Re: [patch V4 part 2 10/18] x86/entry/64: Check IF in __preempt_enable_notrace() thunk

[Thomas Gleixner <tglx@linutronix.de>]

Andy Lutomirski <luto@kernel.org> writes:
> On Tue, May 5, 2020 at 7:14 AM Thomas Gleixner <tglx@linutronix.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().

Duh, yes.

> 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.

Let me stare into that again.

Thanks,

        tglx