Re: [PATCH] printk: Don't discard earlier unprinted messages to make space

[David Woodhouse <dwmw2@infradead.org>]

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On Thu, 2015-10-22 at 11:16 +0100, David Howells wrote:
> printk() currently discards earlier messages to make space for new messages
> arriving.  This has the distinct downside that if the kernel starts
> churning out messages because of some initial incident, the report of the
> initial incident is likely to be lost under a blizzard of:
> 
>         ** NNN printk messages dropped **
> 
> messages from console_unlock().
> 
> The first message generated (typically an oops) is usually the most
> important - the one you want to solve first - so we really want to see
> that.

But wait... didn't I watch you muttering on IRC about the actual bug
you were trying to catch here... and didn't you have a *serial* console
hooked up?

What broke such that serial console stopped giving you *every* message?

Serial console was always *synchronous*.

We could do stuff like...

   printk("Going to do foo...\n");
   outb(foo, baz);
   printk("Did foo and the machine didn't catch fire! Now bar\n");
   outb(bar, baz);
   printk("Done\n");

And with a serial console I could know *precisely* the point at which
the machine locked up.

And I could enable the silly debugging levels on things like JFFS2 and
be sure that with a serial console I could catch *every* printk
reliably — which led to a number of cases where people would reproduce
a bug with a serial console and debugging, mail me a huge log file, and
get a patch back in reply.

We *need* to have a mode where serial console is actually *reliable*,
and we can know that the message has been sent out the port before the
printk() call returns.

What happened to it? And how do we fix it?

-- 
dwmw2


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