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From:   Andy Lutomirski <luto@kernel.org>
Date:   Thu, 7 May 2020 11:02:09 -0700
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Subject: Re: [patch V4 part 1 29/36] x86/mce: Send #MC singal from task work
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Andy Lutomirski <luto@kernel.org>,
        Alexandre Chartre <alexandre.chartre@oracle.com>,
        Frederic Weisbecker <frederic@kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Petr Mladek <pmladek@suse.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Joel Fernandes <joel@joelfernandes.org>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Juergen Gross <jgross@suse.com>,
        Brian Gerst <brgerst@gmail.com>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Will Deacon <will@kernel.org>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>
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On Tue, May 5, 2020 at 7:13 AM Thomas Gleixner <tglx@linutronix.de> wrote:
>
> From: Peter Zijlstra <peterz@infradead.org>
>
> Convert #MC over to using task_work_add(); it will run the same code
> slightly later, on the return to user path of the same exception.

I think this patch is correct, but I think it's only one small and not
that obviously wrong step away from being broken:

>         if ((m.cs & 3) == 3) {
>                 /* If this triggers there is no way to recover. Die hard. */
>                 BUG_ON(!on_thread_stack() || !user_mode(regs));
> -               local_irq_enable();
> -               preempt_enable();
>
> -               if (kill_it || do_memory_failure(&m))
> -                       force_sig(SIGBUS);
> -               preempt_disable();
> -               local_irq_disable();
> +               current->mce_addr = m.addr;
> +               current->mce_status = m.mcgstatus;
> +               current->mce_kill_me.func = kill_me_maybe;
> +               if (kill_it)
> +                       current->mce_kill_me.func = kill_me_now;
> +               task_work_add(current, &current->mce_kill_me, true);

This is fine if the source was CPL3, but it's not going to work if CPL
was 0.  We don't *currently* do this from CPL0, but people keep
wanting to.  So perhaps there should be a comment like:

/*
 * The #MC originated at CPL3, so we know that we will go execute the
task_work before returning to the offending user code.
 */

IOW, if we want to recover from CPL0 #MC, we will need a different mechanism.

I also confess a certain amount of sadness that my beautiful
haha-not-really-atomic-here mechanism isn't being used anymore. :(

--Andy