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From:   Valentin Schneider <vschneid@redhat.com>
To:     Petr Mladek <pmladek@suse.com>
Cc:     "Eric W. Biederman" <ebiederm@xmission.com>,
        linux-kernel@vger.kernel.org, kexec@lists.infradead.org,
        linux-rt-users@vger.kernel.org, Arnd Bergmann <arnd@arndb.de>,
        Thomas Gleixner <tglx@linutronix.de>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Juri Lelli <jlelli@redhat.com>,
        "Luis Claudio R. Goncalves" <lgoncalv@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Vivek Goyal <vgoyal@redhat.com>
Subject: Re: [PATCH v2] panic, kexec: Make __crash_kexec() NMI safe
In-Reply-To: <20220629115539.GB12720@pathway.suse.cz>
References: <20220620111520.1039685-1-vschneid@redhat.com>
 <87r13c7jyp.fsf@email.froward.int.ebiederm.org>
 <xhsmhpmiu5lch.mognet@vschneid.remote.csb>
 <xhsmhmtdw66cr.mognet@vschneid.remote.csb>
 <20220629115539.GB12720@pathway.suse.cz>
Date:   Wed, 29 Jun 2022 13:23:08 +0100
Message-ID: <xhsmhk08z64lv.mognet@vschneid.remote.csb>
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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