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[73.241.217.19]) by smtp.gmail.com with ESMTPSA id i66sm382345pfc.12.2020.07.28.19.41.49 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 28 Jul 2020 19:41:50 -0700 (PDT) Subject: Re: [PATCH] block: fix possible race on blk_get_queue() To: Luis Chamberlain , axboe@kernel.dk Cc: ming.lei@redhat.com, hch@infradead.org, jack@suse.cz, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org References: <20200729015101.31534-1-mcgrof@kernel.org> 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: <74bd5f51-c8bd-a1b8-cf85-d9eeb0a82d16@acm.org> Date: Tue, 28 Jul 2020 19:41:49 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <20200729015101.31534-1-mcgrof@kernel.org> Content-Type: text/plain; charset=utf-8 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 2020-07-28 18:51, Luis Chamberlain wrote: > diff --git a/block/blk-core.c b/block/blk-core.c > index d9d632639bd1..febdd8e8d409 100644 > --- a/block/blk-core.c > +++ b/block/blk-core.c > @@ -605,12 +605,18 @@ EXPORT_SYMBOL(blk_alloc_queue); > */ > bool blk_get_queue(struct request_queue *q) > { > - if (likely(!blk_queue_dying(q))) { > - __blk_get_queue(q); > - return true; > + struct kobject *obj; > + > + obj = __blk_get_queue(q); > + if (!obj) > + return false; > + > + if (unlikely(blk_queue_dying(q))) { > + blk_put_queue(q); > + return false; > } > > - return false; > + return true; > } This change is not sufficient to prevent that the QUEUE_FLAG_DYING flag is set immediately after this function returns. I propose not to modify this function but instead to add a comment that is the responsibility of the caller to prevent that such a race condition occurs. > -static inline void __blk_get_queue(struct request_queue *q) > +static inline struct kobject * __must_check > +__blk_get_queue(struct request_queue *q) > { > - kobject_get(&q->kobj); > + return kobject_get_unless_zero(&q->kobj); > } If a function passes a queue pointer to another function that calls blk_get_queue() then the caller should guarantee that 'q' is valid during the entire duration of the call. In other words, I'm not sure the above change is an improvement. Thanks, Bart.