Re: [PATCH] printk: Add printk_flush() to force buffered text to console

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Fri, Jun 15, 2012 at 07:53:35PM -0400, Steven Rostedt wrote:
> On Fri, 2012-06-15 at 16:13 -0700, Greg Kroah-Hartman wrote:
> 
> > > Please apply Steve's fix, fix it yourself or revert the changes that
> > > regressed printk().
> > 
> > I thought Steve's patch was just a RFC thing, is it really something
> > that everyone wants to see applied?
> 
> It was, and still is, an RFC. I'd like to fix the double printing of the
> time stamps as well.
> 
> It was really a compromise, as the new printk() is a serious
> functionality change. Even though I mostly use trace_printk(), there are
> still times that I like to use printk in some hot spots. For example I
> sometimes to do:
> 
> printk("a");
> some_code();
> printk("b");
> more_code();
> printk("c");
> continued_code();
> [etc]
> 
> 
> And watch a stream of 'abcabcabc...' and see where finally something
> causes the machine to triple fault and reboot.

Yeah, that's not good, but note that because of this change, other
things that were failing are now working properly, we really can't have
one without the other here :(

> Because the bug would cause a triple fault, there would be no time to
> recover data from trace_printk(). But as the code is quite hot, my use
> of printk is to be as small as possible and with no new lines.
> 
> But now this method is gone. Not to mention, digging through the complex
> maze of the new printk, I realize that it's not optimize for speed at
> all. But printing to the serial console makes it not that big of a deal.
> 
> I have patches that just force printk() to be early_printk(), and in
> fact, we have in the -rt patch this:
> 
> http://git.kernel.org/?p=linux/kernel/git/rt/linux-stable-rt.git;a=blobdiff;f=kernel/printk.c;h=5a172f9e5027b7416a82e54dfc29afbda71296ee;hp=c4426061e228c252e226feb67193d80cdba3af5b;hb=865407ef06e61c97cdd9091082ed8038c801f535;hpb=c09d1c02743a7d2df13574d93dea9f21ccf02560
> 
> I had my own patch that did pretty much the same thing, and I was very
> happy to see that Ingo had the same mind set.
> 
> Actually, I think that patch really should go mainline, and give others
> some of the luxury that us -rt folks enjoy.

That would be good.

> But back to this patch. I was irritated that we blamed the wrong code
> because the printk functionality changed. And instead of writing some
> drastic changes that would be controversial, I suggested a
> 'printk_flush()' that would work similar to a fflush(). Yeah, buffering
> is good and lets lines fit together, but there's times where getting the
> partial line out to the screen is more important that keeping it
> together.

I agree, it's frustrating to go down the wrong path, sorry about that.

> If other printks were interleaved and it didn't crash, we wouldn't care
> (or even notice). But if it did crash, I'll even argue seeing the
> interleaved printks would provide a hint to why it crashed.

Ok, so what do you want to do here?  Fix up your RFC patch to not have
the printk stamps?  Or something like the above rt patch?

thanks,

greg k-h