linux-kernel - Re: [PATCH] panic, kexec: Don't mutex_trylock() in __crash

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Date:   Fri, 17 Jun 2022 17:13:01 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Valentin Schneider <vschneid@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kexec@...ts.infradead.org,
        linux-rt-users@...r.kernel.org,
        Eric Biederman <ebiederm@...ssion.com>,
        Arnd Bergmann <arnd@...db.de>, Petr Mladek <pmladek@...e.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Juri Lelli <jlelli@...hat.com>,
        "Luis Claudio R. Goncalves" <lgoncalv@...hat.com>
Subject: Re: [PATCH] panic, kexec: Don't mutex_trylock() in __crash_kexec()

On 2022-06-16 13:37:09 [+0100], Valentin Schneider wrote:
> Regarding the original explanation for the WARN & return:
> 
> I don't get why 2) is a problem - if the lock is acquired by the trylock
> then the critical section will be run without interruption since it
> cannot sleep, the interrupted task may get boosted but that will not
> have any actual impact AFAICT.

boosting an unrelated task is considered wrong. I don't know how bad
it gets in terms of lock chains since a task is set as owner which did
not actually ask for the lock.

> Regardless, even if this doesn't sleep, the ->wait_lock in the slowpath
> isn't NMI safe so this needs changing.

This includes the unlock path which may wake a waiter and deboost.

> I've thought about trying to defer the kexec out of an NMI (or IRQ)
> context, but that pretty much means deferring the panic() which I'm
> not sure is such a great idea.

If we could defer it out of NMI on RT then it would work non-RT, too. If
the system is "stuck" and the NMI is the only to respond then I guess
that it is not a great idea.

Sebastian