linux-kernel - Re: [PATCH bpf-next v2 09/26] rqspinlock: Protect waiters in queue from stalls

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Message-ID: <20250210101730.GI10324@noisy.programming.kicks-ass.net>
Date: Mon, 10 Feb 2025 11:17:30 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Kumar Kartikeya Dwivedi <memxor@...il.com>
Cc: bpf@...r.kernel.org, linux-kernel@...r.kernel.org,
	Barret Rhoden <brho@...gle.com>,
	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>,
	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 09/26] rqspinlock: Protect waiters in queue
 from stalls

On Thu, Feb 06, 2025 at 02:54:17AM -0800, Kumar Kartikeya Dwivedi wrote:
> Implement the wait queue cleanup algorithm for rqspinlock. There are
> three forms of waiters in the original queued spin lock algorithm. The
> first is the waiter which acquires the pending bit and spins on the lock
> word without forming a wait queue. The second is the head waiter that is
> the first waiter heading the wait queue. The third form is of all the
> non-head waiters queued behind the head, waiting to be signalled through
> their MCS node to overtake the responsibility of the head.
> 
> In this commit, we are concerned with the second and third kind. First,
> we augment the waiting loop of the head of the wait queue with a
> timeout. When this timeout happens, all waiters part of the wait queue
> will abort their lock acquisition attempts. 

Why? Why terminate the whole wait-queue?

I *think* I understand, but it would be good to spell out. Also, in the
comment.