Messages in this thread | | | From | Thomas Gleixner <> | Subject | Re: [patch V4 part 2 10/18] x86/entry/64: Check IF in __preempt_enable_notrace() thunk | Date | Sat, 09 May 2020 12:25:34 +0200 |
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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
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