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 5E673449B24; Fri, 25 Sep 2026 07:29:22 +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=1790321363; cv=none; b=nzsWs/bfivOSaGbc7txOA4AXi4nx4SwntACjyiU6PeFlpR6SOVE9OMHWw63N20dNhg2zp/tL65hIBjCgafYm/52EYRD9bpH4pK9tY8zBsMsZ/JKzAoFofE9NIFOdieRUni+p8ju+UHrLEqL4K8h/SLbu/kyCnxG0fdoEd96NvIo= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790321363; c=relaxed/simple; bh=Jy0t0oie/GWyBaf/N3g2nOLwsuUuXeJQOCiKnvp+SD0=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=dilue/3dh7MZOm62H7Cf908UitY7MHUUpfWIQoJcvnF3clvxeaq/wM1LidQlb3yAA0TD50OObXuxOeaHZgZAUU3ShwZHubQYSKTsO9+78hFjPThaC9ngcntzDTrmiHM2XUjnkkluOzsjSnoBhkIrU5UAzKKRA8yKmhQppR9xl2w= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=TPsZEIc/; 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="TPsZEIc/" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 30DE71F000FF; Fri, 25 Sep 2026 07:29:19 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1790321362; bh=Mfnb3GXAJ6qEATTIM7j+kMnju+C9c/oq/1MBEZFo2sk=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=TPsZEIc/WsipxGDE8RhpcTcM1O2qXwgqnk+1A1yxgE0s67tJ2P0+FzqsPD5j/ra8N 3IC54JRZw5rnypO/kwU2ObM8PWWqJ7ENRm1hUrNo5hKBX5/nf++KIQPKKmhfUB9vcy VWZy5xWZ8BMPsmBycV8uzg2vT+k4tTAzRjnUmas9eIsRaxdC9ajviKZ3X7nnghOGE1 zYJ9D3pUxZPwORZDZOoxo9kJzt6hmhcpxd8fKJCRRsdtjKmeX70z2pdMoXJHcqDEWN QLp7HIA1+Z0ZIQ90pHRtsdxQN37aVNzYYd43+8HqfkVpxsMRrCxKhXysQoJV99TxVI 8sjVtYyX7LgXg== Message-ID: Date: Fri, 25 Sep 2026 09:29:16 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH] mm: restrict can_spin_trylock() to preemptible context on PREEMPT_RT Content-Language: en-US To: Alexei Starovoitov , Karl Mehltretter Cc: Harry Yoo , Sebastian Andrzej Siewior , Andrew Morton , Hao Li , Suren Baghdasaryan , Michal Hocko , Amery Hung , Swaraj Gaikwad , Clark Williams , Steven Rostedt , linux-mm@kvack.org, bpf@vger.kernel.org, linux-rt-devel@lists.linux.dev, linux-kernel@vger.kernel.org References: <20260919171443.90512-1-kmehltretter@gmail.com> From: "Vlastimil Babka (SUSE)" Autocrypt: addr=vbabka@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 9/25/26 06:12, Alexei Starovoitov wrote: > On Tue, Sep 22, 2026 at 08:04 AM Karl Mehltretter wrote: >> a) Change MM so _nolock() allocation remains safe and can still succeed >> while pi_lock or an rq lock is held. Existing BPF behavior would >> remain unchanged. Is this feasible, or would it require substantial >> allocator changes? > > I don't think it's feasible. On RT the sheaves lock, n->list_lock and > zone->lock are all rtmutex based. When trylock succeeded and a waiter > showed up rt_spin_unlock() has to wake it. > >> b) Restore BPF local storage's dedicated allocator, and document, with >> debug checks if possible, that _nolock() must not be used in those >> scheduler-lock contexts. > > No. Local storage is not the only one. bpf_arena_alloc_pages(), > bpf_stream_vprintk(), bpf_task_work_schedule_*() call kmalloc_nolock() > as well and the same sched_waking prog can call them. > bpf_mem_alloc was removed from local storage, because it wastes memory > on preallocation. Same answer for d). > >> c) Have MM detect those contexts and return NULL. This prevents the >> deadlock while preserving ordinary raw-lock callers such as arena, >> but task-storage creation from scheduler tracepoints then fails >> deterministically. > > This one. Yeah, but how exactly to do that detection :) > bpf_task_storage_get() with F_CREATE can return NULL and the progs > have to check for it anyway. e) Introduce raw_local_trylock_t... >> Are these the right alternatives? In particular, what behavior is >> intended from the _nolock() API in these scheduler-lock contexts? > > It can be called from any context and it can return NULL. > That's what it does from NMI and hardirq on RT already.