linux-kernel - Re: [PATCH next v3 01/15] um: synchronize kmsg

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Date:   Tue, 02 Mar 2021 09:06:07 +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, Jeff Dike <jdike@...toit.com>,
        Richard Weinberger <richard@....at>,
        Anton Ivanov <anton.ivanov@...bridgegreys.com>,
        Thomas Meyer <thomas@...3r.de>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        linux-um@...ts.infradead.org
Subject: Re: [PATCH next v3 01/15] um: synchronize kmsg_dumper

On 2021-03-01, Petr Mladek <pmladek@...e.com> wrote:
>> > diff --git a/arch/um/kernel/kmsg_dump.c b/arch/um/kernel/kmsg_dump.c
>> > index 6516ef1f8274..4869e2cc787c 100644
>> > --- a/arch/um/kernel/kmsg_dump.c
>> > +++ b/arch/um/kernel/kmsg_dump.c
>> > @@ -1,5 +1,6 @@
>> >  // SPDX-License-Identifier: GPL-2.0
>> >  #include <linux/kmsg_dump.h>
>> > +#include <linux/spinlock.h>
>> >  #include <linux/console.h>
>> >  #include <linux/string.h>
>> >  #include <shared/init.h>
>> > @@ -9,6 +10,7 @@
>> >  static void kmsg_dumper_stdout(struct kmsg_dumper *dumper,
>> >  				enum kmsg_dump_reason reason)
>> >  {
>> > +	static DEFINE_SPINLOCK(lock);
>> >  	static char line[1024];
>> >  	struct console *con;
>> >  	size_t len = 0;
>> > @@ -29,11 +31,16 @@ static void kmsg_dumper_stdout(struct kmsg_dumper *dumper,
>> >  	if (con)
>> >  		return;
>> >  
>> > +	if (!spin_trylock(&lock))
>> 
>> I have almost missed this. It is wrong. The last version correctly
>> used
>> 
>> 	if (!spin_trylock_irqsave(&lock, flags))
>> 
>> kmsg_dump(KMSG_DUMP_PANIC) is called in panic() with interrupts
>> disabled. We have to store the flags here.
>
> Ah, I get always confused with these things. spin_trylock() can
> actually get called in a context with IRQ disabled. So it is not
> as wrong as I thought.
>
> But still. panic() and kmsg_dump() can be called in IRQ context.
> So, this function might be called in IRQ context. So, it feels
> more correct to use the _irqsafe variant here.
>
> I know that there is the trylock so it probably does not matter much.
> Well, the disabled irq might help to serialize the two calls when
> one is in normal context and the other would happen in IRQ one.
>
> As I said, using _irqsafe variant looks better to me.

For the record, the reason I removed the _irqsave for v3 is because I
felt like it was misleading, appearing to be necessary when it is not.

I think anyone could argue both sides. But it really doesn't matter
(especially for arch/um). I will use the _irqsave variant for v4. I am
OK with that.

John Ogness