Re: [PATCH] panic, kexec: Don't mutex_trylock() in __crash_kexec()

From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To: Valentin Schneider <vschneid@redhat.com>
Cc: linux-kernel@vger.kernel.org, kexec@lists.infradead.org,
	linux-rt-users@vger.kernel.org,
	Eric Biederman <ebiederm@xmission.com>,
	Arnd Bergmann <arnd@arndb.de>, Petr Mladek <pmladek@suse.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Juri Lelli <jlelli@redhat.com>,
	"Luis Claudio R. Goncalves" <lgoncalv@redhat.com>
Subject: Re: [PATCH] panic, kexec: Don't mutex_trylock() in __crash_kexec()
Date: Fri, 17 Jun 2022 17:13:01 +0200	[thread overview]
Message-ID: <YqyZ/Uf14qkYtMDX@linutronix.de> (raw)
In-Reply-To: <20220616123709.347053-1-vschneid@redhat.com>

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