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Mon, 19 Oct 2026 19:14:38 -0700 (PDT) Received: from unknown748F3CBA5068 (dynamic-2a02-3100-9c92-6501-cda9-90cd-4e49-75d9.310.pool.telefonica.de. [2a02:3100:9c92:6501:cda9:90cd:4e49:75d9]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-48e1d8208b1sm2879816f8f.33.2026.10.19.19.14.36 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 19 Oct 2026 19:14:38 -0700 (PDT) Date: Tue, 20 Oct 2026 04:14:36 +0200 From: Karl Mehltretter To: Harry Yoo Cc: Peter Zijlstra , Thomas Gleixner , Sebastian Andrzej Siewior , Andrew Morton , Vlastimil Babka , Alexei Starovoitov , Ingo Molnar , Will Deacon , Boqun Feng , Waiman Long , Jonathan Corbet , David Hildenbrand , Johannes Weiner , Shakeel Butt , David Stevens , Daniel Borkmann , Andrii Nakryiko , Martin KaFai Lau , Shuah Khan , Amery Hung , Swaraj Gaikwad , Clark Williams , Steven Rostedt , linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, linux-mm@kvack.org, linux-rt-devel@lists.linux.dev, bpf@vger.kernel.org, linux-kselftest@vger.kernel.org, cgroups@vger.kernel.org Subject: Re: [RFC PATCH v2 0/3] locking, mm: Add atomic allocator trylocks on RT Message-ID: References: <20261005070625.8871-1-kmehltretter@gmail.com> X-Mailing-List: bpf@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Wed, Oct 07, 2026 at 07:15:29PM +0100, Harry Yoo wrote: > On Mon, Oct 05, 2026 at 09:06:22AM +0200, Karl Mehltretter wrote: > > On PREEMPT_RT, a BPF task-storage program attached to sched_waking > > can deadlock when kmalloc_nolock() obtains an rtmutex-backed allocator > > spinlock while try_to_wake_up() holds p->pi_lock. Releasing the allocator > > lock can enter priority-inheritance or wakeup code and re-enter scheduler > > locking. > > > > The first RFC [1] rejected every non-preemptible caller. That prevents the > > deadlock, but it also rejects BPF arena allocation and faults under the > > arena's ordinary raw lock. > > > > This RFC instead adds an atomic owner state for bounded PREEMPT_RT spinlock > > trylocks. Atomic acquisition succeeds only from the completely free state. > > Please don't skip discussion stage and jump into submitting a very > intrusive change across locking and mm, assisted by LLM? :/ > > I don't think attempts to tackle this problem will make progress without > discussing and getting buy-in from (PREEMPT_RT) locking folks. > Sebastian, Peter, Thomas, did you have a chance to look at the locking direction here? Thanks, Karl