Re: [PATCH v2] kvm: fix waitqueue_active without memory barrier in virt/kvm/async_pf.c

[Paolo Bonzini <pbonzini@redhat.com>]

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