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SubjectRe: [PATCH] sysrq: Restore original console_loglevel when sysrq disabled
On Fri, 11 Jan 2019 22:07:29 +0900
Sergey Senozhatsky <sergey.senozhatsky@gmail.com> 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>
>
>
> ---
>
> 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).
>

All this is for another patch and another discussion. This current
patch keeps with what is there and fixes a missing reset of
console_loglevel.

Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

-- Steve

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