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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 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 X-Rspamd-Server: rspam06 X-Stat-Signature: ab7wjc5m4unzdnr9h61bjw5ueis5tyqz X-Rspam-User: X-Rspamd-Queue-Id: 9415E20004 X-HE-Tag: 1790321363-414114 X-HE-Meta: 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 +seIiQfN zi4i9E7DY1h+GVej2EU4J8v54VSytkBWDXV0JSDm4TVNR0/0fGPewWoIRhFKofiFdkyS+1U9x6jmjcpTYdUu3ZwjjETEU8t6liuo5oKa8Bovk9lBVJuvgk3pM+2n/5iplUZWO5cdk+lQ5s7gca7fX0XK3nFjdefbZZtwJvzZ2Yuw6RYsIGXRyTk4aeL/rbh/NrnAs41liIYBFiisXIqPlDE8Ohl0lMm+fOgU/D9egZAy3e7PkQ7/4/WRZlk4BQuIq08vUgm9PEldpaYPHhRtzNLVKcvMxHTNdTF1BpD44ACV1EOuLM5fN/kajUEJI6aW2/wQHRpxFjqCMA7Gqp75J0uye6syCi4x+PHVxrho5kUDUlcju5uCn2xibsDnuaafB3xnh/DxhbjqH+SMYCVut3yauUw== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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.