[PATCH tip/core/rcu 02/40] rcu: Make arch select smp_mb__after_unlock_lock() strength

[Michael Ellerman]

"Paul E. McKenney" <paulmck at linux.vnet.ibm.com> writes:

> On Thu, Apr 13, 2017 at 06:37:57PM +0200, Peter Zijlstra wrote:
>> On Thu, Apr 13, 2017 at 09:26:51AM -0700, Paul E. McKenney wrote:
>> 
>> > ARCH_WEAK_RELEASE_ACQUIRE actually works both ways.
>> > 
>> > To see this, imagine some strange alternate universe in which the Power
>> > hardware guys actually did decide to switch PPC to doing RCsc as you
>> > suggest.  There would still be a lot of Power hardware out there that
>> > still does RCpc.  Therefore, powerpc builds that needed to run on old
>> > Power hardware would select ARCH_WEAK_RELEASE_ACQUIRE, while kernels
>> > built to run only on the shiny new (but mythical) alternate-universe
>> > Power hardware would avoid selecting this Kconfig option.
>> 
>> Ah, but Power software guys could do it today by replacing an LWSYNC
>> with a SYNC in say arch_spin_unlock().
>> 
>> And yes, I know this isn't a popular suggestion, but it would do the
>> trick.
>
> Indeed, there is a fine line between motivating people to move to new
> hardware on the one hand and terminally annoying existing users on
> the other.  ;-)
>
>> Its just that since there's one (PPC) we can sort of pressure them with
>> the pain of being the only ones to hit all the bugs. But the moment more
>> appear (and I'm afraid it'll be MIPS, with the excuse that PPC already
>> does this) it will be ever so much harder to get rid of it.
>> 
>> Then again, maybe I should just give up and accept the Linux kernel has
>> RCpc locks..
>
> As usual, I must defer to the powerpc maintainers on this one.

I reworked my locking tests a bit, to run longer, disable ASLR and a few
other things, and ran them again. They just bang repeatedly on an
uncontended lock, so nothing fancy at all.

Switching the release barrier to sync (from lwsync) slows it down by
about 18%.

So I think that pretty much rules it out, at least on current CPUs.

I'll try and get some more time to make sure I didn't do something
stupid in the test, and maybe do a version that includes some
contention.

cheers