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From:   Valentin Schneider <vschneid@redhat.com>
To:     "Eric W. Biederman" <ebiederm@xmission.com>
Cc:     linux-kernel@vger.kernel.org, kexec@lists.infradead.org,
        linux-rt-users@vger.kernel.org, Arnd Bergmann <arnd@arndb.de>,
        Petr Mladek <pmladek@suse.com>,
        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: <xhsmhpmiu5lch.mognet@vschneid.remote.csb>
References: <20220620111520.1039685-1-vschneid@redhat.com>
 <87r13c7jyp.fsf@email.froward.int.ebiederm.org>
 <xhsmhpmiu5lch.mognet@vschneid.remote.csb>
Date:   Tue, 28 Jun 2022 18:33:08 +0100
Message-ID: <xhsmhmtdw66cr.mognet@vschneid.remote.csb>
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On 27/06/22 13:42, Valentin Schneider wrote:
> On 25/06/22 12:04, Eric W. Biederman wrote:
>> At this point I recommend going back to being ``unconventional'' with
>> the kexec locking and effectively reverting commit 8c5a1cf0ad3a ("kexec:
>> use a mutex for locking rather than xchg()").
>>
>> That would also mean that we don't have to worry about the lockdep code
>> doing something weird in the future and breaking kexec.
>>
>> Your change starting to is atomic_cmpxchng is most halfway to a revert
>> of commit 8c5a1cf0ad3a ("kexec: use a mutex for locking rather than
>> xchg()").  So we might as well go the whole way and just document that
>> the kexec on panic code can not use conventional kernel locking
>> primitives and has to dig deep and build it's own.  At which point it
>> makes no sense for the rest of the kexec code to use anything different.
>>
>
> Hm, I'm a bit torn about that one, ideally I'd prefer to keep "homegrown"
> locking primitives to just where they are needed (loading & kexec'ing), but
> I'm also not immensely fond of the "hybrid" mutex+cmpxchg approach.
>

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)?

Otherwise, we keep the mutexes for functions like those which go nowhere
near an NMI.