Re: [RFC PATCH 0/7] locking/rtqspinlock: Realtime queued spinlocks

[Peter Zijlstra]

On Thu, Jan 05, 2017 at 10:55:55AM -0500, Waiman Long wrote:
> What I am saying that we don't need to change spinlock to rtmutex in a
> -RT kernel. Instead, we can use rtqspinlock for this purpose. All the
> sleeping locks will still be converted to rtmutex.

No-no-no..

> Conversion of rtmutex does allow forced CPU preemption when there is a
> need for that. What rtqspinlock can provide is voluntary preemption
> where the lock waiters explicitly yield the CPU while waiting for the
> lock. I use the need_resched() to detect if CPU yielding is necessary.
> However, if the CPU was in a preempt disabled region before the
> spin_lock() call, we can't yield the CPU. The only way is to raise its
> priority and try to get the lock ASAP.

And here you've lost your finger because the saw-blade didn't stop in
time.

RT very fundamentally relies on the spinlock->rtmutex conversion to
allow preempting things when a higher priority task comes along. A
spinlock, of any kind, requires having preemption disabled while holding
the lock. If the critical section is of unbounded latency, you have
unbounded preemption latency and RT is no more.

Its not about PI on contention, although that helps inversion scenarios.
Its about allowing preemption, which fundamentally requires a sleeping
lock to be used.

Many of the spinlock sections of mainline are not well behaved in an RT
sense and therefore must not disable preemption. Similar for the IRQ
disable regions and hence we have the whole threaded interrupt stuff.

Please stop writing code and read up on things..