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

[John Blackwood <john.blackwood@ccur.com>]

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.