:55 ` Torvald Riegel
2014-03-03 19:20 ` Paul E. McKenney
2014-03-03 20:46 ` Torvald Riegel
2014-03-04 19:00 ` Paul E. McKenney
2014-03-04 21:35 ` Paul E. McKenney
2014-03-05 16:54 ` Torvald Riegel
2014-03-05 18:15 ` Paul E. McKenney
2014-03-07 18:33 ` Torvald Riegel
2014-03-07 19:11 ` Paul E. McKenney
2014-03-05 16:26 ` Torvald Riegel
2014-03-05 18:01 ` Paul E. McKenney
2014-03-07 17:45 ` Torvald Riegel
2014-03-07 19:02 ` Paul E. McKenney
2014-03-03 18:59 ` Torvald Riegel
2014-03-03 15:36 ` Torvald Riegel
2014-02-27 17:50 ` Paul E. McKenney
2014-02-27 19:22 ` Paul E. McKenney
2014-02-28 1:02 ` Paul E. McKenney
2014-03-03 19:01 ` Torvald Riegel
2014-02-20 18:56 ` Paul E. McKenney
2014-02-20 19:45 ` Linus Torvalds
2014-02-20 22:10 ` Paul E. McKenney
2014-02-20 22:52 ` Linus Torvalds
2014-02-21 18:35 ` Michael Matz
2014-02-21 19:13 ` Paul E. McKenney
2014-02-21 22:10 ` Joseph S. Myers
2014-02-21 22:37 ` Paul E. McKenney
2014-02-26 13:09 ` Torvald Riegel
2014-02-26 18:43 ` Joseph S. Myers
2014-02-27 0:52 ` Torvald Riegel
2014-02-24 13:55 ` Michael Matz
2014-02-24 17:40 ` Paul E. McKenney
2014-02-26 13:04 ` Torvald Riegel
2014-02-26 18:27 ` Paul E. McKenney
2014-02-20 18:44 ` Torvald Riegel
2014-02-20 18:56 ` Paul E. McKenney
2014-02-20 18:23 ` Torvald Riegel
[not found] ` <CAHWkzRQZ8+gOGMFNyTKjFNzpUv6d_J1G9KL0x_iCa=YCgvEojQ@mail.gmail.com>
2014-02-21 19:16 ` Linus Torvalds
2014-02-21 19:41 ` Linus Torvalds
2014-02-21 19:48 ` Peter Sewell
[not found] ` <CAHWkzRSO82jU-9dtTEjHaW2FeLcEqdZXxp5Q8cmVTTT9uhZQYw@mail.gmail.com>
2014-02-21 20:22 ` Linus Torvalds
[not found] ` <CAHWkzRRxqhH+DnuQHu9bM4ywGBen3oqtT8W4Xqt1CFAHy2WQRg@mail.gmail.com>
2014-02-21 19:24 ` Paul E. McKenney
2014-02-20 17:54 ` Torvald Riegel
2014-02-20 18:11 ` Paul E. McKenney
2014-02-20 17:49 ` Torvald Riegel
2014-02-20 18:25 ` Linus Torvalds
2014-02-20 19:02 ` Linus Torvalds
2014-02-20 19:06 ` Linus Torvalds
2014-02-20 17:26 ` Torvald Riegel
2014-02-20 18:18 ` Paul E. McKenney
2014-02-22 18:30 ` Torvald Riegel
2014-02-22 20:17 ` Paul E. McKenney
2014-02-20 17:14 ` Torvald Riegel
2014-02-20 17:34 ` Linus Torvalds
2014-02-20 18:12 ` Torvald Riegel
2014-02-20 18:26 ` Paul E. McKenney
2014-02-18 5:01 ` Paul E. McKenney
2014-02-18 15:56 ` Torvald Riegel
2014-02-18 16:51 ` Paul E. McKenney
2014-02-17 20:23 ` Paul E. McKenney
2014-02-17 21:05 ` Torvald Riegel
2014-02-15 17:30 ` Torvald Riegel
2014-02-15 19:15 ` Linus Torvalds
2014-02-17 22:09 ` Torvald Riegel
2014-02-17 22:32 ` Linus Torvalds
2014-02-17 23:17 ` Torvald Riegel
2014-02-18 0:09 ` Linus Torvalds
2014-02-18 15:46 ` Torvald Riegel
2014-02-10 11:48 ` Peter Zijlstra
2014-02-10 11:49 ` Will Deacon
2014-02-10 12:05 ` Peter Zijlstra
2014-02-10 15:04 ` Paul E. McKenney
2014-02-10 16:22 ` Will Deacon
2014-02-07 18:44 ` Torvald Riegel
2014-02-10 0:06 ` Torvald Riegel
2014-02-10 3:51 ` Paul E. McKenney
2014-02-12 5:13 ` Torvald Riegel
2014-02-12 18:26 ` Paul E. McKenney
2014-02-06 21:09 ` Torvald Riegel
2014-02-06 21:55 ` Paul E. McKenney
2014-02-06 22:58 ` Torvald Riegel
2014-02-07 4:06 ` Paul E. McKenney
2014-02-07 9:13 ` Torvald Riegel
2014-02-07 16:44 ` Paul E. McKenney
2014-02-06 22:13 ` Joseph S. Myers
2014-02-06 23:25 ` Torvald Riegel
2014-02-06 23:33 ` Joseph S. Myers
2014-02-07 12:01 ` Will Deacon
2014-02-07 16:47 ` Paul E. McKenney
2014-02-06 19:21 ` Linus Torvalds
[not found] ` <52F93B7C.2090304@tilera.com>
[not found] ` <20140210205719.GY5002@laptop.programming.kicks-ass.net>
2014-02-10 21:08 ` Chris Metcalf
2014-02-10 21:14 ` Peter Zijlstra
2014-02-18 12:12 Peter Sewell
2014-02-18 12:53 ` Peter Zijlstra
2014-02-18 16:08 ` Peter Sewell
2014-02-18 14:56 ` Paul E. McKenney
2014-02-18 15:16 ` Mark Batty
2014-02-18 17:17 ` Paul E. McKenney
2014-02-18 15:33 ` Peter Sewell
2014-02-18 16:47 ` Paul E. McKenney
2014-02-18 17:38 ` Linus Torvalds
2014-02-18 18:21 ` Peter Sewell
2014-02-18 18:49 ` Linus Torvalds
2014-02-18 19:47 ` Paul E. McKenney
2014-02-18 20:46 ` Torvald Riegel
2014-02-18 20:43 ` Torvald Riegel
2014-02-18 21:29 ` Paul E. McKenney
2014-02-18 23:48 ` Peter Sewell
2014-02-19 9:46 ` Torvald Riegel
2014-02-26 3:06 George Spelvin
2014-02-26 5:22 ` Paul E. McKenney
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