linux-kernel - Re: [PATCH next v3 02/15] mtd: mtdoops: synchronize kmsg

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Message-ID: <87tuptq1fc.fsf@jogness.linutronix.de>
Date:   Tue, 02 Mar 2021 11:45:27 +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,
        Miquel Raynal <miquel.raynal@...tlin.com>,
        Richard Weinberger <richard@....at>,
        Vignesh Raghavendra <vigneshr@...com>,
        linux-mtd@...ts.infradead.org
Subject: Re: [PATCH next v3 02/15] mtd: mtdoops: synchronize kmsg_dumper

On 2021-03-01, Petr Mladek <pmladek@...e.com> wrote:
>> The kmsg_dumper can be called from any context and CPU, possibly
>> from multiple CPUs simultaneously. Since the writing of the buffer
>> can occur from a later scheduled work queue, the oops buffer must
>> be protected against simultaneous dumping.
>> 
>> Use an atomic bit to mark when the buffer is protected. Release the
>> protection in between setting the buffer and the actual writing in
>> order for a possible panic (immediate write) to be written during
>> the scheduling of a previous oops (delayed write).
>
> Just to be sure. You did not use spin lock to prevent problems
> with eventual double unlock in panic(). Do I get it correctly,
> please?

I do not understand what possible double unlock you are referring to.

I chose not to use spinlocks because I wanted something that does not
cause any scheduling or preemption side-effects for mtd. The mtd dumper
sometimes dumps directly, sometimes delayed (via scheduled work), and
they use different mtd callbacks in different contexts.

mtd_write() expects to be called in a non-atomic context. The callbacks
can take a mutex.

John Ogness