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Date: Thu, 30 Sep 2021 21:30:55 -0400 From: Alan Stern <stern@...land.harvard.edu> To: Boqun Feng <boqun.feng@...il.com> Cc: "Paul E. McKenney" <paulmck@...nel.org>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, Dan Lustig <dlustig@...dia.com>, Will Deacon <will@...nel.org>, Peter Zijlstra <peterz@...radead.org>, Linus Torvalds <torvalds@...ux-foundation.org>, Alexander Shishkin <alexander.shishkin@...ux.intel.com>, Peter Anvin <hpa@...or.com>, Andrea Parri <parri.andrea@...il.com>, Ingo Molnar <mingo@...nel.org>, Vince Weaver <vincent.weaver@...ne.edu>, Thomas Gleixner <tglx@...utronix.de>, Jiri Olsa <jolsa@...hat.com>, Arnaldo Carvalho de Melo <acme@...hat.com>, Stephane Eranian <eranian@...gle.com>, palmer@...belt.com, paul.walmsley@...ive.com, mpe@...erman.id.au Subject: Re: [PATCH] tools/memory-model: Provide extra ordering for unlock+lock pair on the same CPU On Fri, Oct 01, 2021 at 08:12:56AM +0800, Boqun Feng wrote: > On Thu, Sep 30, 2021 at 04:46:34PM -0400, Alan Stern wrote: > > On Thu, Sep 30, 2021 at 11:17:53AM -0700, Paul E. McKenney wrote: > > > On Thu, Sep 30, 2021 at 11:20:33AM -0400, Alan Stern wrote: > > > > On Thu, Sep 30, 2021 at 09:08:23PM +0800, Boqun Feng wrote: > > > > > A recent discussion[1] shows that we are in favor of strengthening the > > > > > ordering of unlock + lock on the same CPU: a unlock and a po-after lock > > > > > should provide the so-called RCtso ordering, that is a memory access S > > > > > po-before the unlock should be ordered against a memory access R > > > > > po-after the lock, unless S is a store and R is a load. > > > > > > > > > > The strengthening meets programmers' expection that "sequence of two > > > > > locked regions to be ordered wrt each other" (from Linus), and can > > > > > reduce the mental burden when using locks. Therefore add it in LKMM. > > > > > > > > > > [1]: https://lore.kernel.org/lkml/20210909185937.GA12379@rowland.harvard.edu/ > > > > > > > > > > Co-developed-by: Alan Stern <stern@...land.harvard.edu> > > > > > Signed-off-by: Alan Stern <stern@...land.harvard.edu> > > > > > Signed-off-by: Boqun Feng <boqun.feng@...il.com> > > > > > --- > > > > > Alan, > > > > > > > > > > I added the "Co-developed-by" and "Signed-off-by" tags since most of the > > > > > work is done by you. Feel free to let me know if you want to change > > > > > anything. > > > > > > > > It looks good to me. However, do we really want to add these litmus > > > > tests to the kernel source, or would it be better to keep them with > > > > the thousands of other tests in Paul's archives? > > > > > > Either way works for me. But if they are referred to from within the > > > kernel, it is best to have them in the kernel source. Which might be seen > > > as a reason to minimize referring to litmus tests from the kernel. ;-) > > > > In this case the litmus tests are not referred to within the kernel > > source. > > > > I'm OK to drop the litmus tests, but the reason I add the two litmus > tests is that they directly describe one of our memory model rules. The > two litmus tests tells developers "you can use unlock+lock on the same > CPU to order READ->WRITE or WRITE->WRITE", so they are kind of part of > the manual of our memory model. Thoughts? The explanation.txt file already contains example litmus tests (not in a form acceptable to herd7, though) for these things. Alan
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