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...
> 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.
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