linux-kernel - Re: [PATCHv2] printk: queue wake_up_klogd irq

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Message-ID: <20200304152159.2p7d7dnztf433i24@pathway.suse.cz>
Date:   Wed, 4 Mar 2020 16:21:59 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org,
        Lech Perczak <l.perczak@...lintechnologies.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Theodore Ts'o <tytso@....edu>,
        John Ogness <john.ogness@...utronix.de>
Subject: Re: [PATCHv2] printk: queue wake_up_klogd irq_work only if per-CPU
 areas are ready

On Tue 2020-03-03 20:30:02, Sergey Senozhatsky wrote:
> 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. The reason is printk_deferred() being
> called too early (as was pointed out by Petr and John).
> 
> Fix printk_deferred() and do not queue per-CPU irq_work
> before per-CPU areas are initialized.
> 
> [0] https://lore.kernel.org/lkml/aa0732c6-5c4e-8a8b-a1c1-75ebe3dca05b@camlintechnologies.com/
> 
> Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
> Reported-by: Lech Perczak <l.perczak@...lintechnologies.com>
> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Cc: Theodore Ts'o <tytso@....edu>
> Cc: John Ogness <john.ogness@...utronix.de>

Reviewed-by: Petr Mladek <pmladek@...e.com>

Now, the question is whether to hurry this fix into 5.6 or if
it could wait for 5.7.

I think that it could wait because 5.6 is not affected by
the particular printk_deferred(). This patch fixes a long-term
generic problem. But I am open for other opinions.

Best Regards,
Petr