On Thu, 2023-10-26 at 09:38 +0200, Johannes Berg wrote: > On Thu, 2023-10-26 at 07:23 +0000, Vincent Whitchurch wrote: > > > @@ -839,9 +863,7 @@ static u64 timer_read(struct clocksource *cs) > > > */ > > > if (!irqs_disabled() && !in_interrupt() && !in_softirq() && > > > !time_travel_ext_waiting) > > > - time_travel_update_time(time_travel_time + > > > - TIMER_MULTIPLIER, > > > - false); > > > + time_travel_update_time_rel(TIMER_MULTIPLIER); > > > return time_travel_time / TIMER_MULTIPLIER; > > > } > > > > The reason I hesitated with putting the whole of > > time_travel_update_time() under local_irq_save() in my attempt was > > because I didn't quite understand the reason for the !irqs_disabled() > > condition here and the comment just above it about recursion and things > > getting messed up. If it's OK to disable interrupts as this patch does, > > is the !irqs_disabled() condition valid? > > Hmm. I was going to say that's different, because it wants to only > prevent us from doing this while we're *already* in IRQ context, and the > bug you found is calling timer_read() not in IRQ context, but getting an > event queued by the signal. > > But ... now that I think about it, I have a feeling that this was a > workaround for the exact same problem, and I just didn't understand it > at the time? I mean, recursing into our own processing is now impossible > here after this patch - either we're running normally, or the interrupt > cannot hit timer_read() in the middle, same as it cannot hit > time_travel_handle_real_alarm() in the middle now. > > Removing that still seems to work with your test, but it's also not a > good test for this, since there are no devices etc. that could have > interrupts, not sure how to test it right now? > > Maybe I'll add a comment there saying this might no longer be needed?
I tried removing the !irqs_disabled() check and that blew up pretty quickly (below) when running the full roadtest suite. It works fine with your unmodified patch so no need to change the comment. Kernel panic - not syncing: time-travel: time goes backwards 26374790000864 -> 26374790000853 show_stack.cold (arch/um/kernel/sysrq.c:56) dump_stack_lvl (lib/dump_stack.c:107 (discriminator 4)) dump_stack (lib/dump_stack.c:114) panic (kernel/panic.c:262 kernel/panic.c:361) timer_handler.cold (arch/um/kernel/time.c:51 arch/um/kernel/time.c:510 arch/um/kernel/time.c:634) timer_real_alarm_handler (arch/um/os-Linux/signal.c:109) unblock_signals (arch/um/os-Linux/signal.c:338) tick_nohz_idle_exit (kernel/time/tick-sched.c:1364) do_idle (kernel/sched/idle.c:310) cpu_startup_entry (kernel/sched/idle.c:379 (discriminator 1)) kernel_init (init/main.c:1435) 0x60001ce6 0x6000220e 0x60004961 new_thread_handler (arch/um/include/asm/thread_info.h:46 arch/um/kernel/process.c:136) uml_finishsetup (arch/um/kernel/um_arch.c:268) _______________________________________________ linux-um mailing list linux-um@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-um
