On Tue, Sep 17, 2019 at 11:45:19AM +0100, Will Deacon wrote: > Hi, > > [Expanding CC list; original message is here: > https://lore.kernel.org/linux-arm-kernel/BX1W47JXPMR8.58IYW53H6M5N@dragonstone/] > > On Mon, Sep 16, 2019 at 09:35:36PM -0400, Xogium wrote: > > On arm64 in some situations userspace will continue running even after a > > panic. This means any userspace watchdog daemon will continue pinging, > > that service managers will keep running and displaying messages in certain > > cases, and that it is possible to enter via ssh in the now unstable system > > and to do almost anything except reboot/power off and etc. If > > CONFIG_PREEMPT=n is set in the kernel's configuration, the issue is fixed. > > I have reproduced the very same behavior with linux 4.19, 5.2 and 5.3. On > > x86/x86_64 the issue does not seem to be present at all. > > I've managed to reproduce this under both 32-bit and 64-bit ARM kernels. > The issue is that the infinite loop at the end of panic() can run with > preemption enabled (particularly when invoking by echoing 'c' to > /proc/sysrq-trigger), so we end up rescheduling user tasks. On x86, this > doesn't happen because smp_send_stop() disables the local APIC in > native_stop_other_cpus() and so interrupts are effectively masked while > spinning. > > A straightforward fix is to disable preemption explicitly on the panic() > path (diff below), but I've expanded the cc list to see both what others > think, Yep, and it looks like this bug goes back into the dim and distant past. At least to the start of modern git history, 2.6.12-rc2. > but also in case smp_send_stop() is supposed to have the side-effect > of disabling interrupt delivery for the local CPU. That can't fix it. Consider a preemptive non-SMP kernel. smp_send_stop() becomes a no-op there. I'd suggest that a preemptive UP kernel on x86 hardware will suffer this same issue - it will be able to preempt out of this loop and continue running userspace. -- RMK's Patch system: https://www.armlinux.org.uk/developer/patches/ FTTC broadband for 0.8mile line in suburbia: sync at 12.1Mbps down 622kbps up According to speedtest.net: 11.9Mbps down 500kbps up
