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

[Tong Tiangen]

在 2022/10/21 22:41, Guo Ren 写道:
> On Fri, Oct 21, 2022 at 9:46 PM Tong Tiangen <tongtiangen@xxxxxxxxxx> wrote:
> > 在 2022/10/21 21:22, Andrea Parri 写道:
> > > Hi Tong,
> > >
> > > > > > 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.
> > >
> > > >       Consider this implementation:)
> > > >
> > > >       smp_store_mb(&spin_shadow_stack, 0);
> > >
> > > smp_store_mb() has "WRITE_ONCE(); smp_mb()" semantics; so it doesn't
> > > guarantee that the store to spin_shadow_stack is ordered after program
> > > -order earlier memory accesses.
> > >
> > >     Andrea
> > > .
> >
> > Hi Andrea:
> >
> > IIUC, the earlier memory access amoswap.aqrl, here .aqrl guarantee it.
> > But anyway, consider we don't care about performance here, using
> > smp_store_release()(add barrier()) surely right.
> We use smp_store_release() is for:
>          //load per-cpu overflow stack
>          REG_L sp, -8(sp)

Oh, missing that, you're right. This guarantee the spin flag is set 
after the sp is used.

> Not amoswap.
>
> Actually, amoswap.aqrl guarantees nothing because all instructions
> depend on the sp register.
right.

Thanks,
Tong.

> > Thanks,
> > Tong.