From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-wm1-f73.google.com (mail-wm1-f73.google.com [209.85.128.73]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id D2F7821D3D6 for ; Mon, 8 Dec 2025 11:15:16 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.128.73 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1765192519; cv=none; b=aEHlrQ9qVSh8uOyMuvY+BbuQ5rA8v/MdW1I2Jn5eTi0mwBx8B2gj1vLYKT/S0ckSpng/1FU59u73dRr+UoOrfivXzgUjf6+h4Uq7+jVTsGXGisXf9SMFm+5BkdAEygNJAAfyJPXsCUSXg6TpUfOEN3SxCpVmyl+RuDhpF/mCtJc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1765192519; c=relaxed/simple; bh=sj7sNphhYZaVF5qP7MRJ+wFIXaN25bN/qgpFeh/bFkA=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Cc:Content-Type; b=Q+AHTovFsPvzhh1WMza4xYUAqVn08187Fo8K/gRNxYJ43Xb47QeLPyMaKUe9U/5MWTm+JLbNe31FF6W2OLY0QDC6ZYRNfRjR6oaWwB+Sxof/KwTKAR9fAQ3QgYqPGeVwoLB7rcLJ8bEarCIHMnv/KavzyL2Rf2EqorrW8WXYSl0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com; spf=pass smtp.mailfrom=flex--jpiecuch.bounces.google.com; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b=Lueurns3; arc=none smtp.client-ip=209.85.128.73 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=flex--jpiecuch.bounces.google.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="Lueurns3" Received: by mail-wm1-f73.google.com with SMTP id 5b1f17b1804b1-4779b3749a8so33481245e9.1 for ; Mon, 08 Dec 2025 03:15:16 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1765192515; x=1765797315; darn=lists.linux.dev; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:from:to:cc:subject:date:message-id:reply-to; bh=sj7sNphhYZaVF5qP7MRJ+wFIXaN25bN/qgpFeh/bFkA=; b=Lueurns377RC1u0NUgWABIxgq/nUToH4GdcYDDX2sG03Hd/WcBZ+nKAE8bY0BFY2fM 7BwjZp57OAJk4DNawBiBaejhoLg+VfSRpTbN5jg3BKrolGK27xCdAmwSMbBh+RxxPgHb nALtbiaoWlOeBNl21vxCySN+QbTKNzyWyoGwwNSl5OmKzDosfnghmtoIQCQ+3htrzHMO wDRpYcWkU9yegeFl2A5xWs4EH+atjKyhJnC9HSOGApx2FZYknNJfDBWPg2V6ra+r7mvQ IP38vQ/9T0XKaELnd4Ce4tC/YZBJi5stDj0OlMgZ83Bbkhkq9Zxedipkpm7HyOc6v4vt ABog== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1765192515; x=1765797315; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:x-gm-message-state:from:to:cc:subject:date:message-id:reply-to; bh=sj7sNphhYZaVF5qP7MRJ+wFIXaN25bN/qgpFeh/bFkA=; b=PPhWVyFi8H+jYS6N80ZXPbmrQLbA7Y3jZnxwAJAYI8PcQqoeS/I29wGIKbULUZHA3J bpazzstLURsJSsPkrBOMC8AamMvvMGIDecY7vznFNjcI7n5T9lZmtXUcn0wICEclEhjh UPH1IBeGZWcEmbqJtVQVYytxNJXq04vx9fyKBWYpPGvepaM/5ESneQLGntDZzQMsw+uq 449XE6lE/+Xu1jOxR6TOzUa+PYg//2oszlU22Vuud5r6oHBI9FHQxypsUv3ivuO7G+zj rirjZUYuwQ4BFEK2K6c20ZxOF547FllD8XsppR3qSQRT/7oSNSVc+wHILmMDTItLqTSR 0DaA== X-Forwarded-Encrypted: i=1; AJvYcCXYEx8aQ0JxI4FS/bY0nQT/GyWjirJsN0HgpyJdmsGqLm5RZl8aX+1pmY37LlrMdTPFh+N3V6uc+6I=@lists.linux.dev X-Gm-Message-State: AOJu0YyXf1FzxAR9UK8bn300G8E5KbZptw2yiRy2Q19tv9NHkpYYuhH7 axfxMcg8wtPEgwAkcDZi2487F1JG1BGTT3NbBhuCfkTGgB+my8ATVtKjm7praRcPXnRbuchTdEl gNbrsVFUnXBqQug== X-Google-Smtp-Source: AGHT+IHgil5oa21F4CeSi52vizMTZQ9YndgxtOBxLFoV0jCWSTP89KlmMN4RCZcHpaF+jM9XC0kYM0SuwxPNlA== X-Received: from wmma16.prod.google.com ([2002:a05:600c:2250:b0:475:d94e:4d5d]) (user=jpiecuch job=prod-delivery.src-stubby-dispatcher) by 2002:a05:600c:4fd6:b0:479:3876:22a8 with SMTP id 5b1f17b1804b1-47939e1deacmr81009055e9.16.1765192514810; Mon, 08 Dec 2025 03:15:14 -0800 (PST) Date: Mon, 08 Dec 2025 11:15:14 +0000 In-Reply-To: Precedence: bulk X-Mailing-List: sched-ext@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20251206022218.1541878-1-jstultz@google.com> X-Mailer: aerc 0.20.1 Message-ID: Subject: Re: [RFC][PATCH] sched/ext: Avoid null ptr traversal when ->put_prev_task() is called with NULL next From: Kuba Piecuch To: Kuba Piecuch , John Stultz , LKML Cc: Joel Fernandes , Qais Yousef , Ingo Molnar , Peter Zijlstra , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Valentin Schneider , Steven Rostedt , Ben Segall , Zimuzo Ezeozue , Mel Gorman , Will Deacon , Waiman Long , Boqun Feng , "Paul E. McKenney" , Metin Kaya , Xuewen Yan , K Prateek Nayak , Thomas Gleixner , Daniel Lezcano , Suleiman Souhlal , kuyo chang , hupu , Tejun Heo , David Vernet , Andrea Righi , Changwoo Min , , Content-Type: text/plain; charset="UTF-8" On Mon Dec 8, 2025 at 10:10 AM UTC, Kuba Piecuch wrote: > It looks like it's impossible for an outside observer holding a CPU's rq lock > to observe a task that is running on that CPU and isn't queued, i.e. > 'running' implies 'queued' (I'm new to the scheduler so I may be wrong here). A task that blocks in __schedule() can drop the rq lock while picking the next task, which is after try_to_block_task() dequeues prev. So it's very much possible for a task on another CPU to grab the rq lock and observe prev as dequeued but still running.