Messages in this thread | | | Date | Tue, 10 Jul 2018 09:25:55 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH v2] tools/memory-model: Add extra ordering for locks and remove it for ordinary release/acquire |
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On Tue, Jul 10, 2018 at 09:57:17AM -0400, Alan Stern wrote: > On Mon, 9 Jul 2018, Paul E. McKenney wrote: > > > On Mon, Jul 09, 2018 at 04:01:57PM -0400, Alan Stern wrote: > > > More than one kernel developer has expressed the opinion that the LKMM > > > should enforce ordering of writes by locking. In other words, given > > > the following code: > > > > > > WRITE_ONCE(x, 1); > > > spin_unlock(&s): > > > spin_lock(&s); > > > WRITE_ONCE(y, 1); > > > > > > the stores to x and y should be propagated in order to all other CPUs, > > > even though those other CPUs might not access the lock s. In terms of > > > the memory model, this means expanding the cumul-fence relation. > > > > > > Locks should also provide read-read (and read-write) ordering in a > > > similar way. Given: > > > > > > READ_ONCE(x); > > > spin_unlock(&s); > > > spin_lock(&s); > > > READ_ONCE(y); // or WRITE_ONCE(y, 1); > > > > > > the load of x should be executed before the load of (or store to) y. > > > The LKMM already provides this ordering, but it provides it even in > > > the case where the two accesses are separated by a release/acquire > > > pair of fences rather than unlock/lock. This would prevent > > > architectures from using weakly ordered implementations of release and > > > acquire, which seems like an unnecessary restriction. The patch > > > therefore removes the ordering requirement from the LKMM for that > > > case. > > > > > > All the architectures supported by the Linux kernel (including RISC-V) > > > do provide this ordering for locks, albeit for varying reasons. > > > Therefore this patch changes the model in accordance with the > > > developers' wishes. > > > > > > Signed-off-by: Alan Stern <stern@rowland.harvard.edu> > > > > Nice! > > > > However, it doesn't apply against current -rcu. Am I missing a patch? > > Or is this supposed to apply against origin/lkmm? > > I wrote it based on 4.18-rc. However, I can rebase it against your > current dev branch.
Could you please? Against either the dev or lkmm branch should well.
If you don't have time for this, my approach would be to apply against 4.18-rc, then cherry-pick onto my branch, resolving the conflicts and emailing you both the "<<<<"-marked file and my proposed resolution. (Or git might just resolve everything automatically -- that does sometimes happen. But it would still be good to double-check its work, as it sometimes does "interesting" resolutions.)
Thanx, Paul
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