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Registered Address: Red Hat UK Ltd, Amberley Place, 107-111 Peascod Street, Windsor, Berkshire, SI4 1TE, United Kingdom. Registered in England and Wales under Company Registration No. 3798903 From: David Howells In-Reply-To: References: <20250430152554.23646-1-jarkko@kernel.org> To: Jarkko Sakkinen Cc: dhowells@redhat.com, keyrings@vger.kernel.org, Lukas Wunner , Ignat Korchagin , Herbert Xu , "David S. Miller" , Peter Huewe , Jason Gunthorpe , Paul Moore , James Morris , "Serge E. Hallyn" , James Bottomley , Mimi Zohar , 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() Precedence: bulk X-Mailing-List: linux-crypto@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-ID: <1121542.1746310761.1@warthog.procyon.org.uk> Date: Sat, 03 May 2025 23:19:21 +0100 Message-ID: <1121543.1746310761@warthog.procyon.org.uk> X-Scanned-By: MIMEDefang 3.0 on 10.30.177.40 Jarkko Sakkinen wrote: > 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. */ > } > 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. spin_unlock() is semi-permeable, so stuff after it can leak into the inside of it up as far as the spin_lock(). With your change, the garbage collector can no longer guarantee that key_put() will have done with accessing key->user when it sees KEY_FLAG_FINAL_PUT is set. So, NAK on this patch, I think. If you want a second opinion, I'd suggest waving it in front of Paul McKenney. Possibly we only need smp_mb() in the IN_QUOTA branch in key_put(). David