Re: [RFC RESEND v10 03/14] irq & spin_lock: Add counted interrupt disabling/enabling

[Peter Zijlstra <peterz@infradead.org>]

On Tue, May 27, 2025 at 06:21:44PM -0400, Lyude Paul wrote:
> From: Boqun Feng <boqun.feng@gmail.com>
> 
> Currently the nested interrupt disabling and enabling is present by
> _irqsave() and _irqrestore() APIs, which are relatively unsafe, for
> example:
> 
> 	<interrupts are enabled as beginning>
> 	spin_lock_irqsave(l1, flag1);
> 	spin_lock_irqsave(l2, flag2);
> 	spin_unlock_irqrestore(l1, flags1);
> 	<l2 is still held but interrupts are enabled>
> 	// accesses to interrupt-disable protect data will cause races.
> 
> This is even easier to triggered with guard facilities:
> 
> 	unsigned long flag2;
> 
> 	scoped_guard(spin_lock_irqsave, l1) {
> 		spin_lock_irqsave(l2, flag2);
> 	}
> 	// l2 locked but interrupts are enabled.
> 	spin_unlock_irqrestore(l2, flag2);
> 
> (Hand-to-hand locking critical sections are not uncommon for a
> fine-grained lock design)
> 
> And because this unsafety, Rust cannot easily wrap the
> interrupt-disabling locks in a safe API, which complicates the design.
> 
> To resolve this, introduce a new set of interrupt disabling APIs:
> 
> *	local_interrupt_disable();
> *	local_interrupt_enable();
> 
> They work like local_irq_save() and local_irq_restore() except that 1)
> the outermost local_interrupt_disable() call save the interrupt state
> into a percpu variable, so that the outermost local_interrupt_enable()
> can restore the state, and 2) a percpu counter is added to record the
> nest level of these calls, so that interrupts are not accidentally
> enabled inside the outermost critical section.
> 
> Also add the corresponding spin_lock primitives: spin_lock_irq_disable()
> and spin_unlock_irq_enable(), as a result, code as follow:
> 
> 	spin_lock_irq_disable(l1);
> 	spin_lock_irq_disable(l2);
> 	spin_unlock_irq_enable(l1);
> 	// Interrupts are still disabled.
> 	spin_unlock_irq_enable(l2);
> 
> doesn't have the issue that interrupts are accidentally enabled.
> 
> This also makes the wrapper of interrupt-disabling locks on Rust easier
> to design.
> 
> Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
> 
> ---
> V10:
> * Add missing __raw_spin_lock_irq_disable() definition in spinlock.c
> 
> Signed-off-by: Lyude Paul <lyude@redhat.com>

Your SOB is placed wrong, should be below Boqun's. This way it gets
lost.

Also, is there effort planned to fully remove the save/restore variant?
As before, my main objection is adding variants with overlapping
functionality while not cleaning up the pre-existing code.