Messages in this thread | | | Date | Sun, 26 Feb 2023 12:17:31 +0100 | Subject | Re: [PATCH v3] tools/memory-model: Make ppo a subrelation of po | From | Jonas Oberhauser <> |
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On 2/26/2023 4:30 AM, Alan Stern wrote: > On Sat, Feb 25, 2023 at 07:09:05PM -0800, Boqun Feng wrote: >> On Sat, Feb 25, 2023 at 09:29:51PM -0500, Alan Stern wrote: >>> On Sat, Feb 25, 2023 at 05:01:10PM -0800, Paul E. McKenney wrote: >>>> A few other oddities: >>>> >>>> litmus/auto/C-LB-Lww+R-OC.litmus >>>> >>>> Both versions flag a data race, which I am not seeing. It appears >>>> to me that P1's store to u0 cannot happen unless P0's store >>>> has completed. So what am I missing here? >>> The LKMM doesn't believe that a control or data dependency orders a >>> plain write after a marked read. Hence in this test it thinks that P1's >>> store to u0 can happen before the load of x1. I don't remember why we >>> did it this way -- probably we just wanted to minimize the restrictions >>> on when plain accesses can execute. (I do remember the reason for >>> making address dependencies induce order; it was so RCU would work.) >>> >> Because plain store can be optimzed as an "store only if not equal"? >> As the following sentenses in the explanations.txt: >> >> The need to distinguish between r- and w-bounding raises yet another >> issue. When the source code contains a plain store, the compiler is >> allowed to put plain loads of the same location into the object code. >> For example, given the source code: >> >> x = 1; >> >> the compiler is theoretically allowed to generate object code that >> looks like: >> >> if (x != 1) >> x = 1; >> >> thereby adding a load (and possibly replacing the store entirely). >> For this reason, whenever the LKMM requires a plain store to be >> w-pre-bounded or w-post-bounded by a marked access, it also requires >> the store to be r-pre-bounded or r-post-bounded, so as to handle cases >> where the compiler adds a load. > Good guess; maybe that was the reason. [...] > So perhaps the original reason is not valid now > that the memory model explicitly includes tests for stores being > r-pre/post-bounded. > > Alan
I agree, I think you could relax that condition.
Note there's also rw-xbstar (used with fr) which doesn't check for r-pre-bounded, but it should be ok. That's because only reads would be unordered, as a result the read (in the if (x != ..) x=..) should provide the correct value. The store would be issued as necessary, and the issued store would still be ordered correctly w.r.t the read.
Best wishes, jonas
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