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Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.9.0 MIME-Version: 1.0 In-Reply-To: <5d79122d-afcd-9340-df67-d81e1d94dd80@oracle.com> 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 10/29/19 1:40 PM, Bijan Mottahedeh wrote: > > On 10/29/19 12:33 PM, Jens Axboe wrote: >> On 10/29/19 1:31 PM, Bijan Mottahedeh wrote: >>> On 10/29/19 12:27 PM, Jens Axboe wrote: >>>> On 10/29/19 1:23 PM, Bijan Mottahedeh wrote: >>>>> On 10/29/19 12:17 PM, Bijan Mottahedeh wrote: >>>>>> On 10/25/19 7:21 AM, Jens Axboe wrote: >>>>>>> On 10/25/19 8:18 AM, Jens Axboe wrote: >>>>>>>> On 10/25/19 8:07 AM, Jens Axboe wrote: >>>>>>>>> On 10/25/19 7:46 AM, Bijan Mottahedeh wrote: >>>>>>>>>> On 10/24/19 3:31 PM, Jens Axboe wrote: >>>>>>>>>>> On 10/24/19 1:18 PM, Bijan Mottahedeh wrote: >>>>>>>>>>>> On 10/24/19 10:09 AM, Jens Axboe wrote: >>>>>>>>>>>>> On 10/24/19 3:18 AM, Bijan Mottahedeh wrote: >>>>>>>>>>>>>> Running an fio test consistenly crashes the kernel with the >>>>>>>>>>>>>> trace included >>>>>>>>>>>>>> below. The root cause seems to be the code in >>>>>>>>>>>>>> __io_submit_sqe() that >>>>>>>>>>>>>> checks the result of a request for -EAGAIN in polled mode, >>>>>>>>>>>>>> without >>>>>>>>>>>>>> ensuring first that the request has completed: >>>>>>>>>>>>>> >>>>>>>>>>>>>> if (ctx->flags & IORING_SETUP_IOPOLL) { >>>>>>>>>>>>>> if (req->result == -EAGAIN) >>>>>>>>>>>>>> return -EAGAIN; >>>>>>>>>>>>> I'm a little confused, because we should be holding the submission >>>>>>>>>>>>> reference to the request still at this point. So how is it >>>>>>>>>>>>> going away? >>>>>>>>>>>>> I must be missing something... >>>>>>>>>>>> I don't think the submission reference is going away... >>>>>>>>>>>> >>>>>>>>>>>> I *think* the problem has to do with the fact that >>>>>>>>>>>> io_complete_rw_iopoll() which sets REQ_F_IOPOLL_COMPLETED is being >>>>>>>>>>>> called from interrupt context in my configuration and so there is a >>>>>>>>>>>> potential race between updating the request there and checking >>>>>>>>>>>> it in >>>>>>>>>>>> __io_submit_sqe(). >>>>>>>>>>>> >>>>>>>>>>>> My first workaround was to simply poll for >>>>>>>>>>>> REQ_F_IOPOLL_COMPLETED in the >>>>>>>>>>>> code snippet above: >>>>>>>>>>>> >>>>>>>>>>>> if (req->result == --EAGAIN) { >>>>>>>>>>>> >>>>>>>>>>>> poll for REQ_F_IOPOLL_COMPLETED >>>>>>>>>>>> >>>>>>>>>>>> return -EAGAIN; >>>>>>>>>>>> >>>>>>>>>>>> } >>>>>>>>>>>> >>>>>>>>>>>> and that got rid of the problem. >>>>>>>>>>> But that will not work at all for a proper poll setup, where you >>>>>>>>>>> don't >>>>>>>>>>> trigger any IRQs... It only happens to work for this case because >>>>>>>>>>> you're >>>>>>>>>>> still triggering interrupts. But even in that case, it's not a real >>>>>>>>>>> solution, but I don't think that's the argument here ;-) >>>>>>>>>> Sure. >>>>>>>>>> >>>>>>>>>> I'm just curious though as how it would break the poll case because >>>>>>>>>> io_complete_rw_iopoll() would still be called though through polling, >>>>>>>>>> REQ_F_IOPOLL_COMPLETED would be set, and so io_iopoll_complete() >>>>>>>>>> should be able to reliably check req->result. >>>>>>>>> It'd break the poll case because the task doing the submission is >>>>>>>>> generally also the one that finds and reaps completion. Hence if you >>>>>>>>> block that task just polling on that completion bit, you are >>>>>>>>> preventing >>>>>>>>> that very task from going and reaping completions. The condition would >>>>>>>>> never become true, and you are now looping forever. >>>>>>>>> >>>>>>>>>> The same poll test seemed to run ok with nvme interrupts not being >>>>>>>>>> triggered. Anyway, no argument that it's not needed! >>>>>>>>> A few reasons why it would make progress: >>>>>>>>> >>>>>>>>> - You eventually trigger a timeout on the nvme side, as blk-mq >>>>>>>>> finds the >>>>>>>>> request hasn't been completed by an IRQ. But that's a 30 >>>>>>>>> second ordeal >>>>>>>>> before that event occurs. >>>>>>>>> >>>>>>>>> - There was still interrupts enabled. >>>>>>>>> >>>>>>>>> - You have two threads, one doing submission and one doing >>>>>>>>> completions. >>>>>>>>> Maybe using SQPOLL? If that's the case, then yes, it'd still >>>>>>>>> work as >>>>>>>>> you have separate threads for submission and completion. >>>>>>>>> >>>>>>>>> For the "generic" case of just using one thread and IRQs disabled, >>>>>>>>> it'd >>>>>>>>> deadlock. >>>>>>>>> >>>>>>>>>>> I see what the race is now, it's specific to IRQ driven polling. We >>>>>>>>>>> really should just disallow that, to be honest, it doesn't make any >>>>>>>>>>> sense. But let me think about if we can do a reasonable solution >>>>>>>>>>> to this >>>>>>>>>>> that doesn't involve adding overhead for a proper setup. >>>>>>>>>> It's a nonsensical config in a way and so disallowing it would make >>>>>>>>>> the most sense. >>>>>>>>> Definitely. The nvme driver should not set .poll() if it doesn't have >>>>>>>>> non-irq poll queues. Something like this: >>>>>>>> Actually, we already disable polling if we don't have specific poll >>>>>>>> queues: >>>>>>>> >>>>>>>> if (set->nr_maps > HCTX_TYPE_POLL && >>>>>>>> set->map[HCTX_TYPE_POLL].nr_queues) >>>>>>>> blk_queue_flag_set(QUEUE_FLAG_POLL, q); >>>>>>>> >>>>>>>> Did you see any timeouts in your tests? I wonder if the use-after-free >>>>>>>> triggered when the timeout found the request while you had the >>>>>>>> busy-spin >>>>>>>> logic we discussed previously. >>>>>>> Ah, but we still have fops->iopoll() set for that case. So we just won't >>>>>>> poll for it, it'll get completed by IRQ. So I do think we need to handle >>>>>>> this case in io_uring. I'll get back to you. >>>>>>> >>>>>> I ran the same test on linux-next-20191029 in polled mode and got the >>>>>> same free-after-user panic: >>>>>> >>>>>> - I booted with nvme.poll_queues set and verified that all queues >>>>>> except default where of type poll >>>>>> >>>>>> - I added three assertions to verify the following: >>>>>> >>>>>> - nvme_timeout() is not called >>>>>> >>>>>> - io_complete_rw_iopoll() is not called from interrupt context >>>>>> >>>>>> - io_sq_offload_start() is not called with IORING_SETUP_SQPOLL set >>>>>> >>>>>> Is it possible that the race is there also in polled mode since a >>>>>> request submitted by one thread could conceivably be polled for and >>>>>> completed by a different thread, e.g. in >>>>>> io_uring_enter()->io_iopoll_check()? >>>>>> >>>>>> --bijan >>>>>> >>>>>> >>>>> I also tested my RFC again with 1 thread and with queue depths of 1 to >>>>> 1024 in multiples of 8 and didn't see any hangs. >>>>> >>>>> Just to be clear, the busy-spin logic discussed before was only a >>>>> workaround an not in the RFC. >>>> What is your exact test case? >>>> >>> See original cover letter. I can reproduce the failure with numjobs >>> between 8 and 32. >> And how many poll queues are you using? >> > 30 And how many threads/cores in the box? Trying to get a sense for how many CPUs share a single poll queue, if any. -- Jens Axboe