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

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Message-ID: <2426186.1744387151@warthog.procyon.org.uk>
Date: Fri, 11 Apr 2025 16:59:11 +0100
From: David Howells <dhowells@...hat.com>
To: Jarkko Sakkinen <jarkko@...nel.org>
Cc: dhowells@...hat.com, 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

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.

> +		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.

> +			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);

David