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bh=DFll6b5XSR2+mVH3UkBsPiEUvx10UhW2nPsXPtrbYrA=; b=mWmR47h85GurhEqAuf3Z2rpCGZIRnlHI8tadXMhjnig3Tw5z2jabIOEsfNAExLyKTQ 4lbH4nYM/GGveltwzXv4YoRUZLOBMHt7JmW14ojanop4awNrNe5wUcHC5paMPNmbXTQg P3biF+n1taWH70opzrxsYnDdnTgtj6oYEXmif6o9Wkk8NOpXQPwRNUaMDQrNtUtJOdAB ZfZnjcQ/lFfK1rIWknMmuumiv3m3US7nxyw0s5WBmizhuUcKh7UWxh7ootHr1YbwU7a9 S+fQokW+RLIIhPvnBIPmvtSGqE9/EGydAyW7DWVDILdBZZMX67r9lRbG2C2oh4Ez6h2i Emfg== X-Gm-Message-State: APjAAAU4t5KqgjlRT7CsF2yEdXEp1Y6mZ4i8YykUxcPRJdzdwFLH2Y9W 5XDACvyoxI1z86IMGbMNir3xXHby60VpMw== X-Google-Smtp-Source: APXvYqyTjm8aPBRGzwcAJDPXqOuEVqPJ/skVfz9/Bm7zS1Rnk8yMOUyoF+v/SAGagDP+YrzJDCU2Cg== X-Received: by 2002:a92:9e08:: with SMTP id q8mr30863413ili.108.1572378779866; Tue, 29 Oct 2019 12:52:59 -0700 (PDT) Received: from [192.168.1.159] ([65.144.74.34]) by smtp.gmail.com with ESMTPSA id s28sm1442967ioa.88.2019.10.29.12.52.58 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 29 Oct 2019 12:52:58 -0700 (PDT) Subject: Re: [RFC 0/2] io_uring: examine request result only after completion To: Bijan Mottahedeh Cc: linux-block@vger.kernel.org References: <1571908688-22488-1-git-send-email-bijan.mottahedeh@oracle.com> <22fc1057-237b-a9b8-5a57-b7c53166a609@kernel.dk> <201931df-ae22-c2fc-a9c7-496ceb87dff7@oracle.com> <90c23805-4b73-4ade-1b54-dc68010b54dd@kernel.dk> <533409a8-6907-44d8-1b90-a10ec3483c2c@kernel.dk> <6adb9d2d-93f1-f915-7f20-5faa34b06398@kernel.dk> <34f483d9-2a97-30c3-9937-d3596649356c@oracle.com> <47b38d9d-04a3-99f6-c586-e82611d21655@kernel.dk> <057bb6f9-29ec-1160-a1b1-00c57b610282@kernel.dk> <5d79122d-afcd-9340-df67-d81e1d94dd80@oracle.com> <3b71fff1-5b5e-3d33-b701-c7e1b3c9d8b9@oracle.com> From: Jens Axboe Message-ID: Date: Tue, 29 Oct 2019 13:52:57 -0600 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.9.0 MIME-Version: 1.0 In-Reply-To: <3b71fff1-5b5e-3d33-b701-c7e1b3c9d8b9@oracle.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On 10/29/19 1:51 PM, Bijan Mottahedeh wrote: > > On 10/29/19 12:46 PM, Jens Axboe wrote: >> 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. >> > Thread(s) per core:    2 > Core(s) per socket:    8 > Socket(s):             2 OK, so 2*8*2 == 32 threads, hence some threads will share a poll queue. -- Jens Axboe