Messages in this thread | | | Date | Fri, 11 Jan 2019 16:32:56 +0100 | From | Petr Mladek <> | Subject | Re: [PATCH] sysrq: Restore original console_loglevel when sysrq disabled |
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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...
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