Re: Handling panics inside vt(4) callbacks

From: Mark Johnston <markj_at_freebsd.org>
Date: Thu, 13 Apr 2023 15:18:51 UTC
On Wed, Apr 12, 2023 at 10:45:27PM +0200, Jean-Sbastien Pdron wrote:
> Hi!
> 
> While working on the DRM drivers, I don't always get a kernel core dump 
> in case of a panic.
> 
> My hypothesis is that if the DRM driver code called by vt(4) panics, 
> then the panic code might not go through successfully. The reason is 
> because panic(9) prints the reason, a stacktrace and possibly some 
> progress to the console, which calls vt(4) and the DRM driver code again.
> 
> I played with the following patch:
> https://gist.github.com/dumbbell/88d77789bfeb38869268c84c40575f49
> 
> The idea is that before calling "vt_flush()" in "vtterm_done()", I set a 
> global flag to true to indicate that vt(4) is called as part of kdb or a 
> panic. If another panic occurs inside vt_flush(), typically the 
> underlying DRM driver code, "vtterm_done()" is called recursively and 
> "vt_flush()" might trigger the same panic again. If the flag is set, the 
> entire function is skipped instead.
> 
> I test the patch by adding a panic(9) just before "vt_flush()" and I 
> trigger the initial panic with debug.kdb.panic=1. I don't even load a 
> DRM driver. My problem is that in this case, the laptop reboots 
> immediately. However, if I replace panic(9) with a simple printf(9), it 
> works as expected and I get a kernel dump.
> 
> I could not find something in panic(9) code that would reboot the 
> computer in case of a nested panic.

In the case of a nested panic, vpanic() will not set RB_DUMP when it
calls kern_reboot(), so it won't write a kernel dump.  And, if
debug.debugger_on_recursive_panic is not set, the kernel will not try to
re-enter the debugger.  So the kernel will simply reboot.

> Previous versions of the patch called doadump() and rebooted the 
> computer explicitly if the flag was set, but it didn't work either and I 
> thought I could simplify that patch and let panic(9) handle recursion. 
> In other words, I just want to skip most of vt(4) code if vt(4) or DRM 
> crash.

Perhaps we should set RB_DUMP in the case of a recursive panic so long
as dumping == 0, i.e., we did not panic again while trying to dump core.
In fact, kern_reboot() already checks this.

> Does someone spot something wrong in my hypothesis or methodology?
> 
> -- 
> Jean-Sbastien Pdron
> The FreeBSD Project
>