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SubjectRe: [PATCH v2] kvm: fix waitqueue_active without memory barrier in virt/kvm/async_pf.c
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On 09/10/2015 14:55, Peter Zijlstra wrote:
> On Fri, Oct 09, 2015 at 12:21:55PM +0000, Kosuke Tatsukawa wrote:
>
>> + * Memory barrier is required here to make sure change to
>> + * vcpu->async_pf.done is visible from other CPUs. This memory
>> + * barrier pairs with prepare_to_wait's set_current_state()
>
> That is not how memory barriers work; they don't 'make visible'. They
> simply ensure order between operations.
>
> X = Y = 0
>
> CPU0 CPU1
>
> [S] X=1 [S] Y=1
> MB MB
> [L] y=Y [L] x=X
>
> assert(x || y)
>
> The issue of the memory barrier does not mean the store is visible, it
> merely means that the load _must_ happen after the store (in the above
> scenario).
>
> This gives a guarantee that not both x and y can be 0. Because either
> being 0, means the other has not yet executed and must therefore observe
> your store.
>
> Nothing more, nothing less.
>
> So your comment is misleading at best.

Yeah, I will just reword the comment when applying. Thanks Kosuke-san
for your contribution!

Paolo


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