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

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Date:   Wed, 29 Jun 2022 13:23:08 +0100
From:   Valentin Schneider <vschneid@...hat.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        linux-kernel@...r.kernel.org, kexec@...ts.infradead.org,
        linux-rt-users@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Juri Lelli <jlelli@...hat.com>,
        "Luis Claudio R. Goncalves" <lgoncalv@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vivek Goyal <vgoyal@...hat.com>
Subject: Re: [PATCH v2] panic, kexec: Make __crash_kexec() NMI safe

On 29/06/22 13:55, Petr Mladek wrote:
> On Tue 2022-06-28 18:33:08, Valentin Schneider wrote:
>>
>> 8c5a1cf0ad3a ("kexec: use a mutex for locking rather than xchg()") was
>> straightforward enough because it turned
>>
>>         if (xchg(&lock, 1))
>>                 return -EBUSY;
>>
>> into
>>
>>         if (!mutex_trylock(&lock))
>>                 return -EBUSY;
>>
>> Now, most of the kexec_mutex uses are trylocks, except for:
>> - crash_get_memory_size()
>> - crash_shrink_memory()
>>
>> I really don't want to go down the route of turning those into cmpxchg
>> try-loops, would it be acceptable to make those use trylocks (i.e. return
>> -EBUSY if the cmpxchg fails)?
>
> IMHO, -EBUSY is acceptable for both crash_get_memory_size()
> and crash_shrink_memory(). They are used in the sysfs interface.
>
>> Otherwise, we keep the mutexes for functions like those which go nowhere
>> near an NMI.
>
> If we go this way then I would hide the locking into some wrappers,
> like crash_kexec_trylock()/unlock() that would do both mutex
> and xchg. The xchg part might be hidden in a separate wrapper
> __crash_kexec_trylock()/unlock() or
> crash_kexec_atomic_trylock()/unlock().
>

Makes sense, thanks. I've started playing with the trylock/-EBUSY approach,
I'll toss it out if I don't end up hating it.

> Best Regards,
> Petr