From: Herbert Xu <herbert@gondor.apana.org.au>
To: Jarkko Sakkinen <jarkko@kernel.org>
Cc: keyrings@vger.kernel.org, David Howells <dhowells@redhat.com>,
Lukas Wunner <lukas@wunner.de>,
Ignat Korchagin <ignat@cloudflare.com>,
"David S. Miller" <davem@davemloft.net>,
Peter Huewe <peterhuewe@gmx.de>, Jason Gunthorpe <jgg@ziepe.ca>,
Paul Moore <paul@paul-moore.com>,
James Morris <jmorris@namei.org>,
"Serge E. Hallyn" <serge@hallyn.com>,
James Bottomley <James.Bottomley@hansenpartnership.com>,
Mimi Zohar <zohar@linux.ibm.com>,
linux-crypto@vger.kernel.org, linux-kernel@vger.kernel.org,
linux-integrity@vger.kernel.org,
linux-security-module@vger.kernel.org
Subject: Re: [PATCH] KEYS: Reduce smp_mb() calls in key_put()
Date: Sat, 3 May 2025 23:02:57 +0800 [thread overview]
Message-ID: <aBYwIcy5JCOamAkj@gondor.apana.org.au> (raw)
In-Reply-To: <aBYqlBoSq4FwiDKD@kernel.org>
On Sat, May 03, 2025 at 05:39:16PM +0300, Jarkko Sakkinen wrote:
> On Wed, Apr 30, 2025 at 06:25:53PM +0300, Jarkko Sakkinen wrote:
> > Rely only on the memory ordering of spin_unlock() when setting
> > KEY_FLAG_FINAL_PUT under key->user->lock in key_put().
> >
> > Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>
> > ---
> > security/keys/key.c | 6 ++++--
> > 1 file changed, 4 insertions(+), 2 deletions(-)
> >
> > diff --git a/security/keys/key.c b/security/keys/key.c
> > index 7198cd2ac3a3..aecbd624612d 100644
> > --- a/security/keys/key.c
> > +++ b/security/keys/key.c
> > @@ -656,10 +656,12 @@ void key_put(struct key *key)
> > spin_lock_irqsave(&key->user->lock, flags);
> > key->user->qnkeys--;
> > key->user->qnbytes -= key->quotalen;
> > + set_bit(KEY_FLAG_FINAL_PUT, &key->flags);
> > spin_unlock_irqrestore(&key->user->lock, flags);
> > + } else {
> > + set_bit(KEY_FLAG_FINAL_PUT, &key->flags);
> > + smp_mb(); /* key->user before FINAL_PUT set. */
> > }
> > - smp_mb(); /* key->user before FINAL_PUT set. */
> > - set_bit(KEY_FLAG_FINAL_PUT, &key->flags);
>
> Oops, my bad (order swap), sorry. Should have been:
>
> spin_unlock_irqrestore(&key->user->lock, flags);
> } else {
> smp_mb(); /* key->user before FINAL_PUT set. */
You can use smp_mb__before_atomic here as it is equivalent to
smp_mb in this situation.
> }
> set_bit(KEY_FLAG_FINAL_PUT, &key->flags);
>
> Should spin_lock()/unlock() be good enough or what good does smp_mb() do
> in that branch? Just checking if I'm missing something before sending
> fixed version.
I don't think spin_unlock alone is enough to replace an smp_mb.
A spin_lock + spin_unlock would be enough though.
However, looking at the bigger picture this smp_mb looks bogus.
What exactly is it protecting against?
The race condition that this is supposed to fix should have been
dealt with by the set_bit/test_bit of FINAL_PUT alone. I don't
see any point in having this smb_mb at all.
Cheers,
--
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
next prev parent reply other threads:[~2025-05-03 15:03 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-04-30 15:25 [PATCH] KEYS: Reduce smp_mb() calls in key_put() Jarkko Sakkinen
2025-05-03 14:39 ` Jarkko Sakkinen
2025-05-03 15:02 ` Herbert Xu [this message]
2025-05-04 16:55 ` Jarkko Sakkinen
2025-05-03 22:19 ` David Howells
2025-05-04 0:35 ` Herbert Xu
2025-05-04 5:36 ` [PATCH] KEYS: Invert FINAL_PUT bit Herbert Xu
2025-05-04 7:44 ` David Howells
2025-05-04 7:52 ` [v2 PATCH] " Herbert Xu
2025-05-09 9:34 ` kernel test robot
2025-05-09 9:45 ` [v3 " Herbert Xu
2025-05-12 9:19 ` David Howells
2025-05-12 11:42 ` Jarkko Sakkinen
2025-05-12 12:01 ` David Howells
2025-05-12 12:07 ` Herbert Xu
2025-05-04 16:42 ` [PATCH] KEYS: Reduce smp_mb() calls in key_put() Jarkko Sakkinen
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=aBYwIcy5JCOamAkj@gondor.apana.org.au \
--to=herbert@gondor.apana.org.au \
--cc=James.Bottomley@hansenpartnership.com \
--cc=davem@davemloft.net \
--cc=dhowells@redhat.com \
--cc=ignat@cloudflare.com \
--cc=jarkko@kernel.org \
--cc=jgg@ziepe.ca \
--cc=jmorris@namei.org \
--cc=keyrings@vger.kernel.org \
--cc=linux-crypto@vger.kernel.org \
--cc=linux-integrity@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-security-module@vger.kernel.org \
--cc=lukas@wunner.de \
--cc=paul@paul-moore.com \
--cc=peterhuewe@gmx.de \
--cc=serge@hallyn.com \
--cc=zohar@linux.ibm.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).