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Thu, 10 Sep 2026 20:41:05 -0700 (PDT) Received: from spider.bream-herring.ts.net ([103.252.203.158]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-39d9afa367bsm558148a91.0.2026.09.10.20.41.00 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 10 Sep 2026 20:41:04 -0700 (PDT) From: Matthias Goergens To: paulmck@kernel.org, urezki@gmail.com, harry@kernel.org Cc: frederic@kernel.org, neeraj.upadhyay@kernel.org, joelagnelf@nvidia.com, josh@joshtriplett.org, boqun@kernel.org, rostedt@goodmis.org, mathieu.desnoyers@efficios.com, jiangshanlai@gmail.com, qiang.zhang@linux.dev, corbet@lwn.net, skhan@linuxfoundation.org, rdunlap@infradead.org, surenb@google.com, vbabka@kernel.org, rcu@vger.kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org Subject: [PATCH v3 0/1] rcu: make userspace barrier hook drain kvfree_rcu work Date: Fri, 11 Sep 2026 11:40:58 +0800 Message-ID: <20260911034059.3569914-1-matthias.goergens@gmail.com> X-Mailer: git-send-email 2.55.0 In-Reply-To: <6092a9d0-0125-4da4-ac48-85ccaeb83beb@paulmck-laptop> References: <6092a9d0-0125-4da4-ac48-85ccaeb83beb@paulmck-laptop> Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 7bit Extend rcutree.do_rcu_barrier to drain kfree_rcu() batches and per-CPU SLUB sheaves as well as ordinary callbacks. This strengthens the documented test interface, implementing the follow-up proposed when kvfree_rcu_barrier() was added in 2024: https://lore.kernel.org/all/20240820155935.1167988-1-urezki@gmail.com/ While testing bcachefs performance changes, ktest's /proc/allocinfo check falsely reported leaks despite repeated hook writes and a 20-second wait. An earlier prototype let the same unmodified workload pass: eight checkpoints in one VM, after 50 through 400 option changes, found zero retained reconcile_scan objects. The original kernel's retained count eventually fell as a sheaf filled; no unbounded growth or OOM was observed. Following Paul's v2 review, retain the entry sequence snapshot and check it after the unconditional kvfree_rcu() drain. If complete, retain smp_mb() and skip the final ordinary barrier; otherwise, call rcu_barrier(). An unrelated ordinary barrier cannot justify skipping the deferred-free drain. The start-rate throttle remains unconditional. The documentation now scopes completion to work queued before the request, without preventing new concurrent work. This can avoid an extra barrier operation: v2's trailing rcu_barrier() takes a fresh snapshot and need not reuse the internal barrier. No elapsed-time improvement has been measured. Exact-v3 builds and interface smoke checks passed in four fresh two-vCPU TREE_RCU VMs: two with kvfree batching and two with SLUB_TINY (unbatched). Each completed three true requests, remained idle after false, and rejected invalid input with EINVAL. Full kernel logs showed no WARN, oops, panic or RCU-stall diagnostics. The v1/v2 private-cache completion and ordinary-callback regression tests have not been rerun on v3. These smoke tests neither prove concurrency/weak-memory correctness nor force the guarded fallback. v2: https://lore.kernel.org/all/20260910170040.344864-1-matthias.goergens@gmail.com/ Matthias Goergens (1): rcu: make userspace barrier hook drain kvfree_rcu work .../admin-guide/kernel-parameters.txt | 9 ++++-- kernel/rcu/tree.c | 30 ++++++++++++------- 2 files changed, 26 insertions(+), 13 deletions(-) -- 2.55.0