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Wed, 23 Oct 2019 13:58:27 -0700 (PDT) Received: from localhost.localdomain ([2601:647:4000:c3:e5ef:65f1:8f3f:3a78]) by smtp.gmail.com with ESMTPSA id j17sm22736006pfr.70.2019.10.23.13.58.25 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 23 Oct 2019 13:58:25 -0700 (PDT) Subject: Re: [PATCH 2/4] block: Fix a race between blk_poll() and blk_mq_update_nr_hw_queues() To: Ming Lei Cc: Jens Axboe , linux-block@vger.kernel.org, Christoph Hellwig , Christoph Hellwig , Hannes Reinecke , Johannes Thumshirn References: <20191021224259.209542-1-bvanassche@acm.org> <20191021224259.209542-3-bvanassche@acm.org> <20191022094154.GB9037@ming.t460p> From: Bart Van Assche Autocrypt: addr=bvanassche@acm.org; prefer-encrypt=mutual; keydata= mQENBFSOu4oBCADcRWxVUvkkvRmmwTwIjIJvZOu6wNm+dz5AF4z0FHW2KNZL3oheO3P8UZWr LQOrCfRcK8e/sIs2Y2D3Lg/SL7qqbMehGEYcJptu6mKkywBfoYbtBkVoJ/jQsi2H0vBiiCOy fmxMHIPcYxaJdXxrOG2UO4B60Y/BzE6OrPDT44w4cZA9DH5xialliWU447Bts8TJNa3lZKS1 AvW1ZklbvJfAJJAwzDih35LxU2fcWbmhPa7EO2DCv/LM1B10GBB/oQB5kvlq4aA2PSIWkqz4 3SI5kCPSsygD6wKnbRsvNn2mIACva6VHdm62A7xel5dJRfpQjXj2snd1F/YNoNc66UUTABEB AAG0JEJhcnQgVmFuIEFzc2NoZSA8YnZhbmFzc2NoZUBhY20ub3JnPokBOQQTAQIAIwUCVI67 igIbAwcLCQgHAwIBBhUIAgkKCwQWAgMBAh4BAheAAAoJEHFcPTXFzhAJ8QkH/1AdXblKL65M Y1Zk1bYKnkAb4a98LxCPm/pJBilvci6boefwlBDZ2NZuuYWYgyrehMB5H+q+Kq4P0IBbTqTa jTPAANn62A6jwJ0FnCn6YaM9TZQjM1F7LoDX3v+oAkaoXuq0dQ4hnxQNu792bi6QyVdZUvKc macVFVgfK9n04mL7RzjO3f+X4midKt/s+G+IPr4DGlrq+WH27eDbpUR3aYRk8EgbgGKvQFdD CEBFJi+5ZKOArmJVBSk21RHDpqyz6Vit3rjep7c1SN8s7NhVi9cjkKmMDM7KYhXkWc10lKx2 RTkFI30rkDm4U+JpdAd2+tP3tjGf9AyGGinpzE2XY1K5AQ0EVI67igEIAKiSyd0nECrgz+H5 PcFDGYQpGDMTl8MOPCKw/F3diXPuj2eql4xSbAdbUCJzk2ETif5s3twT2ER8cUTEVOaCEUY3 eOiaFgQ+nGLx4BXqqGewikPJCe+UBjFnH1m2/IFn4T9jPZkV8xlkKmDUqMK5EV9n3eQLkn5g lco+FepTtmbkSCCjd91EfThVbNYpVQ5ZjdBCXN66CKyJDMJ85HVr5rmXG/nqriTh6cv1l1Js T7AFvvPjUPknS6d+BETMhTkbGzoyS+sywEsQAgA+BMCxBH4LvUmHYhpS+W6CiZ3ZMxjO8Hgc ++w1mLeRUvda3i4/U8wDT3SWuHcB3DWlcppECLkAEQEAAYkBHwQYAQIACQUCVI67igIbDAAK CRBxXD01xc4QCZ4dB/0QrnEasxjM0PGeXK5hcZMT9Eo998alUfn5XU0RQDYdwp6/kMEXMdmT oH0F0xB3SQ8WVSXA9rrc4EBvZruWQ+5/zjVrhhfUAx12CzL4oQ9Ro2k45daYaonKTANYG22y //x8dLe2Fv1By4SKGhmzwH87uXxbTJAUxiWIi1np0z3/RDnoVyfmfbbL1DY7zf2hYXLLzsJR mSsED/1nlJ9Oq5fALdNEPgDyPUerqHxcmIub+pF0AzJoYHK5punqpqfGmqPbjxrJLPJfHVKy goMj5DlBMoYqEgpbwdUYkH6QdizJJCur4icy8GUNbisFYABeoJ91pnD4IGei3MTdvINSZI5e Message-ID: <322f024f-6756-aa29-28ac-a17aa8499279@acm.org> Date: Wed, 23 Oct 2019 13:58:24 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.1.2 MIME-Version: 1.0 In-Reply-To: <20191022094154.GB9037@ming.t460p> Content-Type: text/plain; charset=windows-1252 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On 2019-10-22 02:41, Ming Lei wrote: > On Mon, Oct 21, 2019 at 03:42:57PM -0700, Bart Van Assche wrote: >> +int blk_poll(struct request_queue *q, blk_qc_t cookie, bool spin) >> +{ >> + int ret; >> + >> + if (!percpu_ref_tryget(&q->q_usage_counter)) >> + return 0; >> + ret = __blk_poll(q, cookie, spin); >> + blk_queue_exit(q); >> + >> + return ret; >> +} > > IMO, this change isn't required. Caller of blk_poll is supposed to > hold refcount of the request queue, then the related hctx data structure > won't go away. When the hctx is in transient state, there can't be IO > to be polled, and it is safe to call into IO path. > > BTW, .poll is absolutely the fast path, we should be careful to add code > in this path. Hi Ming, I'm not sure whether all blk_poll() callers really hold a refcount on the request queue. Anyway, I will convert this code change into a comment that explains that blk_poll() callers must hold a request queue reference. Bart.