netdev - Re: [PATCH] rhashtable: Fix potential deadlock by moving schedule

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Message-ID: <Z09nGHvk5YJABZ1d@slm.duckdns.org>
Date: Tue, 3 Dec 2024 10:16:24 -1000
From: Tejun Heo <tj@...nel.org>
To: Breno Leitao <leitao@...ian.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Thomas Graf <tgraf@...g.ch>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Hao Luo <haoluo@...gle.com>, Josh Don <joshdon@...gle.com>,
	Barret Rhoden <brho@...gle.com>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rhashtable: Fix potential deadlock by moving
 schedule_work outside lock

On Thu, Nov 28, 2024 at 04:16:25AM -0800, Breno Leitao wrote:
> Move the hash table growth check and work scheduling outside the
> rht lock to prevent a possible circular locking dependency.
> 
> The original implementation could trigger a lockdep warning due to
> a potential deadlock scenario involving nested locks between
> rhashtable bucket, rq lock, and dsq lock. By relocating the
> growth check and work scheduling after releasing the rth lock, we break
> this potential deadlock chain.
> 
> This change expands the flexibility of rhashtable by removing
> restrictive locking that previously limited its use in scheduler
> and workqueue contexts.
> 
> Import to say that this calls rht_grow_above_75(), which reads from
> struct rhashtable without holding the lock, if this is a problem, we can
> move the check to the lock, and schedule the workqueue after the lock.
> 
> Fixes: f0e1a0643a59 ("sched_ext: Implement BPF extensible scheduler class")
> Suggested-by: Tejun Heo <tj@...nel.org>
> Signed-off-by: Breno Leitao <leitao@...ian.org>

  Acked-by: Tejun Heo <tj@...nel.org>

This solves a possible deadlock for sched_ext and makes rhashtable more
useful and I don't see any downsides.

Andrew, can you please pick up this one?

Thanks.

-- 
tejun