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, but also in case smp_send_stop() is supposed to have the side-effect > of disabling interrupt delivery for the local CPU. > > Will > > --->8 > > diff --git a/kernel/panic.c b/kernel/panic.c > index 057540b6eee9..02d0de31c42d 100644 > --- a/kernel/panic.c > +++ b/kernel/panic.c > @@ -179,6 +179,7 @@ void panic(const char *fmt, ...) > * after setting panic_cpu) from invoking panic() again. > */ > local_irq_disable(); > + preempt_disable_notrace(); > > /* > * It's possible to come here directly from a panic-assertion and > > _______________________________________________ > linux-arm-kernel mailing list > linux-arm-kernel@xxxxxxxxxxxxxxxxxxx > http://lists.infradead.org/mailman/listinfo/linux-arm-kernel When you run with panic=... it will send you to a loop earlier in the panic code before local_irq_disable() is hit, working around the bug. A patch like this would make the behaviour the same: diff --git a/kernel/panic.c b/kernel/panic.c index 4d9f55bf7d38..92abbb5f8d38 100644 --- a/kernel/panic.c +++ b/kernel/panic.c @@ -331,7 +331,6 @@ void panic(const char *fmt, ...) /* Do not scroll important messages printed above */ suppress_printk = 1; - local_irq_enable(); for (i = 0; ; i += PANIC_TIMER_STEP) { touch_softlockup_watchdog(); if (i >= i_next) {
