linux-kernel - Re: [patch V4 part 2 10/18] x86/entry/64: Check IF in __preempt_enable

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Date:   Mon, 11 May 2020 21:48:35 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Petr Mladek <pmladek@...e.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Brian Gerst <brgerst@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Will Deacon <will@...nel.org>
Subject: Re: [patch V4 part 2 10/18] x86/entry/64: Check IF in
 __preempt_enable_notrace() thunk

On Fri, 8 May 2020 17:10:09 -0700
Andy Lutomirski <luto@...nel.org> wrote:

> On Tue, May 5, 2020 at 7:14 AM Thomas Gleixner <tglx@...utronix.de> wrote:
> >
> > The preempt_enable_notrace() ASM thunk is called from tracing, entry code
> > RCU and other places which are already in or going to be in the noinstr
> > section which protects sensitve code from being instrumented.  
> 
> This text and $SUBJECT agree that you're talking about
> preempt_enable_notrace(), but:
> 
> > +       THUNK preempt_schedule_notrace_thunk, preempt_schedule_notrace, check_if=1  
> 
> You actually seem to be changing preempt_schedule_notrace().
> 
> The actual code in question has this comment:
> 
> /**
>  * preempt_schedule_notrace - preempt_schedule called by tracing
>  *
>  * The tracing infrastructure uses preempt_enable_notrace to prevent
>  * recursion and tracing preempt enabling caused by the tracing
>  * infrastructure itself. But as tracing can happen in areas coming
>  * from userspace or just about to enter userspace, a preempt enable
>  * can occur before user_exit() is called. This will cause the scheduler
>  * to be called when the system is still in usermode.
>  *
>  * To prevent this, the preempt_enable_notrace will use this function
>  * instead of preempt_schedule() to exit user context if needed before
>  * calling the scheduler.
>  */
> 
> Which is no longer really applicable to x86 -- in the state that this
> comment nonsensically refers to as "userspace", x86 *always* has IRQs
> off, which means that preempt_enable() will not schedule.
> 
> So I'm guessing that the issue you're solving is that we have
> redundant preempt disable/enable pairs somewhere in the bowels of
> tracing code that is called with IRQs off, and objtool is now
> complaining.  Could the actual code in question be fixed to assert
> that IRQs are off instead of disabling preemption?  If not, can you
> fix the $SUBJECT and changelog and perhaps add a comment to the code
> as to *why* you're checking IF?  Otherwise some intrepid programmer is
> going to notice it down the road, wonder if it's optimizing anything
> useful at all, and get rid of it.

The commit that added that code is this:

  29bb9e5a75684106a37593ad75ec75ff8312731b

And it may not be applicable anymore, especially after Thomas's
patches. I'll go and stare at that some more. A lot has changed since
2013 ;-)

-- Steve