On Fri, 8 May 2020 17:10:09 -0700 Andy Lutomirski <[email protected]> wrote:
> On Tue, May 5, 2020 at 7:14 AM Thomas Gleixner <[email protected]> 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
