linux-kernel - Re: [PATCH printk v4 0/6] printk: remove safe buffers

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Message-ID: <YP+1PqaKTfZXHjmU@alley>
Date:   Tue, 27 Jul 2021 09:26:54 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Eric Biederman <ebiederm@...ssion.com>,
        Nicholas Piggin <npiggin@...il.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Cédric Le Goater <clg@...d.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        Tiezhu Yang <yangtiezhu@...ngson.cn>,
        Alexey Kardashevskiy <aik@...abs.ru>,
        Yue Hu <huyue2@...ong.com>, linuxppc-dev@...ts.ozlabs.org,
        kexec@...ts.infradead.org, Russell King <linux@...linux.org.uk>,
        Ingo Molnar <mingo@...hat.com>, Marc Zyngier <maz@...nel.org>,
        Valentin Schneider <valentin.schneider@....com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Mike Rapoport <rppt@...nel.org>,
        "Wolfram Sang (Renesas)" <wsa+renesas@...g-engineering.com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Xiongwei Song <sxwjean@...il.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nick Terrell <terrelln@...com>,
        Vipin Sharma <vipinsh@...gle.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Daniel Borkmann <daniel@...earbox.net>,
        Vlastimil Babka <vbabka@...e.cz>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH printk v4 0/6] printk: remove safe buffers

On Thu 2021-07-15 21:39:53, John Ogness wrote:
> Hi,
> 
> Here is v4 of a series to remove the safe buffers. v3 can be
> found here [0]. The safe buffers are no longer needed because
> messages can be stored directly into the log buffer from any
> context.
> 
> However, the safe buffers also provided a form of recursion
> protection. For that reason, explicit recursion protection is
> implemented for this series.
> 
> The safe buffers also implicitly provided serialization
> between multiple CPUs executing in NMI context. This was
> particularly necessary for the nmi_backtrace() output. This
> serializiation is now preserved by using the printk cpulock.
> 
> With the removal of the safe buffers, there is no need for
> extra NMI enter/exit tracking. So this is also removed
> (which includes removing the config option CONFIG_PRINTK_NMI).
> 
> And finally, there are a few places in the kernel that need to
> specify code blocks where all printk calls are to be deferred
> printing. Previously the NMI tracking API was being (mis)used
> for this purpose. This series introduces an official and
> explicit interface for such cases. (Note that all deferred
> printing will be removed anyway, once printing kthreads are
> introduced.)
> 
> John Ogness (6):
>   lib/nmi_backtrace: explicitly serialize banner and regs
>   printk: track/limit recursion
>   printk: remove safe buffers
>   printk: remove NMI tracking
>   printk: convert @syslog_lock to mutex
>   printk: syslog: close window between wait and read

The entire patchset has been committed into printk/linux.git,
branch rework/printk_safe-removal.

Note that I have updated the 4th patch as discussed, see
https://lore.kernel.org/r/20210721120026.y3dqno24ahw4sazy@pathway.suse.cz
https://lore.kernel.org/r/20210721130852.zrjnti6b3fwjgdzj@pathway.suse.cz

Best Regards,
Petr