linux-kernel - Re: [PATCH 5.14 232/849] leds: trigger: use RCU to protect the led

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Message-ID: <YZO6YwBAaOwatrQ8@kroah.com>
Date:   Tue, 16 Nov 2021 15:04:19 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Pavel Machek <pavel@....cz>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Johannes Berg <johannes.berg@...el.com>,
        Sasha Levin <sashal@...nel.org>
Subject: Re: [PATCH 5.14 232/849] leds: trigger: use RCU to protect the
 led_cdevs list

On Tue, Nov 16, 2021 at 12:41:47PM +0100, Pavel Machek wrote:
> On Mon 2021-11-15 17:55:15, Greg Kroah-Hartman wrote:
> > From: Johannes Berg <johannes.berg@...el.com>
> > 
> > [ Upstream commit 2a5a8fa8b23144d14567d6f8293dd6fbeecee393 ]
> > 
> > Even with the previous commit 27af8e2c90fb
> > ("leds: trigger: fix potential deadlock with libata")
> > to this file, we still get lockdep unhappy, and Boqun
> > explained the report here:
> > https://lore.kernel.org/r/YNA+d1X4UkoQ7g8a@boqun-archlinux
> > 
> > Effectively, this means that the read_lock_irqsave() isn't
> > enough here because another CPU might be trying to do a
> > write lock, and thus block the readers.
> > 
> > This is all pretty messy, but it doesn't seem right that
> > the LEDs framework imposes some locking requirements on
> > users, in particular we'd have to make the spinlock in the
> > iwlwifi driver always disable IRQs, even if we don't need
> > that for any other reason, just to avoid this deadlock.
> > 
> > Since writes to the led_cdevs list are rare (and are done
> > by userspace), just switch the list to RCU. This costs a
> > synchronize_rcu() at removal time so we can ensure things
> > are correct, but that seems like a small price to pay for
> > getting lock-free iterations and no deadlocks (nor any
> > locking requirements imposed on users.)
> > 
> > Signed-off-by: Johannes Berg <johannes.berg@...el.com>
> > Signed-off-by: Pavel Machek <pavel@....cz>
> > Signed-off-by: Sasha Levin <sashal@...nel.org>
> 
> Please drop. We discussed this with Johannes, and it was not marked
> for stable on purpose. Bug is rather obscure and change did not have
> enough testing.

Now dropped from 5.14.y and 5.15.y

thanks,

greg k-h