linux-kernel - Re: [PATCH 1/4] blkcg: Drop unnecessary RCU read [un]locks from blkg_conf

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Date:   Mon, 9 Jan 2023 10:48:55 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Christoph Hellwig <hch@....de>
Cc:     axboe@...nel.dk, josef@...icpanda.com, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/4] blkcg: Drop unnecessary RCU read [un]locks from
 blkg_conf_prep/finish()

Hello, Christoph.

On Sun, Jan 08, 2023 at 06:02:40PM +0100, Christoph Hellwig wrote:
> On Thu, Jan 05, 2023 at 11:24:29AM -1000, Tejun Heo wrote:
> > Holding the queue lock now implies RCU read lock, so no need to use
> > rcu_read_[un]lock() explicitly. This shouldn't cause any behavior changes.
> 
> How so?

Now that all RCU flavors have been combined, holding a spin lock, disabling
irq, disabling preemption all imply RCU read lock.

> > While at it, drop __acquires() annotation on the queue lock too. The
> > __acquires() part was already out of sync and it doesn't catch anything that
> > lockdep can't.
> 
> This makes sparse even more unhappy than it was before.  For now
> please keep the annotation.

I can drop the changes but this actually bothers me. The annotation has been
broken for a *long* time and nobody noticed. Furthermore, I can't remember a
time when __acquires/__releases notation caught anything that lockdep
couldn't trivially and can't even think of a way how it could. AFAICS, these
annotations don't contribute anything other than preservation of themselves.
I don't see why we would want to keep them.

Thanks.

-- 
tejun