linux-kernel - Re: synchronization model: was: Re: [PATCH printk-rework 09/14] printk: introduce a kmsg

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Message-ID: <878s7d1nkn.fsf@jogness.linutronix.de>
Date:   Wed, 24 Feb 2021 16:40:24 +0100
From:   John Ogness <john.ogness@...utronix.de>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org
Subject: Re: synchronization model: was: Re: [PATCH printk-rework 09/14] printk: introduce a kmsg_dump iterator

On 2021-02-24, John Ogness <john.ogness@...utronix.de> wrote:
> The @active flag is useless. It should be removed.

I would like to clarify my statement, because the @active flag _did_
protect the arch/um dumper until now. (Although it didn't actually
matter because arch/um does not have SMP or preemption support.)

In mainline we have 6 dumpers. They can be classified as follows:

1. Dumpers that provide their own synchronization to protect against
   parallel or nested dump() calls.

   - arch/powerpc/kernel/nvram_64.c
   - fs/pstore/platform.c
   - arch/um/kernel/kmsg_dump.c (after this series)

2. Dumpers that are safe because they only dump on KMSG_DUMP_PANIC,
   which (currently) can never happen in parallel or nested.

   - arch/powerpc/platforms/powernv/opal-kmsg.c
   - drivers/hv/vmbus_drv.c

3. Dumpers that are unsafe and even @active did not provide the needed
   synchronization.

   - drivers/mtd/mtdoops.c

In all 6 dumpers, @action does not provide any help. That is why it can
be removed.

But I am concerned about drivers/mtd/mtdoops.c that does not have any
synchronization. Since my series is adding sychronization to
arch/um/kernel/kmsg_dump.c, I suppose it should also add it to
drivers/mtd/mtdoops.c also.

And rather than moving the useless @active from kmsg_dumper to
kmsg_dump_iter, I should just drop it.

Unless there are any objections, I will make these changes for my v3.

John Ogness