Re: [PATCH V2] x86/sgx: Fix free page accounting

[Jarkko Sakkinen <jarkko@kernel.org>]

On Wed, 2021-11-10 at 19:16 +0000, Luck, Tony wrote:
> > > > The consequence of sgx_nr_free_pages not being protected is that
> > > > its value may not accurately reflect the actual number of free
> > > > pages on the system, impacting the availability of free pages in
> > > > support of many flows. The problematic scenario isu when the
> > > 
> > > In non-atomicity is not a problem, when it is not a problem :-)
> 
> This is most definitely a problem.
> 
> start with sgx_nr_free_pages == 100
> 
> Now have a cpu on node0 allocate a page at the same time as another
> cpu on node1 allcoates a page. Each holds the relevent per-node lock,
> but that doesn't stop both CPUs from accessing the global together:
> 
>         CPU on node0            CPU on node1
>         sgx_nr_free_pages--;            sgx_nr_free_pages--;
> 
> What is the value of sgx_nr_free_pages now? We want it to be 98,
> but it could be 99.
> 
> Rinse, repeat thousands of times. Eventually the value of sgx_nr_free_pages
> has not even a close connection to the number of free pages.
> 
> -Tony

Yeah, so I figured this (see my follow-up response to Reinette) but
such description is lacking from the commit message.

/Jarkko