Hi Alan, On Tue, Jul 10, 2018 at 02:18:13PM -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 <st...@rowland.harvard.edu>
Thanks, I'm happy with this version of the patch: Reviewed-by: Will Deacon <will.dea...@arm.com> Will
