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[47.55.120.4]) by smtp.gmail.com with ESMTPSA id 6a1803df08f44-88993b59838sm63654226d6.13.2025.12.15.16.57.05 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 15 Dec 2025 16:57:05 -0800 (PST) Received: from jgg by wakko with local (Exim 4.97) (envelope-from ) id 1vVJNB-000000008Lt-0Rrp; Mon, 15 Dec 2025 20:57:05 -0400 Date: Mon, 15 Dec 2025 20:57:05 -0400 From: Jason Gunthorpe To: wujing Cc: Leon Romanovsky , linux-rdma@vger.kernel.org, linux-kernel@vger.kernel.org, Qiliang Yuan Subject: Re: [PATCH] IB/core: Fix ABBA deadlock in rdma_dev_exit_net Message-ID: <20251216005705.GB31492@ziepe.ca> References: Precedence: bulk X-Mailing-List: linux-rdma@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Thu, Dec 11, 2025 at 04:08:13PM +0800, wujing wrote: > Classic ABBA deadlock due to inconsistent lock ordering between > rdma_dev_exit_net() and rdma_dev_init_net(): > > Thread A (cleanup_net workqueue -> kworker/u256:1): > rdma_dev_exit_net(): > down_write(&rdma_nets_rwsem) <- held at line rdma_dev_exit_net+0x60 > down_read(&devices_rwsem) <- waiting (shown in rwsem_down_write_slowpath) This isn't right, it unlocked the &rdma_nets_rwsem: down_write(&rdma_nets_rwsem); /* * Prevent the ID from being re-used and hide the id from xa_for_each. */ ret = xa_err(xa_store(&rdma_nets, rnet->id, NULL, GFP_KERNEL)); WARN_ON(ret); up_write(&rdma_nets_rwsem); <------ down_read(&devices_rwsem); It is not nested and there is not a dependency. > Thread B (stress-ng-clone processes): > rdma_dev_init_net(): > down_read(&devices_rwsem) <- held at line rdma_dev_init_net+0x120 > down_read(&rdma_nets_rwsem) <- waiting (blocked by pending writer from Thread A) This one is nested though. I don't know what your bug is, but it is not some trivial ABBA deadlock, lockdep would have found something like that ages ago. Jason