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	Tue, 05 May 2026 22:50:29 +0000
Date: Tue, 5 May 2026 23:50:28 +0100
From: Al Viro <viro@zeniv.linux.org.uk>
To: Frederic Weisbecker <frederic@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	linux-fsdevel@vger.kernel.org,
	Christian Brauner <brauner@kernel.org>, Jan Kara <jack@suse.cz>,
	NeilBrown <neil@brown.name>, 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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Sender: Al Viro <viro@ftp.linux.org.uk>

On Tue, May 05, 2026 at 11:40:44PM +0200, Frederic Weisbecker wrote:

> > FWIW, back in 2015 spin_lock() did *not* provide an RCU read-side critical
> > area, according to https://lwn.net/Articles/652156/; would be nice to have
> > the history mentioned in D/RCU/whatisRCU.rst - at least to the level of
> > "once upon a time one *did* need an explicit rcu_read_lock() in addition
> > to spin_lock() - spin_lock() or preempt_disable() alone was not enough;
> > that has changed in <...>".
> > 
> > I'm not familiar enough with that code for that kind of archaeology ;-/
> > Could somebody help with that?  Paul?
> 
> That's quite outdated. Now preempt disabled, IRQs disabled, and spinlocks
> are all implicit RCU read sides. On non RT spinlocks disable preemption and
> on RT they call rcu_read_lock() internally.

Out of curiosity, how much headache do bitlocks cause on RT?  I mean, they
are unconditionally disabling preemption, RT or no RT...