Re: [ofa-general] [PATCH] [WORKAROUND] CONFIG_PREEMPT_RT and ib_umad_close() issue

John Blackwood
2007-09-17T23:41:43Z

Roland Dreier wrote:
> Thanks for the explanation...
>
> > But basically, with CONFIG_PREEMPT_RT enabled, the lock points, such as
> > aqcuiring a spinlock, potentially become places where the current task
> > may be context switched out / preempted.
> >
> > Therefore, when a call is made to lock a spinlock for example, the
> > caller should not currently have irqs disabled, or preemption disabled,
> > since a context switch may occur.
>
> this doesn't seem relevant here...

Hi Roland,

right. just some background info.

> > void fastcall rt_downgrade_write(struct rw_semaphore *rwsem)
> > {
> > BUG();
> > }
>
> this seems to be the problem... the -rt patch turns downgrade_write()
> into a BUG().
>
> I need to look at the locking in user_mad.c again, but I think it may
> be possible to replace both places that do downgrade_write() with
> up_write() followed by down_read().
>
> - R.

that sounds like it would be a good solution for both preempt rt and
non-preempt rt kernels.

thanks again for looking at this for us.