Re: sleep under spinlock, sequencer.c, 2.6.12.5

Peter T. Breuer
2005-08-22T21:09:13Z

"Also sprach Nish Aravamudan:"
> On 8/19/05, Alan Cox <alan@lxorguk.ukuu.org.uk> wrote:
> > On Gwe, 2005-08-19 at 10:13 +0200, Peter T. Breuer wrote:
> > > The following "sleep under spinlock" is still present as of linux
> > > 2.6.12.5 in sound/oss/sequencer.c in midi_outc:
> > >
> > >
> > > n = 3 * HZ; /* Timeout */
> > >
> > > spin_lock_irqsave(&lock,flags);
> > > while (n && !midi_devs[dev]->outputc(dev, data)) {
> > > interruptible_sleep_on_timeout(&seq_sleeper, HZ/25);
> > > n--;
> > > }
> > > spin_unlock_irqrestore(&lock,flags);
> > >
> > >
> > > I haven't thought about it, just noted it. It's been there forever
> > > (some others in the sound architecture have been gradually disappearing
> > > as newer kernels come out).
> >
> > Yep thats a blind substition of lock_kernel in an old tree it seems.
> > Probably my fault. Should drop it before the sleep and take it straight
> > after.
>
> Also, the use of n makes no sense. Indicates total sleep for 3

Well spotted.

> seconds, but actually sleep for 40 milliseconds 3*HZ times
> (potentially)?

I presume it should be

n -= HZ/25;

(and "n > 0", of course).

> In any case, probably should be:
>
> timeout = jiffies + 3*HZ;
>
> spin_lock_irqsave(&lock, flags);
> while (time_before(jiffies, timeout) && !midi_devs[dev]->outputc(dev, data)) {
> spin_unlock_irqrestore(&lock, flags);
> interruptible_sleep_on_timeout(&seq_sleeper, msecs_to_jiffies(40));

Well, you'd know. Is there something there really not taken care of by
"HZ"?

> spin_lock_irqsave(&lock, flags);
> }
> spin_lock_irqrestore(&lock, flags);

Peter