Re: [PATCH v2 01/20] locking/rt: Use raw_spin_lock_irqsave() in __rwbase_read_unlock()

[Sean Christopherson <seanjc@google.com>]

On Fri, May 29, 2026, Peter Zijlstra wrote:
> On Fri, May 29, 2026 at 09:32:14PM +0200, Peter Zijlstra wrote:
> > On Fri, May 29, 2026 at 09:50:55AM -0700, Sean Christopherson wrote:
> > > From: David Woodhouse <dwmw@amazon.co.uk>
> > > 
> > > __rwbase_read_unlock() uses raw_spin_lock_irq()/raw_spin_unlock_irq()
> > > which unconditionally disables and re-enables interrupts. When
> > > read_unlock() is called from hardirq context (e.g. after a successful
> > > read_trylock() in a timer callback), the raw_spin_unlock_irq()
> > > incorrectly re-enables interrupts within the hardirq handler.
> > > 
> > > This causes lockdep warnings ('hardirqs_on_prepare' from hardirq
> > > context) and can lead to IRQ state corruption.
> > > 
> > > Using read_trylock() in hardirq context on PREEMPT_RT is safe because
> > > it does not record the lock owner. The read_unlock() acquires the
> > > wait_lock which is hardirq safe. This change additionally allows
> > > rwlock_t during early boot.
> 
> Forgot to reply to this; it is safe with this implementation. If we were
> to ever do reader owner tracking this goes sideways real fast.
> 
> I really think this is a very bad approach.
> 
> > > Switch to raw_spin_lock_irqsave()/raw_spin_unlock_irqrestore() to
> > > preserve the caller's IRQ state.
> > > 
> > > Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
> > > Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> > > Signed-off-by: Sean Christopherson <seanjc@google.com>
> > 
> > We have very specifically not supported the: trylock+unlock from hardirq
> > (although typically this comes up for mutex). Specifically with PI this
> > can lead to trying to boost the idle thread.
> > 
> > Consider doing this from an interrupt that hits idle, then idle becomes
> > the 'owner' of a successful acquisition. This is absolutely broken.

I assume the only alternative is to implement raw versions of rwlock?  Or do I
understand all of this even less than I thought? :-)