linux-kernel - Re: [BUG RT] dump-capture kernel not executed for panic in interrupt context

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Message-ID: <20200907162338.GN1362448@hirez.programming.kicks-ass.net>
Date:   Mon, 7 Sep 2020 18:23:38 +0200
From:   peterz@...radead.org
To:     Joerg Vehlow <lkml@...coder.de>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Huang Ying <ying.huang@...el.com>,
        linux-kernel@...r.kernel.org,
        Joerg Vehlow <joerg.vehlow@...-tech.de>
Subject: Re: [BUG RT] dump-capture kernel not executed for panic in interrupt
 context

On Mon, Sep 07, 2020 at 02:03:09PM +0200, Joerg Vehlow wrote:
> 
> 
> On 9/7/2020 1:46 PM, peterz@...radead.org wrote:
> > I think it's too complicated for that is needed, did you see my
> > suggestion from a year ago? Did i miss something obvious?
> > 
> This one? https://lore.kernel.org/linux-fsdevel/20191219090535.GV2844@hirez.programming.kicks-ass.net/
> 
> I think it may be a bit incorrect?
> According to the original comment in __crash_kexec, the mutex was used to
> prevent a sys_kexec_load, while crash_kexec is executed. Your proposed patch
> does not lock the mutex in crash_kexec.

Sure, but any mutex taker will (spin) wait for panic_cpu==CPU_INVALID.
And if the mutex is already held, we'll not run __crash_kexec() just
like the trylock() would do today.

> This does not cover the original use
> case anymore. The only thing that is protected now are two panicing cores at
> the same time.

I'm not following. AFAICT it does exactly what the old code did.
Although maybe I didn't replace all kexec_mutex users, I now see that
thing isn't static.

> Actually, this implementation feels even more hacky to me....

It's more minimal ;-) It's simpler in that it only provides the required
semantics (as I understand them) and does not attempt to implement a
more general trylock() like primitive that isn't needed.

Also, read the kexec_lock() implementation you posted and explain to me
what happens when kexec_busy is elevated. Also note the lack of
confusing loops in my code.