linux-kernel - Re: [PATCH 0/2 v4] Init the hashed pointer from a worker.

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Date:   Thu, 29 Sep 2022 14:43:33 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, Theodore Ts'o <tytso@....edu>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        "Jason A . Donenfeld " <Jason@...c4.com>,
        John Ogness <john.ogness@...utronix.de>,
        Mike Galbraith <efault@....de>,
        Peter Zijlstra <peterz@...radead.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 0/2 v4] Init the hashed pointer from a worker.

On Tue 2022-09-27 12:49:10, Sebastian Andrzej Siewior wrote:
> This is a mini series to initialize the random value, needed for the %p
> format argument, upfront during boot instead on demand. The latter is
> problematic on PREEMPT_RT if the first user happens to be in an atomic
> region.
> 
> v3…v4:
>     - Added a __read_mostly.
>     - Added Jason's Acked-by for 2/2 after talking to him at Plumbers.
>       While we were discussion several ways of tackling this differently
>       and the possible problems/ side effects that this may cause we
>       happen to notice that the current way of doing things is also a
>       problem if the first printk("%p\n") user happens to be in NMI
>       context.
>       Therefore I leave it to the vsprintf/ printk maintainer to decide
>       if this is -stable material or not. I'm not aware of any NMI code
>       path using %p but then it is not officially forbidden.
>       Assuming unknown_nmi_error() contains %p format the string, then
>       the backtrace at the end of the email will be printed.

JFYI, both patches are committed into printk/linux.git,
branch for-6.1-hash-pointer-init

I have just added a note into the 2nd patch about that it also
prevents deadlock when printk("%p", ptr) is called under the lock
used by get_random_bytes(), see
https://git.kernel.org/pub/scm/linux/kernel/git/printk/linux.git/commit/?h=for-6.1-hash-pointer-init&id=6f0ac3b52a9075b7291a72fb338d08491c1f0a64

I did not modify the code. The system_unbound_wq can and should be
fixed separately.

Best Regards,
Petr