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Date:   Mon, 11 May 2020 21:48:35 -0400
From:   Steven Rostedt <>
To:     Andy Lutomirski <>
Cc:     Thomas Gleixner <>,
        LKML <>, X86 ML <>,
        "Paul E. McKenney" <>,
        Alexandre Chartre <>,
        Frederic Weisbecker <>,
        Paolo Bonzini <>,
        Sean Christopherson <>,
        Masami Hiramatsu <>,
        Petr Mladek <>,
        Joel Fernandes <>,
        Boris Ostrovsky <>,
        Juergen Gross <>,
        Brian Gerst <>,
        Mathieu Desnoyers <>,
        Josh Poimboeuf <>,
        Will Deacon <>
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 <> wrote:

> On Tue, May 5, 2020 at 7:14 AM Thomas Gleixner <> 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:


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

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