On Tue, Sep 17, 2019 at 11:45:19AM +0100, Will Deacon wrote:

quoted

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.

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

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) {