[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-Id: <20F4C097-24B4-416B-95EE-AC11F5952B44@in.tum.de>
Date: Fri, 15 Jul 2022 14:27:28 +0200
From: Paul Heidekrüger <Paul.Heidekrueger@...tum.de>
To: Alan Stern <stern@...land.harvard.edu>
Cc: clang-built-linux <llvm@...ts.linux.dev>,
linux-toolchains@...r.kernel.org,
Andrea Parri <parri.andrea@...il.com>,
Will Deacon <will@...nel.org>,
Peter Zijlstra <peterz@...radead.org>,
Boqun Feng <boqun.feng@...il.com>,
Nicholas Piggin <npiggin@...il.com>,
David Howells <dhowells@...hat.com>,
Jade Alglave <j.alglave@....ac.uk>,
Luc Maranget <luc.maranget@...ia.fr>,
"Paul E. McKenney" <paulmck@...nel.org>,
Akira Yokosawa <akiyks@...il.com>,
Daniel Lustig <dlustig@...dia.com>,
Joel Fernandes <joel@...lfernandes.org>,
Nathan Chancellor <nathan@...nel.org>,
Nick Desaulniers <ndesaulniers@...gle.com>,
Tom Rix <trix@...hat.com>, Palmer Dabbelt <palmer@...belt.com>,
LKML <linux-kernel@...r.kernel.org>,
linux-arch <linux-arch@...r.kernel.org>,
Marco Elver <elver@...gle.com>,
Charalampos Mainas <charalampos.mainas@...il.com>,
Pramod Bhatotia <pramod.bhatotia@...tum.de>,
Soham Chakraborty <s.s.chakraborty@...elft.nl>,
Martin Fink <martin.fink@...tum.de>
Subject: Re: [PATCH RFC] tools/memory-model: Adjust ctrl dependency definition
Alan Stern <stern@...land.harvard.edu> wrote:
> On Mon, Jun 27, 2022 at 11:47:43AM +0200, Paul Heidekrüger wrote:
>>> On 21. Jun 2022, at 16:24, Alan Stern <stern@...land.harvard.edu> wrote:
>>>
>>> On Tue, Jun 21, 2022 at 01:59:27PM +0200, Paul Heidekrüger wrote:
>>>> OK. So, LKMM limits the scope of control dependencies to its arm(s), hence
>>>> there is a control dependency from the last READ_ONCE() before the loop
>>>> exists to the WRITE_ONCE().
>>>>
>>>> But then what about the following:
>>>>
>>>>> int *x, *y;
>>>>>
>>>>> int foo()
>>>>> {
>>>>> /* More code */
>>>>>
>>>>> if(READ_ONCE(x))
>>>>> return 42;
>>>>>
>>>>> /* More code */
>>>>>
>>>>> WRITE_ONCE(y, 42);
>>>>>
>>>>> /* More code */
>>>>>
>>>>> return 0;
>>>>> }
>>>>
>>>> The READ_ONCE() determines whether the WRITE_ONCE() will be executed at all,
>>>> but the WRITE_ONCE() doesn't lie in the if condition's arm.
>>>
>>> So in this case the LKMM would not recognize that there's a control
>>> dependency, even though it clearly exists.
>>
>> Oh, that's unfortunate.
>>
>> Then I would still argue that the "at all" definition is misleading. This
>
> I agree, and I would welcome a patch improving the definition. Perhaps
> something along the lines of what I wrote earlier in this email thread.
I have just been thinking about how to word this patch; am I correct in
assuming that the LKMM does not deal with loop conditions? Or in other
words, there is no way for a loop condition to impose a ctrl dependency on
any WRITE_ONCE's in the loop body? It are only if and switch statements the
LKMM is concerned with in the case of ctrl dependencies?
Many thanks,
Paul
>> time in the other direction as I had initially proposed though, as the above
>> example is a case where "at all" holds true, but LKMM doesn't cover it. Or
>> do you think that caveating this in litmus-tests.txt, e.g. via the patch we
>> had recently worked out [1], is enough?
>
> No, the explanation should be improved.
>
> Alan
Powered by blists - more mailing lists