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[195.250.132.146]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-42f7cbff352sm26981207f8f.17.2025.12.08.10.11.32 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 08 Dec 2025 10:11:32 -0800 (PST) Date: Mon, 8 Dec 2025 19:11:31 +0100 From: Michal =?utf-8?Q?Koutn=C3=BD?= To: Shakeel Butt Cc: Tejun Heo , Johannes Weiner , "Paul E . McKenney" , JP Kobryn , Yosry Ahmed , linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, Meta kernel team Subject: Re: [PATCH v2] cgroup: rstat: use LOCK CMPXCHG in css_rstat_updated Message-ID: References: <20251205200106.3909330-1-shakeel.butt@linux.dev> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha512; protocol="application/pgp-signature"; boundary="xxdwkhj7rhflr6ss" Content-Disposition: inline In-Reply-To: <20251205200106.3909330-1-shakeel.butt@linux.dev> X-Stat-Signature: ss3qg718g78cr777g3hxzu3dtf8fgfkf X-Rspam-User: X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: 21D1F4000B X-HE-Tag: 1765217494-578919 X-HE-Meta: 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 OMJoiai2 d5gFgKwVMRjzi2eXCDB1dTOC35zdOXLOmtXK1HM7Gzq6UPcEuSMXUQi3OWeHFxfbe4t6HAMJ9NeOf9hP7b6TaZM8tNDfe3J6zEAM97Q+junCFOIj9nJmyuMNlE7I1o4ipAtQDQNdCXvuPLRjdsMYdZjn9HQDpEoMldlpIPvGPtdsE6AlzT7L3WECdO28VfSDK9ki9yY0dwsWCDPlyZepNzBwyi7Nd+7nyYAkEWn12f/fZrbijCGyeX8EIz0npGbF01TXdOGTMYlU1Blx3IBhAaL1WfwCUYTIIWsE1vknRQ0tXGAHMyxXjCffkpm9cP7a372BbHzsNevoUhd5xBL7J2taghNprk0xk8J9QKNScDsB7HzDgshsVghyblYeTNmomybjHMvVx9401mTWoZ7ac/Io9y99/VQ81+9yKGnsvw9C4yUf+aijDIpxd9YHpO2ocnO/LCpPtBBJCqqYJL8NXuJzOTACQWnPQ+8Ur 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: --xxdwkhj7rhflr6ss Content-Type: text/plain; protected-headers=v1; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable Subject: Re: [PATCH v2] cgroup: rstat: use LOCK CMPXCHG in css_rstat_updated MIME-Version: 1.0 On Fri, Dec 05, 2025 at 12:01:06PM -0800, Shakeel Butt wrote: > On x86-64, this_cpu_cmpxchg() uses CMPXCHG without LOCK prefix which > means it is only safe for the local CPU and not for multiple CPUs. =2E.. > The CMPXCNG without LOCK on CPU A is not safe and thus we need LOCK > prefix. Does it mean that this_cpu_cmpxchg() is generally useless? (It appears so from your analysis.) > Now concurrently CPU B is running the flusher and it calls > llist_del_first_init() for CPU A and got rstatc_pcpu->lnode of cgroup C > which was added by the IRQ/NMI updater. Or it's rather the case where rstat code combines both this_cpu_* and remote access from the flusher. Documentation/core-api/this_cpu_ops.rst washes its hands with: | Please note that accesses by remote processors to a per cpu area are | exceptional situations and may impact performance and/or correctness | (remote write operations) of local RMW operations via this_cpu_*. I see there's currently only one other user of that in kernel/scs.c (__scs_alloc() vs scs_cleanup() without even WRITE_ONCE, but the race would involve CPU hotplug, so its impact may be limited(?)). I think your learnt-the-hard-way discovery should not only be in cgroup.c but also in this this_cpu_ops.rst document to be wary especially with this_cpu_cmpxchg (when dealing with pointers and not more tolerable counters). > Consider this scenario: Updater for cgroup stat C on CPU A in process > context is after llist_on_list() check and before this_cpu_cmpxchg() in > css_rstat_updated() where it get interrupted by IRQ/NMI. In the IRQ/NMI > context, a new updater calls css_rstat_updated() for same cgroup C and > successfully inserts rstatc_pcpu->lnode. >=20 > Now imagine CPU B calling init_llist_node() on cgroup C's > rstatc_pcpu->lnode of CPU A and on CPU A, the process context updater > calling this_cpu_cmpxchg(rstatc_pcpu->lnode) concurrently. Sounds feasible to me. Thanks, Michal --xxdwkhj7rhflr6ss Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iJEEABYKADkWIQRCE24Fn/AcRjnLivR+PQLnlNv4CAUCaTcU0RsUgAAAAAAEAA5t YW51MiwyLjUrMS4xMSwyLDIACgkQfj0C55Tb+Ah/WgD+KoG41+NA0shwqndSMfLf NKaf3TLNxpT5rbMR72AmSkIA/0gUzIXL2v+ADYvYPkK/71qUo+l1A7Ki6u8nCkZi f5gN =Aghw -----END PGP SIGNATURE----- --xxdwkhj7rhflr6ss--