Re: panic() logic

From: Anirban Sinha
Date: Sat Oct 18 2008 - 22:44:21 EST

Hi Andi:

Thanks for replying.
On 18-Oct-08, at 12:56 AM, Andi Kleen wrote:

"Ani Sinha" <kernel@xxxxxxxxxxx> writes:

I noticed an issue with the panic() firing on a back core in SMP
lately. We are mostly working on mips architectures but it might
effect other archs as well. Therefore, I am putting forward my
thoughts and comments to the whole linux community. In the following,
by front core I mean core#0 and by back core I mean other cores.

Why exactly is the "front core" special?

I am not exactly a firmware (CFE) guy but if I understand it correctly, all the interrupts are tied to the front core and cfe_exit() can only be called from the front core. I have written to the other guy who specializes in the CFE area and I will get back to you when I get an answer from him.

smp_send_stop basically marks all the other cores as 'down' and
updates the cpu bitmap. One implication of this is that you can not do
an IPI later on to other cores (smp_send_function() does a
'for_earch_online_cpu'). This makes sense since you should not be
allowed to do anything on a down cpu.

This part of the logic is in Linux and is arch independent.

But what if a particular
architecture had logic to do specific things for the front core and
other things on the back cores as a part of 'graceful reboot' process?

Is that logic in Linux or in the platform?

This logic is in arch specific code.

Normally it's best to not rely on any specific CPU for panic.
What do you do when that CPU is so broken that it cannot
process IPIs anymore?

Agreed. That is why in my pseudo code I have a block (a comment really) telling you do do absolutely bare minimal things that you must do in a panic situation on the current core (without relying on IPIs to succeed on other cores).. What this bare minimum will be is a matter of debate. Getting a message out to the console saying that something bad has occurred (with details of the crash) can perhaps be a part of that minumum hunk of code.

Currently, the arch independent logic defeats the main purpose of the arch dependent emergency_restart() function which is to restart the system. If a panic occurs on a back core, the kernel halts with the message "rebooting in 5 sec) and someone has to physically press the reset button. In a vast majority of the cases, we do have a perfectly sane and functional front core and we are just not able to gracefully reboot the system because we are limited by the way panic() handles things. If there are other archs that does a similar specific operation for the front core as a part of 'emergency restart', they are all defeated.

All I am trying to say is that perhaps there is a window of possibility where we can better handle a kernel panic. I am not saying that we should rearrange code in order to just accommodate mips archs, but if it can be done without much pain and objection, then lets just do it.





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