linux-kernel - Re: [PATCH v8] KEYS: Add a list for unreferenced keys

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Message-ID: <Z_l9f45aO3CqYng_@kernel.org>
Date: Fri, 11 Apr 2025 23:37:19 +0300
From: Jarkko Sakkinen <jarkko@...nel.org>
To: David Howells <dhowells@...hat.com>
Cc: keyrings@...r.kernel.org, Jarkko Sakkinen <jarkko.sakkinen@...nsys.com>,
	stable@...r.kernel.org, Lukas Wunner <lukas@...ner.de>,
	Ignat Korchagin <ignat@...udflare.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	"David S. Miller" <davem@...emloft.net>,
	Peter Huewe <peterhuewe@....de>, Jason Gunthorpe <jgg@...pe.ca>,
	Paul Moore <paul@...l-moore.com>, James Morris <jmorris@...ei.org>,
	"Serge E. Hallyn" <serge@...lyn.com>,
	James Bottomley <James.Bottomley@...senpartnership.com>,
	Mimi Zohar <zohar@...ux.ibm.com>, linux-crypto@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-integrity@...r.kernel.org,
	linux-security-module@...r.kernel.org
Subject: Re: [PATCH v8] KEYS: Add a list for unreferenced keys

On Fri, Apr 11, 2025 at 04:59:11PM +0100, David Howells wrote:
> Jarkko Sakkinen <jarkko@...nel.org> wrote:
> 
> > +	spin_lock_irqsave(&key_graveyard_lock, flags);
> > +	list_splice_init(&key_graveyard, &graveyard);
> > +	spin_unlock_irqrestore(&key_graveyard_lock, flags);
> 
> I would wrap this bit in a check to see if key_graveyard is empty so that we
> can avoid disabling irqs and taking the lock if the graveyard is empty.

Can do, and does make sense.

> 
> > +		if (!refcount_inc_not_zero(&key->usage)) {
> 
> Sorry, but eww.  You're going to wangle the refcount twice on every key on the
> system every time the gc does a pass.  Further, in some cases inc_not_zero is
> not the fastest op in the world.

One could alternatively "test_bit(KEY_FLAG_FINAL_PUT, &key->flags)) &&
!refcount_inc_not_zero(&key->usage))" without mb() on either side and
set_bit() could be at the beginning of key_put().

Race at worst would be an extra refcount_inc_not_zero() but not often.

> 
> > +			spin_lock_irqsave(&key_graveyard_lock, flags);
> > +			list_add_tail(&key->graveyard_link, &key_graveyard);
> > +			spin_unlock_irqrestore(&key_graveyard_lock, flags);
> >  			schedule_work(&key_gc_work);
> 
> This is going to enable and disable interrupts twice and that can be
> expensive, depending on the arch.  I wonder if it would be better to do:
> 
> 			local_irq_save(flags);
> 			spin_lock(&key_graveyard_lock);
> 			list_add_tail(&key->graveyard_link, &key_graveyard);
> 			spin_unlock(&key_graveyard_lock);
> 			schedule_work(&key_gc_work);
> 			local_irq_restore(flags);

I like this but shouldn't this also comprehend the quota update before
(just asking for completeness sake)?

> 
> David
> 
> 

BR, Jarkko