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Date:   Wed, 13 May 2020 20:13:49 -0400
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        x86 <x86@kernel.org>, paulmck <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>,
        "Joel Fernandes, Google" <joel@joelfernandes.org>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Juergen Gross <jgross@suse.com>,
        Brian Gerst <brgerst@gmail.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Will Deacon <will@kernel.org>
Subject: Re: [patch V4 part 1 14/36] x86/entry: Get rid of
 ist_begin/end_non_atomic()
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On Wed, 13 May 2020 18:57:25 -0400 (EDT)
Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:

> ----- On May 5, 2020, at 9:16 AM, Thomas Gleixner tglx@linutronix.de wrote:
> 
> > This is completely overengineered and definitely not an interface which
> > should be made available to anything else than this particular MCE case.  
> 
> This patch introduces a significant change under the radar (not explained
> in the changelog): it turns preempt_enable_no_resched() into preempt_enable().
> 
> Why, and why was it a no_resched() in the first place ? Was it for performance
> or correctness reasons ?

I believe the reason for no_resched, is because it was within a
local_irq_disable() section, which means it couldn't schedule anyway.

-- Steve


> > --- a/arch/x86/kernel/cpu/mce/core.c
> > +++ b/arch/x86/kernel/cpu/mce/core.c
> > @@ -1352,13 +1352,15 @@ void notrace do_machine_check(struct pt_
> > 
> > 	/* Fault was in user mode and we need to take some action */
> > 	if ((m.cs & 3) == 3) {
> > -		ist_begin_non_atomic(regs);
> > +		/* 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();
> > -		ist_end_non_atomic();
> > 	} else {
> > 		if (!fixup_exception(regs, X86_TRAP_MC, error_code,


> > -void ist_begin_non_atomic(struct pt_regs *regs)
> > -{
> > -	BUG_ON(!user_mode(regs));
> > -
> > -	/*
> > -	 * Sanity check: we need to be on the normal thread
> > stack.  This
> > -	 * will catch asm bugs and any attempt to use
> > ist_preempt_enable
> > -	 * from double_fault.
> > -	 */
> > -	BUG_ON(!on_thread_stack());
> > -
> > -	preempt_enable_no_resched();
> > -}
> > -
> > -/**
> > - * ist_end_non_atomic() - begin a non-atomic section in an IST
> > exception
> > - *
> > - * Ends a non-atomic section started with ist_begin_non_atomic().
> > - */
> > -void ist_end_non_atomic(void)
> > -{
> > -	preempt_disable();
> > -}
> > -
> > int is_valid_bugaddr(unsigned long addr)
> > {
> >  	unsigned short ud;  
>