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	Fri, 08 May 2026 14:47:54 +0000
Date: Fri, 8 May 2026 15:47:54 +0100
From: Al Viro <viro@zeniv.linux.org.uk>
To: "Paul E. McKenney" <paulmck@kernel.org>
Cc: NeilBrown <neil@brown.name>, Frederic Weisbecker <frederic@kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	linux-fsdevel@vger.kernel.org,
	Christian Brauner <brauner@kernel.org>, Jan Kara <jack@suse.cz>,
	Boqun Feng <boqun@kernel.org>,
	Joel Fernandes <joelagnelf@nvidia.com>,
	Uladzislau Rezki <urezki@gmail.com>
Subject: Re: [RFC PATCH 20/25] d_walk(): shrink rcu_read_lock() scope
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On Thu, May 07, 2026 at 04:21:28PM -0700, Paul E. McKenney wrote:

> Alternatively, if the bitlock acquisition is always a trylock-style
> operation with no tight loop around the trylock, it should not be
> necessary to disable preemption.  (Give or take the expectations of the
> code in the critical section, of course!)

In dcache we have two of those - hash chains in the main dentry hash
(insertion and removal only; RCU for traversals) and, more unpleasantly,
hash chains in the in-lookup hash.  Those have search-and-insert-new
and removal, the former with full traversal of the chain under the
lock.

The entries are there for duration of ->lookup(), so it stuffing a lot
of them in there might be feasible with a bunch of lookups for different
names on a stuck network filesystem (each with a separate thread blocked
waiting for server to reply)...