From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id E7C3F3C3F68; Thu, 16 Jul 2026 21:31:01 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784237463; cv=none; b=PIbj5GK/QmdagZ0Korzn8KOcvrteybMby25i8f69r1HvxpbI3VIBKRHIfKKoAAHgDW5SqDgzMOI3NNt/2t2G87J6QQtqjC/JWmjCKSOtkdrNKCWPxE9KyES8xDf5DDPusijrAOmalBP+91S9UB0x4z4Z+oNE63f0woUjxz8UDxE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784237463; c=relaxed/simple; bh=5XcRkEatVXnCaiaZDch8URxqKDgAzOMTWN03R+Wgkmg=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=DACGG+zXraOgjvVu8o4d2m1Jv8HWDXJt1GKkpybT3ks8AQKZ2xIR58+3jwZIb/duQfJ95QRPp6ogb4+6kUDp96HHr0raDyonBFw88DYJIc5fAXlGVaPQ27BiUV7kqwA0ERmcoBbXf6vyHJMjyFek3Pr1y2iwi4WX5aWplu/PUAc= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=gVF6cmHz; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="gVF6cmHz" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 778C71F000E9; Thu, 16 Jul 2026 21:31:01 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1784237461; bh=egTboFz70l3OBqWVwTUDzfHwtFuMvrqkhwzSSovfjVE=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=gVF6cmHzSSQVZnvNJz7pPT97RGsnL+KYe+mqZaaVbZ4qz34a2rCXuBKUqU6Y6rdYC OJ+cj4cARdaeZOdRjLXRfIWAdYUzrt9JEKlP7AruKdV18XzL8mepjRFY6ToLnkqk1a gHFH7EcbV7/+j+l7FXl9TsZs1U0hOYnWPc/UASCsFDUtsVCyg4Cw2x3vUq1ljvvG3c V+cFpyyLUFa6hqc5/mpctktAJajxoTf+lwMEVu9nW9fUiBX3Q59J6FbX7jQ09DZObd jff8wP8SzFA+SVPtDo6yaus6smu5RW3TbYaFLE6bZVt7ExIr6Xznbc/PcRK8vrYtqa ogadb8lrr3Ejw== From: Tejun Heo To: David Vernet , Andrea Righi , Changwoo Min Cc: sched-ext@lists.linux.dev, Emil Tsalapatis , linux-kernel@vger.kernel.org, Tejun Heo , stable@vger.kernel.org Subject: [PATCH 2/4] sched_ext: Take cgroup_lock() first in scx_cgroup_lock() Date: Thu, 16 Jul 2026 11:30:56 -1000 Message-ID: <20260716213058.1739522-3-tj@kernel.org> X-Mailer: git-send-email 2.55.0 In-Reply-To: <20260716213058.1739522-1-tj@kernel.org> References: <20260716213058.1739522-1-tj@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit scx_cgroup_lock() write-locks scx_cgroup_ops_rwsem and then takes cgroup_lock(), which can deadlock through kernfs: scx enable/disable cgroup rmdir cpu.weight write ------------------ ------------ ---------------- cgroup_lock() percpu_down_write(rwsem) cgroup_lock() kernfs_get_active() percpu_down_read(rwsem) kernfs_drain() The enable path waits for the rmdir to release cgroup_mutex. The rmdir, deactivating the cpu controller's files, waits in kernfs_drain() for the write's active reference. The write, in scx_group_set_weight(), waits for the rwsem behind the pending writer. Take cgroup_lock() first. The set_* paths take no cgroup locks inside the read side, so a pending write-lock then only waits for read sections that always run to completion, and no dependency from the rwsem back to cgroup_mutex remains. Fixes: a5bd6ba30b33 ("sched_ext: Use cgroup_lock/unlock() to synchronize against cgroup operations") Cc: stable@vger.kernel.org # v6.18+ Signed-off-by: Tejun Heo --- kernel/sched/ext/ext.c | 9 +++++++-- 1 file changed, 7 insertions(+), 2 deletions(-) diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c index 46f135bddd46..58898cd0727b 100644 --- a/kernel/sched/ext/ext.c +++ b/kernel/sched/ext/ext.c @@ -4580,20 +4580,25 @@ static struct cgroup *root_cgroup(void) return &cgrp_dfl_root.cgrp; } +/* + * cgroup_lock() must nest outside the rwsem write side: a writer waiting + * for cgroup_mutex deadlocks with cgroup teardown, which holds it while + * draining a set_* file write blocked on the rwsem behind the writer. + */ static void scx_cgroup_lock(void) { + cgroup_lock(); #ifdef CONFIG_EXT_GROUP_SCHED percpu_down_write(&scx_cgroup_ops_rwsem); #endif - cgroup_lock(); } static void scx_cgroup_unlock(void) { - cgroup_unlock(); #ifdef CONFIG_EXT_GROUP_SCHED percpu_up_write(&scx_cgroup_ops_rwsem); #endif + cgroup_unlock(); } #else /* CONFIG_EXT_GROUP_SCHED || CONFIG_EXT_SUB_SCHED */ static inline struct cgroup *root_cgroup(void) { return NULL; } -- 2.55.0