Re: [RFC PATCH] sys_membarrier(): system/process-wide memory barrier (x86) (v12)

[Peter Zijlstra]

On Tue, Mar 17, 2015 at 01:45:25AM +0000, Mathieu Desnoyers wrote:
> Let's go through a memory ordering scenario to highlight my reasoning
> there.
> 
> Let's consider the following memory barrier scenario performed in
> user-space on an architecture with very relaxed ordering. PowerPC comes
> to mind.
> 
> https://lwn.net/Articles/573436/
> scenario 12:
> 
> CPU 0                   CPU 1
> CAO(x) = 1;             r3 = CAO(y);
> cmm_smp_wmb();          cmm_smp_rmb();
> CAO(y) = 1;             r4 = CAO(x);
> 
> BUG_ON(r3 == 1 && r4 == 0)

WTF is CAO() ? and that ridiculous cmm_ prefix on the barriers.

> We tweak it to use sys_membarrier on CPU 1, and a simple compiler
> barrier() on CPU 0:
> 
> CPU 0                   CPU 1
> CAO(x) = 1;             r3 = CAO(y);
> barrier();              sys_membarrier();
> CAO(y) = 1;             r4 = CAO(x);
> 
> BUG_ON(r3 == 1 && r4 == 0)

That hardly seems like a valid substitution; barrier() is not a valid
replacement of a memory barrier is it? Esp not on PPC.

> Now if CPU 1 executes sys_membarrier while CPU 0 is preempted after both
> stores, we have:
> 
> CPU 0                           CPU 1
> CAO(x) = 1;
>   [1st store is slow to
>    reach other cores]
> CAO(y) = 1;
>   [2nd store reaches other
>    cores more quickly]
> [preempted]
>                                 r3 = CAO(y)
>                                   (may see y = 1)
>                                 sys_membarrier()
> Scheduler changes rq->curr.
>                                 skips CPU 0, because rq->curr has
>                                   been updated.
>                                 [return to userspace]
>                                 r4 = CAO(x)
>                                   (may see x = 0)
>                                 BUG_ON(r3 == 1 && r4 == 0) -> fails.
> load_cr3, with implied
>   memory barrier, comes
>   after CPU 1 has read "x".
> 
> The only way to make this scenario work is if a memory barrier is added
> before updating rq->curr. (we could also do a similar scenario for the
> needed barrier after store to rq->curr).

Hmmm.. like that. Light begins to dawn.

So I think in this case we're good with the smp_mb__before_spinlock() we
have; but do note its not a full MB even though the name says so.

Its basically: WMB + ACQUIRE, which theoretically can leak a read in,
but nobody sane _delays_ reads, you want to speculate reads, not
postpone.

Also, it lacks the transitive property.

> Would you see it as acceptable if we start by implementing
> only the non-expedited sys_membarrier() ?

Sure.

> Then we can add
> the expedited-private implementation after rq->curr becomes
> available through RCU.

Yeah, or not at all; I'm still trying to get Paul to remove the
expedited nonsense from the kernel RCU bits; and now you want it in
userspace too :/
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