Re: panic: data abort in critical section or under mutex (was: Re: panic: Unknown kernel exception 0 esr_el1 2000000 (on 14-CURRENT/aarch64 Feb 28))

On Mon, Mar 07, 2022 at 04:25:22PM +0000, Andrew Turner wrote:
> 
> > On 7 Mar 2022, at 15:13, Mark Johnston <[email protected]> wrote:
> > ...
> > A (the?) problem is that the compiler is treating "pc" as an alias
> > for x18, but the rmlock code assumes that the pcpu pointer is loaded
> > once, as it dereferences "pc" outside of the critical section.  On
> > arm64, if a context switch occurs between the store at _rm_rlock+144 and
> > the load at +152, and the thread is migrated to another CPU, then we'll
> > end up using the wrong CPU ID in the rm->rm_writecpus test.
> > 
> > I suspect the problem is unique to arm64 as its get_pcpu()
> > implementation is different from the others in that it doesn't use
> > volatile-qualified inline assembly.  This has been the case since
> > https://cgit.freebsd.org/src/commit/?id=63c858a04d56529eddbddf85ad04fc8e99e73762 <https://cgit.freebsd.org/src/commit/?id=63c858a04d56529eddbddf85ad04fc8e99e73762>
> > .
> > 
> > I haven't been able to reproduce any crashes running poudriere in an
> > arm64 AWS instance, though.  Could you please try the patch below and
> > confirm whether it fixes your panics?  I verified that the apparent
> > problem described above is gone with the patch.
> 
> Alternatively (or additionally) we could do something like the following. There are only a few MI users of get_pcpu with the main place being in rm locks.
> 
> diff --git a/sys/arm64/include/pcpu.h b/sys/arm64/include/pcpu.h
> index 09f6361c651c..59b890e5c2ea 100644
> --- a/sys/arm64/include/pcpu.h
> +++ b/sys/arm64/include/pcpu.h
> @@ -58,7 +58,14 @@ struct pcpu;
> 
>  register struct pcpu *pcpup __asm ("x18");
> 
> -#define        get_pcpu()      pcpup
> +static inline struct pcpu *
> +get_pcpu(void)
> +{
> +       struct pcpu *pcpu;
> +
> +       __asm __volatile("mov   %0, x18" : "=&r"(pcpu));
> +       return (pcpu);
> +}
> 
>  static inline struct thread *
>  get_curthread(void)

Indeed, I think this is probably the best solution.