Re: [PATCH v3] tools/memory-model: Add extra ordering for locks and remove it for ordinary release/acquire

Will Deacon
2018-07-11T09:43:06Z

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 <stern@rowland.harvard.edu>

Thanks, I'm happy with this version of the patch:

Reviewed-by: Will Deacon <will.deacon@arm.com>

Will