linux-kernel - Re: [PATCH 0/5] Make it easier to use nested locking for interrupt descriptors

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Message-ID: <87r04eolz5.ffs@tglx>
Date: Mon, 03 Feb 2025 20:40:46 +0100
From: Thomas Gleixner <tglx@...utronix.de>
To: Bart Van Assche <bvanassche@....org>
Cc: Peter Zijlstra <peterz@...radead.org>, linux-kernel@...r.kernel.org,
 Bart Van Assche <bvanassche@....org>
Subject: Re: [PATCH 0/5] Make it easier to use nested locking for interrupt
 descriptors

Bart!

On Mon, Feb 03 2025 at 09:59, Bart Van Assche wrote:
> The number of irq_set_lockdep_class() calls and IRQ_GC_INIT_NESTED_LOCK
> occurrences in the kernel tree shows that nested locking is common for
> interrupt descriptors.

I can see why you think it's cumbersome to nest interrupts, but calling
it common is pretty exaggerated. If my mental arithmetic is not
completely off, then this affects less than 5% of the interrupt
chip/handler implementations.

The restriction is there on purpose because nesting interrupts is the
exception and not the common case. If there is no real reason, i.e. a
hardware constraint, then interrupt nesting is a bug by definition.

> This patch series makes it easier to use nested
> locking by removing the function irq_set_lockdep_class() and also the
> IRQ_GC_INIT_NESTED_LOCK flag.
>
> As one can tell from the diffstat, this patch series removes more code
> than it adds.

Sure, but with that it removes a debug mechanism for the common case,
which is not nested. Line count is not a measure for quality, neither in
one nor in the other direction.

So if you want to go there, then you have to come up with a proper
explanation why removing the 'yell, if not annotated nesting'
functionality is not a problem and the 'simplificaiton' is actually
worth it.

Thanks,

        tglx