linux-kernel - Re: [PATCH v2] panic, kexec: Make __crash

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Message-ID: <xhsmhsfnu5giq.mognet@vschneid.remote.csb>
Date:   Fri, 24 Jun 2022 14:37:33 +0100
From:   Valentin Schneider <vschneid@...hat.com>
To:     Baoquan He <bhe@...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>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Juri Lelli <jlelli@...hat.com>,
        "Luis Claudio R. Goncalves" <lgoncalv@...hat.com>
Subject: Re: [PATCH v2] panic, kexec: Make __crash_kexec() NMI safe

On 24/06/22 09:30, Baoquan He wrote:
> On 06/20/22 at 12:15pm, Valentin Schneider wrote:
>> @@ -94,14 +94,20 @@ static int do_kexec_load(unsigned long entry, unsigned long nr_segments,
>>  	/*
>>  	 * Because we write directly to the reserved memory region when loading
>>  	 * crash kernels we need a mutex here to prevent multiple crash kernels
>> -	 * from attempting to load simultaneously, and to prevent a crash kernel
>> -	 * from loading over the top of a in use crash kernel.
>> -	 *
>> -	 * KISS: always take the mutex.
>> +	 * from attempting to load simultaneously.
>>  	 */
>>  	if (!mutex_trylock(&kexec_mutex))
>>  		return -EBUSY;
>
> So kexec_mutex is degenerated to only avoid simultaneous loading,
> should we rename to reflect that?, e.g kexec_load_mutex.
>

It's also serializing crash_get_memory_size() and crash_shrink_memory();
more generally it should still be the preferred serialization mechanism as
it's a "proper" lock visible by instrumentation, the atomic variable is a
side character for the NMI case.