From: Petr Mladek <pmladek@suse.com>
To: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
Jiri Slaby <jslaby@suse.com>,
Steven Rostedt <rostedt@goodmis.org>,
Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH] sysrq: Restore original console_loglevel when sysrq disabled
Date: Fri, 11 Jan 2019 16:32:56 +0100 [thread overview]
Message-ID: <20190111153256.emsne4s37g2dgt4g@pathway.suse.cz> (raw)
In-Reply-To: <20190111130729.GA31790@tigerII.localdomain>
On Fri 2019-01-11 22:07:29, Sergey Senozhatsky wrote:
> On (01/11/19 13:45), Petr Mladek wrote:
> > The sysrq header line is printed with an increased loglevel
> > to provide users some positive feedback.
> >
> > The original loglevel is not restored when the sysrq operation
> > is disabled. This bug was introduced in 2.6.12 (pre-git-history)
> > by the commit ("Allow admin to enable only some of the Magic-Sysrq
> > functions").
>
>
> Good find, and the patch looks OK to me. A small comment below.
> FWIW,
> Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Thanks.
> A side note (nitpick, etc.); it's Friday night in here, I'm enjoying
> my beer; so maybe I'm wrong about the whole thing.
>
>
> > @@ -553,6 +553,7 @@ void __handle_sysrq(int key, bool check_mask)
> > op_p->handler(key);
> > } else {
> > pr_cont("This sysrq operation is disabled.\n");
> > + console_loglevel = orig_log_level;
> > }
>
> This looks a bit racy.
>
> When we do
>
> printk("FOO\n");
> console_loglevel = XYZ;
>
> We don't have any real guarantees that printk("FOO\n") will print
> anything straight ahead. It is possible that console_sem is already
> locked and the owner is preempted, so by the time the console_sem
> owner picks up that FOO\n messages, console_loglevel is back to
> orig_log_level and suppress_message_printing() will just tell us
> to skip the message.
>
> Do we need pr_cont() there? Maybe we can just have a normal pr_err()
> which would always tell that "key" sysrq is disabled? (we also
> would need to change the error message a bit).
The same problem is with the sysrq header line. It uses the trick
with console_loglevel by intention. We want to show it but
it is not really an error message, see the commit
fb144adc517d9ebe8fd ("sysrq: add commentary on why we use
the console loglevel over using KERN_EMERG").
Best Regards,
Petr
PS: I am going to resend this patch as part of a patchset.
I was working on one more fix. It looked independent
and questionable. I wanted to send it as RFC separately
but there is a clash in the end...
next prev parent reply other threads:[~2019-01-11 15:32 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-01-11 12:45 [PATCH] sysrq: Restore original console_loglevel when sysrq disabled Petr Mladek
2019-01-11 13:07 ` Sergey Senozhatsky
2019-01-11 15:32 ` Petr Mladek [this message]
2019-01-14 5:36 ` Sergey Senozhatsky
2019-01-14 14:37 ` Petr Mladek
2019-01-11 16:24 ` Steven Rostedt
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