linux-kernel - Re: [PATCH] riscv: fix race when vmap stack overflow

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Date:   Fri, 21 Oct 2022 10:36:35 +0200
From:   Andrea Parri <parri.andrea@...il.com>
To:     Guo Ren <guoren@...nel.org>
Cc:     Jisheng Zhang <jszhang@...nel.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] riscv: fix race when vmap stack overflow

> > > +     atomic_set_release(&spin_shadow_stack, 0);
> >
> > Have not really looked the details: should there be a matching acquire?
> 
> I use atomic_set_release here, because I need earlier memory
> operations finished to make sure the sp is ready then set the spin
> flag.
> 
> The following memory operations order is not important, because we
> just care about sp value.
> 
> Also, we use relax amoswap before, because sp has naturelly
> dependency. But giving them RCsc is okay here, because we don't care
> about performance here.

Thanks for the clarification.

I'm not really suggesting to add unneeded synchronization, even more
so in local/private constructs as in this case.  It just felt odd to
see the release without a pairing acquire, so I asked.  ;-)

Thanks,
  Andrea


> eg:
>  handle_kernel_stack_overflow:
> +1:     la sp, spin_shadow_stack
> +       amoswap.w.aqrl sp, sp, (sp)
> +       bnez sp, 1b
> +
> ....
> +     smp_store_release(&spin_shadow_stack, 0);
> +     smp_mb();