Interrupts disabled for too long in printk

[Mathieu Desnoyers <compudj@krystal.dyndns.org>]

Hi,

I ran some experiments with my kernel tracer (LTTng : http://ltt.polymtl.ca)
that showed missing interrupts. I wrote a small paper to show how to use my
tracer to solve this kind of problem which I presented at the CE Linux Form
last April.

http://tree.celinuxforum.org/CelfPubWiki/ELC2006Presentations?action=AttachFile&do=get&target=celf2006-desnoyers.pdf

It shows that, when the serial console is activated, the following code disables
interrupts for up to 15ms. On a system configured with a 250HZ timer (each 4ms),
it means that 3 scheduler ticks are lost.

In the current git :

kernel/printk.c: release_console_sem()

        for ( ; ; ) {
----->          spin_lock_irqsave(&logbuf_lock, flags);
                wake_klogd |= log_start - log_end;
                if (con_start == log_end)
                        break;                  /* Nothing to print */
                _con_start = con_start;
                _log_end = log_end;
                con_start = log_end;            /* Flush */
                spin_unlock(&logbuf_lock);
                call_console_drivers(_con_start, _log_end);
----->          local_irq_restore(flags);
        }

I guess interrupts are disabled for a good reason (to protect this spinlock for
being taken by a nested interrupt handler. One way I am thinking to fix this
problem would be to do a spin try lock and fail if it is already taken.

Mathieu


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