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

Peter Zijlstra
2011-06-10T09:43:08Z

On Fri, 2011-06-10 at 11:40 +0200, Peter Zijlstra wrote:
> 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.

Ah, not so, another cpu could start printing stuff.

So what we need is another test of log_start - log_end after up().