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

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Date:   Tue, 2 Mar 2021 11:12:41 +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, 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 Tue 2021-03-02 09:06:07, John Ogness wrote:
> 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.

Yeah, I could imagine myself to have different opinion another day
or year. I am sorry for the noise.

> But it really doesn't matter
> (especially for arch/um). I will use the _irqsave variant for v4. I am
> OK with that.

Please do, if it is easy. You are going to send v4 anyway.

Best Regards,
Petr