From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 44C0827979A; Wed, 8 Apr 2026 18:37:32 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775673452; cv=none; b=pB6MdmxaRHXKUX7h9q+1ekMEJkDHeyQPVIKMMEhDrGhEY54VYfFcY6ppShhCOXXzeqcGorV0s+THkYbXIOvE/SolXDiCy0NA4dlp7SER53mVwtrAJMnacNalO9WVy/Al6yUJt/NT4fohHrtgdGeAJYCzdoGfdD+O+U2N++Usolk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775673452; c=relaxed/simple; bh=ujUtUitJ9prwfI+J/Ad8lYPLG4JoXqujVXoArB8DEho=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=MfhixkfzFut98mC45cOMZSldUUNYpw5yVXueaHkQFS7rINRWNGx26Rc5PDfGDEVCc6jIHAYFKfc2mem61xfwrt3AFMj/nOPOX9nM6f0uGXD/uebLcSUxNu8/eDFYmyfKjaJNHfAOEjvQwo5jNU4HdK+3qSaS9Df0YkYtTfsh0Ds= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=nO94OMoB; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="nO94OMoB" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 8B55FC2BC87; Wed, 8 Apr 2026 18:37:31 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1775673451; bh=ujUtUitJ9prwfI+J/Ad8lYPLG4JoXqujVXoArB8DEho=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=nO94OMoBwfLAdeHvvnyjkoCZToNOsmO0TZDyJMwu36BsPIlwWSfRZZFFbdS+jyyqe hR5xgtZTJJTd/fXqnnkUfifpabFpGN1nEO+hAjKHxuYp4WWUo44Midk7D2+KrP9eDg 8+dYPYHL5beouFcoWhmTobqHvUIYO8QPpI7ZA0HQ= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, Changwoo Min , Andrea Righi , Tejun Heo Subject: [PATCH 6.18 237/277] sched_ext: Fix is_bpf_migration_disabled() false negative on non-PREEMPT_RCU Date: Wed, 8 Apr 2026 20:03:42 +0200 Message-ID: <20260408175942.707235386@linuxfoundation.org> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20260408175933.836769063@linuxfoundation.org> References: <20260408175933.836769063@linuxfoundation.org> User-Agent: quilt/0.69 X-stable: review X-Patchwork-Hint: ignore Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 6.18-stable review patch. If anyone has any objections, please let me know. ------------------ From: Changwoo Min commit 0c4a59df370bea245695c00aaae6ae75747139bd upstream. Since commit 8e4f0b1ebcf2 ("bpf: use rcu_read_lock_dont_migrate() for trampoline.c"), the BPF prolog (__bpf_prog_enter) calls migrate_disable() only when CONFIG_PREEMPT_RCU is enabled, via rcu_read_lock_dont_migrate(). Without CONFIG_PREEMPT_RCU, the prolog never touches migration_disabled, so migration_disabled == 1 always means the task is truly migration-disabled regardless of whether it is the current task. The old unconditional p == current check was a false negative in this case, potentially allowing a migration-disabled task to be dispatched to a remote CPU and triggering scx_error in task_can_run_on_remote_rq(). Only apply the p == current disambiguation when CONFIG_PREEMPT_RCU is enabled, where the ambiguity with the BPF prolog still exists. Fixes: 8e4f0b1ebcf2 ("bpf: use rcu_read_lock_dont_migrate() for trampoline.c") Cc: stable@vger.kernel.org # v6.18+ Link: https://lore.kernel.org/lkml/20250821090609.42508-8-dongml2@chinatelecom.cn/ Signed-off-by: Changwoo Min Reviewed-by: Andrea Righi Signed-off-by: Tejun Heo Signed-off-by: Greg Kroah-Hartman --- kernel/sched/ext_idle.c | 31 +++++++++++++++++++------------ 1 file changed, 19 insertions(+), 12 deletions(-) --- a/kernel/sched/ext_idle.c +++ b/kernel/sched/ext_idle.c @@ -861,25 +861,32 @@ static bool check_builtin_idle_enabled(s * code. * * We can't simply check whether @p->migration_disabled is set in a - * sched_ext callback, because migration is always disabled for the current - * task while running BPF code. + * sched_ext callback, because the BPF prolog (__bpf_prog_enter) may disable + * migration for the current task while running BPF code. * - * The prolog (__bpf_prog_enter) and epilog (__bpf_prog_exit) respectively - * disable and re-enable migration. For this reason, the current task - * inside a sched_ext callback is always a migration-disabled task. + * Since the BPF prolog calls migrate_disable() only when CONFIG_PREEMPT_RCU + * is enabled (via rcu_read_lock_dont_migrate()), migration_disabled == 1 for + * the current task is ambiguous only in that case: it could be from the BPF + * prolog rather than a real migrate_disable() call. * - * Therefore, when @p->migration_disabled == 1, check whether @p is the - * current task or not: if it is, then migration was not disabled before - * entering the callback, otherwise migration was disabled. + * Without CONFIG_PREEMPT_RCU, the BPF prolog never calls migrate_disable(), + * so migration_disabled == 1 always means the task is truly + * migration-disabled. + * + * Therefore, when migration_disabled == 1 and CONFIG_PREEMPT_RCU is enabled, + * check whether @p is the current task or not: if it is, then migration was + * not disabled before entering the callback, otherwise migration was disabled. * * Returns true if @p is migration-disabled, false otherwise. */ static bool is_bpf_migration_disabled(const struct task_struct *p) { - if (p->migration_disabled == 1) - return p != current; - else - return p->migration_disabled; + if (p->migration_disabled == 1) { + if (IS_ENABLED(CONFIG_PREEMPT_RCU)) + return p != current; + return true; + } + return p->migration_disabled; } static s32 select_cpu_from_kfunc(struct scx_sched *sch, struct task_struct *p,