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b=BqcVsecxmXNy4fg5mqGk7N0ypwHtyajS81nsbXd5i2Wu2JIBduH8S6XXVmRPLmT3g HKoSujXLfVVRD73FwlUyDSd4AOVs1oJDfsy3KwTF1RdwfXDRYgxe1P0uLaXkRlJ3jE lzsYtMv8NcvuHKlhWPA/VZFDRk5O4/3MfNIfxYhfUSD+9c3plZRiyFJCkjExVJYfsr IpcQPdJRGzG0qB3tzsOqSxtGfjOa/QC9cwlOkWIZvd046tDwLJDuGuQg25VISH2BPO g33ELEp6mj95IliGKNd/jRqTK5yLbX8wtTIqd2/p8jAe0hjq/rZ0pfdOEMZpRX8rlR YMjMUcStMfhQg== Date: Wed, 7 Oct 2026 19:15:29 +0200 From: Harry Yoo To: Karl Mehltretter 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20261005070625.8871-1-kmehltretter@gmail.com> X-Rspamd-Queue-Id: 2588DA000E X-Rspam-User: X-Rspamd-Server: rspam05 X-Stat-Signature: qjuwojubdbxaq63ntn8w7q7bpcbbsqfr X-HE-Tag: 1791393332-625848 X-HE-Meta: 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 gN9xm0cD OIBcXaSZ/DDuzdprufcXJ96VN3lw7V0L+nEy3eK0z+OA9xm78/3c2VDuXqFb3lXZiOTD3/V6p06j+1szN7+ZfxYb6ZR5zoVXtPaHjyyPtw4jPzKNNXxAKprYnGHDmfPGVV4riS+TT4LaCEvKxASVd5lNeYHhmswZdLXtB2mX/EyEtEFfSAh3RzvWvFYU+h2JBxZNd2GihHaz6qgeJcL6UQsgiyG0RAaFCHvNnvrOmdV6OQ12WGwJ8b6pfJWjvo+RTyzV2 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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. > A regular waiter sets HAS_WAITERS before waiting, which prevents a later > atomic owner from barging. Atomic release preserves HAS_WAITERS and does > not enter priority inheritance or wake a task. Preemption and local > interrupts remain disabled for the atomic-owner section. -- Cheers, Harry / Hyeonggon