From: Sergey Senozhatsky <senozhatsky@chromium.org>
To: John Ogness <john.ogness@linutronix.de>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>,
Petr Mladek <pmladek@suse.com>,
Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
Steven Rostedt <rostedt@goodmis.org>,
Thomas Gleixner <tglx@linutronix.de>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH next v1 1/3] printk: track/limit recursion
Date: Mon, 22 Mar 2021 20:13:51 +0900 [thread overview]
Message-ID: <YFh770T97jwQLRKu@google.com> (raw)
In-Reply-To: <87mtuvmpcl.fsf@jogness.linutronix.de>
On (21/03/22 11:53), John Ogness wrote:
> On 2021-03-21, Sergey Senozhatsky <senozhatsky@chromium.org> wrote:
> >> @@ -2055,6 +2122,9 @@ int vprintk_store(int facility, int level,
> >> */
> >> ts_nsec = local_clock();
> >>
> >> + if (!printk_enter_irqsave(&irqflags))
> >> + return 0;
> >
> > I guess it can be interesting to somehow signal us that we had
> > printk() recursion overflow, and how many messages we lost.
>
> Honestly, if we hit 3 levels of recursion, we are probably dealing with
> an infinite recursion issue.
I tend to agree.
> I do not see the value of counting the overflows in that case. The
> logged messages at that recursion level would ben enough to point
> us to the problem.
>
> > 3 levels of recursion seem like reasonable limit, but I maybe wouldn't
> > mind one extra level.
>
> With 3 levels, we will see all the messages of:
>
> printk -> WARN_ON -> WARN_ON -> WARN_ON
Well, not necessarily this simple.
printk
vsprintf
handle_foo_specifier
printk
call_console_drivers
timekeeping
printk
vsprintf
We saw in the past that enabling CONFIG_DEBUG_OBJECTS (if I'm not
mistaken) can add quite a bit of extra printk recursion paths.
We also have other CONFIG_DEBUG_* config options that can pop up as
recursive printk-s here and there. For instance, from vsprintf::foo_specifier()
where we escape from printk() to various kernel subsystems: net, block,
etc.
Maybe sometimes on level 3+ we'll see something interesting,
but I've no strong opinion on this.
> Keep in mind that each additional level causes the reading of the logs
> to be significantly more complex. Each level increases the output
> exponentially:
Yes, I realize that. That's why I suggested that maybe recursive
printk-s can have some special extra prefix. Recursive printk-s
will interleave with whatever is being printed on this_cpu, so
prefix might be helpful.
-ss
next prev parent reply other threads:[~2021-03-22 11:14 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-16 23:33 [PATCH next v1 0/3] printk: remove safe buffers John Ogness
2021-03-16 23:33 ` [PATCH next v1 1/3] printk: track/limit recursion John Ogness
2021-03-21 5:34 ` Sergey Senozhatsky
2021-03-22 10:53 ` John Ogness
2021-03-22 11:13 ` Sergey Senozhatsky [this message]
2021-03-22 15:07 ` Petr Mladek
2021-03-22 14:49 ` Petr Mladek
2021-03-23 21:32 ` John Ogness
2021-03-24 8:41 ` Petr Mladek
2021-03-16 23:33 ` [PATCH next v1 2/3] printk: remove safe buffers John Ogness
2021-03-21 5:26 ` Sergey Senozhatsky
2021-03-22 11:16 ` John Ogness
2021-03-22 18:02 ` Petr Mladek
2021-03-22 21:58 ` John Ogness
2021-03-23 9:46 ` Petr Mladek
2021-03-23 10:47 ` Petr Mladek
2021-03-26 11:12 ` John Ogness
2021-03-29 10:04 ` Petr Mladek
2021-03-29 15:10 ` John Ogness
2021-03-29 15:13 ` John Ogness
2021-03-16 23:33 ` [PATCH next v1 3/3] printk: convert @syslog_lock to spin_lock John Ogness
2021-03-23 12:01 ` Petr Mladek
2021-03-26 11:23 ` John Ogness
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