Re: [PATCH 1/3] printk: Release console_sem after logbuf_lock

Peter Zijlstra
2011-06-10T09:41:06Z

On Fri, 2011-06-10 at 11:33 +0200, Ingo Molnar wrote:
> * Andrew Morton <akpm@linux-foundation.org> wrote:
>
> > > Some notes:
> >
> > Stupid thing doesn't explain the magical locking though :( I'm
> > 99.9% sure that putting an up() inside a spinlock_irq()ed region
> > was deliberate.
>
> My guess would be it's done so that pending irqs that have queued up
> during our current printk-ing activities do not hit us with the
> console still locked.

Ah, so we already flushed the buffer, but have console_sem locked, so
any interrupt that comes in and prints something will place it in the
buffer but find console_sem is taken, so not flush it.

Then when we're back to doing up() the buffer is filled and nobody will
flush it.

I guess, we can write it like:

spin_unlock(&logbuf_lock);
up(&console_sem);
local_irq_restore(flags);

which would keep interrupt disabled over up(), but have the logbuf_lock
dropped.