linux-kernel - Re: [PATCH bpf-next v2 11/26] rqspinlock: Add deadlock detection and recovery

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Message-ID: <20250210103625.GK10324@noisy.programming.kicks-ass.net>
Date: Mon, 10 Feb 2025 11:36:25 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Kumar Kartikeya Dwivedi <memxor@...il.com>
Cc: bpf@...r.kernel.org, linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Will Deacon <will@...nel.org>, Waiman Long <llong@...hat.com>,
	Alexei Starovoitov <ast@...nel.org>,
	Andrii Nakryiko <andrii@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Martin KaFai Lau <martin.lau@...nel.org>,
	Eduard Zingerman <eddyz87@...il.com>,
	"Paul E. McKenney" <paulmck@...nel.org>, Tejun Heo <tj@...nel.org>,
	Barret Rhoden <brho@...gle.com>, Josh Don <joshdon@...gle.com>,
	Dohyun Kim <dohyunkim@...gle.com>,
	linux-arm-kernel@...ts.infradead.org, kernel-team@...a.com
Subject: Re: [PATCH bpf-next v2 11/26] rqspinlock: Add deadlock detection and
 recovery

On Thu, Feb 06, 2025 at 02:54:19AM -0800, Kumar Kartikeya Dwivedi wrote:
> +	/*
> +	 * Find the CPU holding the lock that we want to acquire. If there is a
> +	 * deadlock scenario, we will read a stable set on the remote CPU and
> +	 * find the target. This would be a constant time operation instead of
> +	 * O(NR_CPUS) if we could determine the owning CPU from a lock value, but
> +	 * that requires increasing the size of the lock word.
> +	 */

Is increasing the size of rqspinlock_t really a problem? For the kernel
as a whole there's very little code that really relies on spinlock_t
being u32 (lockref is an example that does care).

And it seems to me this thing might benefit somewhat significantly from
adding this little extra bit.