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Fri, 16 Apr 2021 06:16:46 -0700 (PDT) To: Jens Axboe , io-uring@vger.kernel.org Cc: Dennis Zhou , Tejun Heo , Christoph Lameter , Joakim Hassila References: <8d04fa58-d8d0-8760-a6aa-d2bd6d66d09d@gmail.com> <36df5986-0716-b7e7-3dac-261a483d074a@kernel.dk> 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 0/2] fix hangs with shared sqpoll Message-ID: Date: Fri, 16 Apr 2021 14:12:34 +0100 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: <36df5986-0716-b7e7-3dac-261a483d074a@kernel.dk> 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 16/04/2021 14:04, Jens Axboe wrote: > On 4/15/21 6:26 PM, Pavel Begunkov wrote: >> On 16/04/2021 01:22, Pavel Begunkov wrote: >>> Late catched 5.12 bug with nasty hangs. Thanks Jens for a reproducer. >> >> 1/2 is basically a rip off of one of old Jens' patches, but can't >> find it anywhere. If you still have it, especially if it was >> reviewed/etc., may make sense to go with it instead > > I wonder if we can do something like the below instead - we don't > care about a particularly stable count in terms of wakeup > reliance, and it'd save a nasty sync atomic switch. But we care about it being monotonous. There are nuances with it. I think, non sync'ed summing may put it to eternal sleep. Are you looking to save on switching? It's almost always is already dying with prior ref_kill > > Totally untested... > > > diff --git a/fs/io_uring.c b/fs/io_uring.c > index 6c182a3a221b..9edbcf01ea49 100644 > --- a/fs/io_uring.c > +++ b/fs/io_uring.c > @@ -8928,7 +8928,7 @@ static void io_uring_cancel_sqpoll(struct io_ring_ctx *ctx) > atomic_inc(&tctx->in_idle); > do { > /* read completions before cancelations */ > - inflight = tctx_inflight(tctx, false); > + inflight = percpu_ref_sum(&ctx->refs); > if (!inflight) > break; > io_uring_try_cancel_requests(ctx, current, NULL); > @@ -8939,7 +8939,7 @@ static void io_uring_cancel_sqpoll(struct io_ring_ctx *ctx) > * avoids a race where a completion comes in before we did > * prepare_to_wait(). > */ > - if (inflight == tctx_inflight(tctx, false)) > + if (inflight == percpu_ref_sum(&ctx->refs)) > schedule(); > finish_wait(&tctx->wait, &wait); > } while (1); > diff --git a/include/linux/percpu-refcount.h b/include/linux/percpu-refcount.h > index 16c35a728b4c..2f29f34bc993 100644 > --- a/include/linux/percpu-refcount.h > +++ b/include/linux/percpu-refcount.h > @@ -131,6 +131,7 @@ void percpu_ref_kill_and_confirm(struct percpu_ref *ref, > void percpu_ref_resurrect(struct percpu_ref *ref); > void percpu_ref_reinit(struct percpu_ref *ref); > bool percpu_ref_is_zero(struct percpu_ref *ref); > +long percpu_ref_sum(struct percpu_ref *ref); > > /** > * percpu_ref_kill - drop the initial ref > diff --git a/lib/percpu-refcount.c b/lib/percpu-refcount.c > index a1071cdefb5a..b09ed9fdd32d 100644 > --- a/lib/percpu-refcount.c > +++ b/lib/percpu-refcount.c > @@ -475,3 +475,31 @@ void percpu_ref_resurrect(struct percpu_ref *ref) > spin_unlock_irqrestore(&percpu_ref_switch_lock, flags); > } > EXPORT_SYMBOL_GPL(percpu_ref_resurrect); > + > +/** > + * percpu_ref_sum - return approximate ref counts > + * @ref: perpcu_ref to sum > + * > + * Note that this should only really be used to compare refs, as by the > + * very nature of percpu references, the value may be stale even before it > + * has been returned. > + */ > +long percpu_ref_sum(struct percpu_ref *ref) > +{ > + unsigned long __percpu *percpu_count; > + long ret; > + > + rcu_read_lock(); > + if (__ref_is_percpu(ref, &percpu_count)) { > + ret = atomic_long_read(&ref->data->count); > + } else { > + int cpu; > + > + ret = 0; > + for_each_possible_cpu(cpu) > + ret += *per_cpu_ptr(percpu_count, cpu); > + } > + rcu_read_unlock(); > + > + return ret; > +} > -- Pavel Begunkov