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 98CEB21D5B0 for ; Tue, 17 Mar 2026 17:21:55 +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=1773768115; cv=none; b=u34BlBdY3IlFsC7qE3TRxaOralMB2uZFPaNsbPAxN2hodjb7LZIN3Fkg9RSBD6Pm8lSCp6Nrau9iik4R6Hg6CpGdj7efm2JgF6H1B/5/bpoQdsmLRbqzVtN8RK6d3Gzo+jepxm59efLd6UFkKXpne+1GtKtlAuzC0rypMX5Ctq4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773768115; c=relaxed/simple; bh=KEepDmj13mgjR6mpMdfctSF7L/2p9WNhIY3oNAEYRXo=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=aTDne+oziEs0kNAAONpvSprTM8Fx3txhpUdazf1rN2waM44L/oKYghj1MZAlY9f4kWJuV7lP6yrWiI43sA4pnnYeIczilryBhjtrSU4/fdqiYB0ykmiGsv5aAxEHWwwO7LAgFrta8sgKTVNrK9Vy3xtORqvKMj2kkm6jVUAbzoU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Jh76FciQ; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="Jh76FciQ" Received: by smtp.kernel.org (Postfix) with ESMTPSA id EF4B7C2BCB0; Tue, 17 Mar 2026 17:21:54 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1773768115; bh=KEepDmj13mgjR6mpMdfctSF7L/2p9WNhIY3oNAEYRXo=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=Jh76FciQi/ORB5A2T8sfspBTgDpa6/nyG3iOZ2mWOMc8/eYsxA03coZCaBv801rfF paUuKy44Na5D3CUsTtF30XCG+4IRkHWfeX9T6o6QROmelG3cdYJOUmyLKWggk0Pxje JTONJ1f9OeNkgbDOslzqmSbfjtFlAT0JXsXiKpdCbnHQ0C7+P9ANky9eFVgyBcMVpZ vG57a5/mCAfXXzjdzGBgmSoMICgr+UlwbhFkMsa0HpX28lx3CA+hDJWO4UftaVB+Z1 BGufuzwHNWz6SkKZmJjW0h/4P20vFgOtEpfebWaNts0bobs6obuKxgTFFhnAMhpXTB pFhURC1PCCaJQ== From: Sasha Levin To: stable@vger.kernel.org Cc: Tejun Heo , Andrea Righi , Sasha Levin Subject: [PATCH 6.18.y 2/2] sched_ext: Disable preemption between scx_claim_exit() and kicking helper work Date: Tue, 17 Mar 2026 13:21:52 -0400 Message-ID: <20260317172152.239864-2-sashal@kernel.org> X-Mailer: git-send-email 2.51.0 In-Reply-To: <20260317172152.239864-1-sashal@kernel.org> References: <2026031736-mystify-skating-036b@gregkh> <20260317172152.239864-1-sashal@kernel.org> Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit From: Tejun Heo [ Upstream commit 83236b2e43dba00bee5b82eb5758816b1a674f6a ] scx_claim_exit() atomically sets exit_kind, which prevents scx_error() from triggering further error handling. After claiming exit, the caller must kick the helper kthread work which initiates bypass mode and teardown. If the calling task gets preempted between claiming exit and kicking the helper work, and the BPF scheduler fails to schedule it back (since error handling is now disabled), the helper work is never queued, bypass mode never activates, tasks stop being dispatched, and the system wedges. Disable preemption across scx_claim_exit() and the subsequent work kicking in all callers - scx_disable() and scx_vexit(). Add lockdep_assert_preemption_disabled() to scx_claim_exit() to enforce the requirement. Fixes: f0e1a0643a59 ("sched_ext: Implement BPF extensible scheduler class") Cc: stable@vger.kernel.org # v6.12+ Reviewed-by: Andrea Righi Signed-off-by: Tejun Heo Signed-off-by: Sasha Levin --- kernel/sched/ext.c | 12 ++++++++++++ 1 file changed, 12 insertions(+) diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c index a10344f00411d..e565e622cd5ca 100644 --- a/kernel/sched/ext.c +++ b/kernel/sched/ext.c @@ -4068,10 +4068,19 @@ static void scx_disable_workfn(struct kthread_work *work) scx_bypass(false); } +/* + * Claim the exit on @sch. The caller must ensure that the helper kthread work + * is kicked before the current task can be preempted. Once exit_kind is + * claimed, scx_error() can no longer trigger, so if the current task gets + * preempted and the BPF scheduler fails to schedule it back, the helper work + * will never be kicked and the whole system can wedge. + */ static bool scx_claim_exit(struct scx_sched *sch, enum scx_exit_kind kind) { int none = SCX_EXIT_NONE; + lockdep_assert_preemption_disabled(); + if (!atomic_try_cmpxchg(&sch->exit_kind, &none, kind)) return false; @@ -4094,6 +4103,7 @@ static void scx_disable(enum scx_exit_kind kind) rcu_read_lock(); sch = rcu_dereference(scx_root); if (sch) { + guard(preempt)(); scx_claim_exit(sch, kind); kthread_queue_work(sch->helper, &sch->disable_work); } @@ -4416,6 +4426,8 @@ static void scx_vexit(struct scx_sched *sch, { struct scx_exit_info *ei = sch->exit_info; + guard(preempt)(); + if (!scx_claim_exit(sch, kind)) return; -- 2.51.0