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Tue, 15 Dec 2020 03:11:53 -0800 (PST) To: Xiaoguang Wang , io-uring@vger.kernel.org Cc: axboe@kernel.dk, joseph.qi@linux.alibaba.com References: <20201214154941.10907-1-xiaoguang.wang@linux.alibaba.com> <23ec9427-e904-f87f-2345-d040d9b10673@gmail.com> <39f3c4a2-a81d-b4cd-c01f-d4bf59d708d8@linux.alibaba.com> From: Pavel Begunkov Autocrypt: addr=asml.silence@gmail.com; prefer-encrypt=mutual; keydata= mQINBFmKBOQBEAC76ZFxLAKpDw0bKQ8CEiYJRGn8MHTUhURL02/7n1t0HkKQx2K1fCXClbps bdwSHrhOWdW61pmfMbDYbTj6ZvGRvhoLWfGkzujB2wjNcbNTXIoOzJEGISHaPf6E2IQx1ik9 6uqVkK1OMb7qRvKH0i7HYP4WJzYbEWVyLiAxUj611mC9tgd73oqZ2pLYzGTqF2j6a/obaqha +hXuWTvpDQXqcOZJXIW43atprH03G1tQs7VwR21Q1eq6Yvy2ESLdc38EqCszBfQRMmKy+cfp W3U9Mb1w0L680pXrONcnlDBCN7/sghGeMHjGKfNANjPc+0hzz3rApPxpoE7HC1uRiwC4et83 CKnncH1l7zgeBT9Oa3qEiBlaa1ZCBqrA4dY+z5fWJYjMpwI1SNp37RtF8fKXbKQg+JuUjAa9 Y6oXeyEvDHMyJYMcinl6xCqCBAXPHnHmawkMMgjr3BBRzODmMr+CPVvnYe7BFYfoajzqzq+h EyXSl3aBf0IDPTqSUrhbmjj5OEOYgRW5p+mdYtY1cXeK8copmd+fd/eTkghok5li58AojCba jRjp7zVOLOjDlpxxiKhuFmpV4yWNh5JJaTbwCRSd04sCcDNlJj+TehTr+o1QiORzc2t+N5iJ NbILft19Izdn8U39T5oWiynqa1qCLgbuFtnYx1HlUq/HvAm+kwARAQABtDFQYXZlbCBCZWd1 bmtvdiAoc2lsZW5jZSkgPGFzbWwuc2lsZW5jZUBnbWFpbC5jb20+iQJOBBMBCAA4FiEE+6Ju PTjTbx479o3OWt5b1Glr+6UFAlmKBOQCGwMFCwkIBwIGFQgJCgsCBBYCAwECHgECF4AACgkQ Wt5b1Glr+6WxZA//QueaKHzgdnOikJ7NA/Vq8FmhRlwgtP0+E+w93kL+ZGLzS/cUCIjn2f4Q Mcutj2Neg0CcYPX3b2nJiKr5Vn0rjJ/suiaOa1h1KzyNTOmxnsqE5fmxOf6C6x+NKE18I5Jy xzLQoktbdDVA7JfB1itt6iWSNoOTVcvFyvfe5ggy6FSCcP+m1RlR58XxVLH+qlAvxxOeEr/e aQfUzrs7gqdSd9zQGEZo0jtuBiB7k98t9y0oC9Jz0PJdvaj1NZUgtXG9pEtww3LdeXP/TkFl HBSxVflzeoFaj4UAuy8+uve7ya/ECNCc8kk0VYaEjoVrzJcYdKP583iRhOLlZA6HEmn/+Gh9 4orG67HNiJlbFiW3whxGizWsrtFNLsSP1YrEReYk9j1SoUHHzsu+ZtNfKuHIhK0sU07G1OPN 2rDLlzUWR9Jc22INAkhVHOogOcc5ajMGhgWcBJMLCoi219HlX69LIDu3Y34uIg9QPZIC2jwr 24W0kxmK6avJr7+n4o8m6sOJvhlumSp5TSNhRiKvAHB1I2JB8Q1yZCIPzx+w1ALxuoWiCdwV M/azguU42R17IuBzK0S3hPjXpEi2sK/k4pEPnHVUv9Cu09HCNnd6BRfFGjo8M9kZvw360gC1 reeMdqGjwQ68o9x0R7NBRrtUOh48TDLXCANAg97wjPoy37dQE7e5Ag0EWYoE5AEQAMWS+aBV IJtCjwtfCOV98NamFpDEjBMrCAfLm7wZlmXy5I6o7nzzCxEw06P2rhzp1hIqkaab1kHySU7g dkpjmQ7Jjlrf6KdMP87mC/Hx4+zgVCkTQCKkIxNE76Ff3O9uTvkWCspSh9J0qPYyCaVta2D1 Sq5HZ8WFcap71iVO1f2/FEHKJNz/YTSOS/W7dxJdXl2eoj3gYX2UZNfoaVv8OXKaWslZlgqN jSg9wsTv1K73AnQKt4fFhscN9YFxhtgD/SQuOldE5Ws4UlJoaFX/yCoJL3ky2kC0WFngzwRF Yo6u/KON/o28yyP+alYRMBrN0Dm60FuVSIFafSqXoJTIjSZ6olbEoT0u17Rag8BxnxryMrgR dkccq272MaSS0eOC9K2rtvxzddohRFPcy/8bkX+t2iukTDz75KSTKO+chce62Xxdg62dpkZX xK+HeDCZ7gRNZvAbDETr6XI63hPKi891GeZqvqQVYR8e+V2725w+H1iv3THiB1tx4L2bXZDI DtMKQ5D2RvCHNdPNcZeldEoJwKoA60yg6tuUquvsLvfCwtrmVI2rL2djYxRfGNmFMrUDN1Xq F3xozA91q3iZd9OYi9G+M/OA01husBdcIzj1hu0aL+MGg4Gqk6XwjoSxVd4YT41kTU7Kk+/I 5/Nf+i88ULt6HanBYcY/+Daeo/XFABEBAAGJAjYEGAEIACAWIQT7om49ONNvHjv2jc5a3lvU aWv7pQUCWYoE5AIbDAAKCRBa3lvUaWv7pfmcEACKTRQ28b1y5ztKuLdLr79+T+LwZKHjX++P 4wKjEOECCcB6KCv3hP+J2GCXDOPZvdg/ZYZafqP68Yy8AZqkfa4qPYHmIdpODtRzZSL48kM8 LRzV8Rl7J3ItvzdBRxf4T/Zseu5U6ELiQdCUkPGsJcPIJkgPjO2ROG/ZtYa9DvnShNWPlp+R uPwPccEQPWO/NP4fJl2zwC6byjljZhW5kxYswGMLBwb5cDUZAisIukyAa8Xshdan6C2RZcNs rB3L7vsg/R8UCehxOH0C+NypG2GqjVejNZsc7bgV49EOVltS+GmGyY+moIzxsuLmT93rqyII 5rSbbcTLe6KBYcs24XEoo49Zm9oDA3jYvNpeYD8rDcnNbuZh9kTgBwFN41JHOPv0W2FEEWqe JsCwQdcOQ56rtezdCJUYmRAt3BsfjN3Jn3N6rpodi4Dkdli8HylM5iq4ooeb5VkQ7UZxbCWt UVMKkOCdFhutRmYp0mbv2e87IK4erwNHQRkHUkzbsuym8RVpAZbLzLPIYK/J3RTErL6Z99N2 m3J6pjwSJY/zNwuFPs9zGEnRO4g0BUbwGdbuvDzaq6/3OJLKohr5eLXNU3JkT+3HezydWm3W OPhauth7W0db74Qd49HXK0xe/aPrK+Cp+kU1HRactyNtF8jZQbhMCC8vMGukZtWaAwpjWiiH bA== Subject: Re: [PATCH] io_uring: hold uring_lock to complete faild polled io in io_wq_submit_work() Message-ID: <3cdaee97-444d-a0cf-05cd-1f75436ce1be@gmail.com> Date: Tue, 15 Dec 2020 11:08:35 +0000 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.3.0 MIME-Version: 1.0 In-Reply-To: <39f3c4a2-a81d-b4cd-c01f-d4bf59d708d8@linux.alibaba.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org On 15/12/2020 02:28, Xiaoguang Wang wrote: > hi, > >> On 14/12/2020 15:49, Xiaoguang Wang wrote: >>> io_iopoll_complete() does not hold completion_lock to complete polled >>> io, so in io_wq_submit_work(), we can not call io_req_complete() directly, >>> to complete polled io, otherwise there maybe concurrent access to cqring, >>> defer_list, etc, which is not safe. Commit dad1b1242fd5 ("io_uring: always >>> let io_iopoll_complete() complete polled io") has fixed this issue, but >>> Pavel reported that IOPOLL apart from rw can do buf reg/unreg requests( >>> IORING_OP_PROVIDE_BUFFERS or IORING_OP_REMOVE_BUFFERS), so the fix is >>> not good. >>> >>> Given that io_iopoll_complete() is always called under uring_lock, so here >>> for polled io, we can also get uring_lock to fix this issue. >> >> One thing I don't like is that io_wq_submit_work() won't be able to >> publish an event while someone polling io_uring_enter(ENTER_GETEVENTS), >> that's because both take the lock. The problem is when the poller waits >> for an event that is currently in io-wq (i.e. io_wq_submit_work()). >> The polling loop will eventually exit, so that's not a deadlock, but >> latency,etc. would be huge. > In this patch, we just hold uring_lock for polled io in error path, so I think > normally it maybe not an issue, and seems that the critical section is not > that big, so it also may not result in huge latecy. To give a bit of a context, it breaks out of the io_iopoll_check()'s loop with need_resched(). Anyway, fair enough, probably should be dealt separately. > I also noticed that current codes also hold uring_lock in io_wq_submit_work() > call chain: > ==> io_wq_submit_work() > ====> io_issue_sqe() > ======> io_provide_buffers() > ========> io_ring_submit_lock(ctx, !force_nonblock); Yep, that's not good either, though I care more about rw. > > Regards, > Xiaoguang Wang >> >>> >>> Fixes: dad1b1242fd5 ("io_uring: always let io_iopoll_complete() complete polled io") >>> Signed-off-by: Xiaoguang Wang >>> --- >>>   fs/io_uring.c | 25 +++++++++++++++---------- >>>   1 file changed, 15 insertions(+), 10 deletions(-) >>> >>> diff --git a/fs/io_uring.c b/fs/io_uring.c >>> index f53356ced5ab..eab3d2b7d232 100644 >>> --- a/fs/io_uring.c >>> +++ b/fs/io_uring.c >>> @@ -6354,19 +6354,24 @@ static struct io_wq_work *io_wq_submit_work(struct io_wq_work *work) >>>       } >>>         if (ret) { >>> +        bool iopoll_enabled = req->ctx->flags & IORING_SETUP_IOPOLL; >>> + >>>           /* >>> -         * io_iopoll_complete() does not hold completion_lock to complete >>> -         * polled io, so here for polled io, just mark it done and still let >>> -         * io_iopoll_complete() complete it. >>> +         * io_iopoll_complete() does not hold completion_lock to complete polled >>> +         * io, so here for polled io, we can not call io_req_complete() directly, >>> +         * otherwise there maybe concurrent access to cqring, defer_list, etc, >>> +         * which is not safe. Given that io_iopoll_complete() is always called >>> +         * under uring_lock, so here for polled io, we also get uring_lock to >>> +         * complete it. >>>            */ >>> -        if (req->ctx->flags & IORING_SETUP_IOPOLL) { >>> -            struct kiocb *kiocb = &req->rw.kiocb; >>> +        if (iopoll_enabled) >>> +            mutex_lock(&req->ctx->uring_lock); >>>   -            kiocb_done(kiocb, ret, NULL); >>> -        } else { >>> -            req_set_fail_links(req); >>> -            io_req_complete(req, ret); >>> -        } >>> +        req_set_fail_links(req); >>> +        io_req_complete(req, ret); >>> + >>> +        if (iopoll_enabled) >>> +            mutex_unlock(&req->ctx->uring_lock); >>>       } >>>         return io_steal_work(req); >>> >> -- Pavel Begunkov