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

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Date:   Tue, 2 Mar 2021 13:48:23 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     John Ogness <john.ogness@...utronix.de>
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 Tue 2021-03-02 11:45:27, John Ogness wrote:
> 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 was wrong. I meant the tricks that are under in console drivrers,
for example:

static void mvebu_uart_console_write(struct console *co, const char *s,
				     unsigned int count)
{
	int locked = 1;

	if (oops_in_progress)
		locked = spin_trylock_irqsave(&port->lock, flags);
	else
		spin_lock_irqsave(&port->lock, flags);

	/* do the job */

	if (locked)
		spin_unlock_irqrestore(&port->lock, flags);
}

But this is not a problem here because the kmsg dumper bails out
when the lock could not be taken.

> 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.

Makes sense. Could you please mention this in the commit message?

Best Regards,
Petr