linux-kernel - Re: [PATCH printk v2 26/38] kdb: use srcu console list iterator

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Message-ID: <Y1f53hZkpyAgfz7/@alley>
Date:   Tue, 25 Oct 2022 16:59:42 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Doug Anderson <dianders@...omium.org>
Cc:     John Ogness <john.ogness@...utronix.de>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org,
        Jason Wessel <jason.wessel@...driver.com>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Aaron Tomlin <atomlin@...hat.com>,
        kgdb-bugreport@...ts.sourceforge.net
Subject: Re: [PATCH printk v2 26/38] kdb: use srcu console list iterator

On Mon 2022-10-24 17:47:25, Doug Anderson wrote:
> Hi,
> 
> On Wed, Oct 19, 2022 at 7:56 AM John Ogness <john.ogness@...utronix.de> wrote:
> >
> > Guarantee safe iteration of the console list by using SRCU.
> >
> > Signed-off-by: John Ogness <john.ogness@...utronix.de>
> > ---
> >  kernel/debug/kdb/kdb_io.c | 5 ++++-
> >  1 file changed, 4 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/debug/kdb/kdb_io.c b/kernel/debug/kdb/kdb_io.c
> > index 550fe8b456ec..5c0bd93c3574 100644
> > --- a/kernel/debug/kdb/kdb_io.c
> > +++ b/kernel/debug/kdb/kdb_io.c
> > @@ -545,6 +545,7 @@ static void kdb_msg_write(const char *msg, int msg_len)
> >  {
> >         struct console *c;
> >         const char *cp;
> > +       int cookie;
> >         int len;
> >
> >         if (msg_len == 0)
> > @@ -558,7 +559,8 @@ static void kdb_msg_write(const char *msg, int msg_len)
> >                 cp++;
> >         }
> >
> > -       for_each_console(c) {
> > +       cookie = console_srcu_read_lock();
> > +       for_each_console_srcu(c) {
> 
> Maybe it wouldn't hurt to also have a comment saying that normally the
> console_srcu_read_lock() wouldn't be enough given that we're poking
> into each individual console and calling ->write() but that we're
> relying on the fact that all the other CPUs are stopped at the moment
> and thus we should be safe.

True. I guess that the SRCU lock is not really needed from the same
reason.

Well, the SRCU walk makes sense. It makes sure that the list can be
safely traversed. I mean that pointers are updated and read in
the right order with the right barriers.

Yes, it would be nice to add a comment.

Best Regards,
Petr