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Wed, 29 Jan 2025 11:53:09 -0800 (PST) Received: from localhost ([2a00:79e0:9d:4:524e:3286:ba64:f255]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-38c2a1885besm17686399f8f.49.2025.01.29.11.53.08 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 29 Jan 2025 11:53:08 -0800 (PST) From: Jann Horn Date: Wed, 29 Jan 2025 20:53:03 +0100 Subject: [PATCH] sched: Clarify wake_up_q()'s write to task->wake_q.next Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable Message-Id: <20250129-sched-wakeup-prettier-v1-1-2f51f5f663fa@google.com> X-B4-Tracking: v=1; b=H4sIAB6HmmcC/x2MSwqAMBDFriKzdkAH/F5FXJT2qYOg0voD8e4Wl yEkDwV4RaA2ecjj1KDrEiFPE7KTWUawusgkmRRZLg0HO8HxZWYcG28e+67wbKWuSnG1A0CxjWL Q+/92/ft+Cr5QrmcAAAA= X-Change-ID: 20250129-sched-wakeup-prettier-c28762d8deee To: Ingo Molnar , Peter Zijlstra , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Valentin Schneider Cc: linux-kernel@vger.kernel.org, Jann Horn X-Mailer: b4 0.15-dev X-Developer-Signature: v=1; a=ed25519-sha256; t=1738180384; l=1295; i=jannh@google.com; s=20240730; h=from:subject:message-id; bh=wNpLHO6Ho0iQWCKLMbBlQJLflykh3AtzSgS+lNMlW8o=; b=cJ9E/3jov7vnCKDj9LQZUmS93eFMMeIM7tue5IpikIhTJlef4+6YIi7gZUNfWFjONzAgXtGtV TTtZLv5soRSCVXtfCT5uWab/cO3xQANsQtNZhiLnorN0clww88IO2fT X-Developer-Key: i=jannh@google.com; a=ed25519; pk=AljNtGOzXeF6khBXDJVVvwSEkVDGnnZZYqfWhP1V+C8= Clarify that wake_up_q() does an atomic write to task->wake_q.next, after which a concurrent __wake_q_add() can immediately overwrite task->wake_q.next again. Signed-off-by: Jann Horn --- a minor change; I figured we should at least be using WRITE_ONCE() here, and I might as well change the comments a little bit while I'm touching this --- kernel/sched/core.c | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-) diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 9142a0394d46605e96e10cef97cce02d741f6c93..ce64652858703826dca510479f5= 63a28c2fb2405 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -1063,9 +1063,10 @@ void wake_up_q(struct wake_q_head *head) struct task_struct *task; =20 task =3D container_of(node, struct task_struct, wake_q); - /* Task can safely be re-inserted now: */ node =3D node->next; - task->wake_q.next =3D NULL; + /* pairs with cmpxchg_relaxed() in __wake_q_add() */ + WRITE_ONCE(task->wake_q.next, NULL); + /* Task can safely be re-inserted now. */ =20 /* * wake_up_process() executes a full barrier, which pairs with --- base-commit: 05dbaf8dd8bf537d4b4eb3115ab42a5fb40ff1f5 change-id: 20250129-sched-wakeup-prettier-c28762d8deee --=20 Jann Horn