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Date:   Fri, 21 Oct 2022 10:36:35 +0200
From:   Andrea Parri <parri.andrea@gmail.com>
To:     Guo Ren <guoren@kernel.org>
Cc:     Jisheng Zhang <jszhang@kernel.org>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] riscv: fix race when vmap stack overflow
Message-ID: <Y1JaE/ot91Z0KXuC@andrea>
References: <20221019154727.2395-1-jszhang@kernel.org>
 <Y1HZFcBo21SQzXVj@andrea>
 <CAJF2gTRAEX_jQ_w5H05dyafZzHq+P5j05TJ=C+v+OL__GQam4A@mail.gmail.com>
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> > > +     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();