On Tue, Sep 17, 2019 at 11:51:36AM +0100, Russell King - ARM Linux admin wrote: > On Tue, Sep 17, 2019 at 11:45:19AM +0100, Will Deacon wrote: > > [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. You're right; I managed to reproduce this locally on my xeon box. Will
