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Tue, 22 Apr 2025 10:46:59 -0700 (PDT) Received: from localhost ([2603:7000:c01:2716:365a:60ff:fe62:ff29]) by smtp.gmail.com with UTF8SMTPSA id d75a77b69052e-47ae9c16ba0sm57824681cf.4.2025.04.22.10.46.58 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 22 Apr 2025 10:46:58 -0700 (PDT) Date: Tue, 22 Apr 2025 13:46:57 -0400 From: Johannes Weiner To: Gregory Price Cc: linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@meta.com, longman@redhat.com, mhocko@kernel.org, roman.gushchin@linux.dev, shakeel.butt@linux.dev, muchun.song@linux.dev, tj@kernel.org, mkoutny@suse.com, akpm@linux-foundation.org Subject: Re: [PATCH] cpuset: relax locking on cpuset_node_allowed Message-ID: <20250422174657.GD1853@cmpxchg.org> References: <20250422012616.1883287-3-gourry@gourry.net> <20250422043055.1932434-1-gourry@gourry.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20250422043055.1932434-1-gourry@gourry.net> X-Rspam-User: X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: 2618616000D X-Stat-Signature: w6rcprc6gg84y6s7fg5cjh9umwrotrbm X-HE-Tag: 1745344019-440637 X-HE-Meta: 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 SzhIbXu1 G6oEFiczwGJz9EeftXCTyHn2CwZxQx+Qky66PlmWjWs/qMqSvyzvmRCaTeRMb4/H0IMhEYSnUdY0tcT45y2tvminsKcj4rIoNEdOAXwcfIwJDToDymeXzmx8c3Lvmtiqk3STaZrX8bi2DjiRtGmMgS6+q1F7AtHf+Rb5fGlwy2IYfi9e42WrxUwtpYK9Xi8whDIEdz8LkGzp9Br983cz6+9uvs3QAGAwkF5eiAQMG/aZc11t+c9+v3vAfzt+PFeeyoOB/dmg8swrUJ7uzAH6w0rSpb/T7vuha8Twvt89NLYKwtYqAlO48bUgxLVu+/CFMy+eTom0TO6aysP1WLI2HVZJ9MCfEtOYHIiVMRKXV4odi+K4eSsa3d8QK8k8UTWSmoPibgFUWoV3L0O39DCoZfmFo9NuNSOJB4Jyd3DN8eshKLg+EwyJWzdEZ3Duvi3lKKEU/1CPOFwsbTFMCUutTDTFnTRY1TzMgEt9RIygsADRlE5t+CjyN9FXUzhYW3GkX2VeWLdooLmb5vdDscRfiMo+q+rE/OLufFsm4bnVMCrpGKn4dZNqARLQJkB60AZKehurQ9Up0m/CjqeqDtHId8SgENnFyoK62sFtBIVhFMy1F6M/CfgQtjif02w== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, Apr 22, 2025 at 12:30:55AM -0400, Gregory Price wrote: > The cgroup_get_e_css reference protects the css->effective_mems, and > calls of this interface would be subject to the same race conditions > associated with a non-atomic access to cs->effective_mems. > > So while this interface cannot make strong guarantees of correctness, > it can therefore avoid taking a global or rcu_read_lock for performance. > > Drop the rcu_read_lock from cpuset_node_allowed. > > Suggested-by: Shakeel Butt > Suggested-by: Waiman Long > Signed-off-by: Gregory Price > --- > kernel/cgroup/cpuset.c | 14 ++++++++++++-- > 1 file changed, 12 insertions(+), 2 deletions(-) > > diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c > index c52348bfd5db..1dc41758c62c 100644 > --- a/kernel/cgroup/cpuset.c > +++ b/kernel/cgroup/cpuset.c > @@ -4181,10 +4181,20 @@ bool cpuset_node_allowed(struct cgroup *cgroup, int nid) > if (!css) > return true; > > + /* > + * Normally, accessing effective_mems would require the cpuset_mutex > + * or RCU read lock - but node_isset is atomic and the reference ^^^^^^^^^^^^^ This should be callback_lock. rcu_read_lock() was intended for css lifetime - which is ensured by css_get_e_css() - not a stable mask. Otherwise looks good, makes sense to lampshade the lockless access. Acked-by: Johannes Weiner