RE: [BUG][2.6.8.1] serial driver hangs SMP kernel, but not the UPkernel

Stuart MacDonald
2004-11-01T15:36:30Z

From: Paul Fulghum [mailto:paulkf@microgate.com]
> Stuart MacDonald wrote:
> > I always thought the whole point of low_latency was to make the
> > receive-path very fast, which means specifically allowing the flip
> > routine to run from the ISR. So checking for calling from
> the ISR and
> > specifically disallowing that is basically negating the
> entire raison
> > d'etre for low_latency.
>
> I was thought it was to speed processing if the
> caller was already in process context. Maybe the
> real intentions are lost to history.

Best person to ask may be Ted; he was once the serial maintainer. Ted?

> Moving forward, Alan stated that the flip
> routine should not be called in interrupt context.
> His last post concerning some transient state
> of low_latency has confused me.

I didn't follow that either, but I wasn't reading too closely.

> Currently, with the 8250 driver and N_TTY
> line discipline, calling the flip routine from
> ISR causes an SMP deadlock. There are two paths that
> cause this:
> 1. low_latency is set
> 2. flip buffer becomes full
>
> So calling the flip routine from the ISR may work
> with some specific drivers, but it would be
> dangerous to assume this works in all cases.

I haven't looked at the 2.6 serial rewrite in depth yet, but the
problem always existed in the 2.4 driver. I got around the problem by
checking for interrupt context and taking the locks or not at a much
earlier stage.

..Stu