linux-kernel - Re: [PATCH 4.19 130/133] printk: queue wake_up_klogd irq

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Message-ID: <20200720220506.GB11552@amd>
Date:   Tue, 21 Jul 2020 00:05:06 +0200
From:   Pavel Machek <pavel@...x.de>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Lech Perczak <l.perczak@...lintechnologies.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Jann Horn <jannh@...gle.com>, Petr Mladek <pmladek@...e.com>,
        Theodore Tso <tytso@....edu>,
        John Ogness <john.ogness@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 4.19 130/133] printk: queue wake_up_klogd irq_work only
 if per-CPU areas are ready

Hi!

> From: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
> 
> commit ab6f762f0f53162d41497708b33c9a3236d3609e upstream.
> 
> printk_deferred(), similarly to printk_safe/printk_nmi, does not
> immediately attempt to print a new message on the consoles, avoiding
> calls into non-reentrant kernel paths, e.g. scheduler or timekeeping,
> which potentially can deadlock the system.
> 
> Those printk() flavors, instead, rely on per-CPU flush irq_work to print
> messages from safer contexts.  For same reasons (recursive scheduler or
> timekeeping calls) printk() uses per-CPU irq_work in order to wake up
> user space syslog/kmsg readers.
> 
> However, only printk_safe/printk_nmi do make sure that per-CPU areas
> have been initialised and that it's safe to modify per-CPU irq_work.
> This means that, for instance, should printk_deferred() be invoked "too
> early", that is before per-CPU areas are initialised, printk_deferred()
> will perform illegal per-CPU access.
> 
> Lech Perczak [0] reports that after commit 1b710b1b10ef ("char/random:
> silence a lockdep splat with printk()") user-space syslog/kmsg readers
> are not able to read new kernel messages.

Is this still needed in 4.19? 1b710b1b10ef was reverted in 4.19, so
there should not be any user-visible problems...

Best regards,
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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