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Thu, 13 Jun 2019 08:53:17 +0000 (UTC) To: Amit Shah , qemu-devel@nongnu.org References: <20190611172032.19143-1-lvivier@redhat.com> <8c2d26799074a46fb5f2aaae7dc4e951ec8318a2.camel@infradead.org> From: Laurent Vivier Openpgp: preference=signencrypt Autocrypt: addr=lvivier@redhat.com; prefer-encrypt=mutual; keydata= mQINBFYFJhkBEAC2me7w2+RizYOKZM+vZCx69GTewOwqzHrrHSG07MUAxJ6AY29/+HYf6EY2 WoeuLWDmXE7A3oJoIsRecD6BXHTb0OYS20lS608anr3B0xn5g0BX7es9Mw+hV/pL+63EOCVm SUVTEQwbGQN62guOKnJJJfphbbv82glIC/Ei4Ky8BwZkUuXd7d5NFJKC9/GDrbWdj75cDNQx UZ9XXbXEKY9MHX83Uy7JFoiFDMOVHn55HnncflUncO0zDzY7CxFeQFwYRbsCXOUL9yBtqLer Ky8/yjBskIlNrp0uQSt9LMoMsdSjYLYhvk1StsNPg74+s4u0Q6z45+l8RAsgLw5OLtTa+ePM JyS7OIGNYxAX6eZk1+91a6tnqfyPcMbduxyBaYXn94HUG162BeuyBkbNoIDkB7pCByed1A7q q9/FbuTDwgVGVLYthYSfTtN0Y60OgNkWCMtFwKxRaXt1WFA5ceqinN/XkgA+vf2Ch72zBkJL RBIhfOPFv5f2Hkkj0MvsUXpOWaOjatiu0fpPo6Hw14UEpywke1zN4NKubApQOlNKZZC4hu6/ 8pv2t4HRi7s0K88jQYBRPObjrN5+owtI51xMaYzvPitHQ2053LmgsOdN9EKOqZeHAYG2SmRW LOxYWKX14YkZI5j/TXfKlTpwSMvXho+efN4kgFvFmP6WT+tPnwARAQABtCNMYXVyZW50IFZp dmllciA8bHZpdmllckByZWRoYXQuY29tPokCOAQTAQIAIgUCVgVQgAIbAwYLCQgHAwIGFQgC CQoLBBYCAwECHgECF4AACgkQ8ww4vT8vvjwpgg//fSGy0Rs/t8cPFuzoY1cex4limJQfReLr SJXCANg9NOWy/bFK5wunj+h/RCFxIFhZcyXveurkBwYikDPUrBoBRoOJY/BHK0iZo7/WQkur 6H5losVZtrotmKOGnP/lJYZ3H6OWvXzdz8LL5hb3TvGOP68K8Bn8UsIaZJoeiKhaNR0sOJyI YYbgFQPWMHfVwHD/U+/gqRhD7apVysxv5by/pKDln1I5v0cRRH6hd8M8oXgKhF2+rAOL7gvh jEHSSWKUlMjC7YwwjSZmUkL+TQyE18e2XBk85X8Da3FznrLiHZFHQ/NzETYxRjnOzD7/kOVy gKD/o7asyWQVU65mh/ECrtjfhtCBSYmIIVkopoLaVJ/kEbVJQegT2P6NgERC/31kmTF69vn8 uQyW11Hk8tyubicByL3/XVBrq4jZdJW3cePNJbTNaT0d/bjMg5zCWHbMErUib2Nellnbg6bc 2HLDe0NLVPuRZhHUHM9hO/JNnHfvgiRQDh6loNOUnm9Iw2YiVgZNnT4soUehMZ7au8PwSl4I KYE4ulJ8RRiydN7fES3IZWmOPlyskp1QMQBD/w16o+lEtY6HSFEzsK3o0vuBRBVp2WKnssVH qeeV01ZHw0bvWKjxVNOksP98eJfWLfV9l9e7s6TaAeySKRRubtJ+21PRuYAxKsaueBfUE7ZT 7ze5Ag0EVgUmGQEQALxSQRbl/QOnmssVDxWhHM5TGxl7oLNJms2zmBpcmlrIsn8nNz0rRyxT 460k2niaTwowSRK8KWVDeAW6ZAaWiYjLlTunoKwvF8vP3JyWpBz0diTxL5o+xpvy/Q6YU3BN efdq8Vy3rFsxgW7mMSrI/CxJ667y8ot5DVugeS2NyHfmZlPGE0Nsy7hlebS4liisXOrN3jFz asKyUws3VXek4V65lHwB23BVzsnFMn/bw/rPliqXGcwl8CoJu8dSyrCcd1Ibs0/Inq9S9+t0 VmWiQWfQkz4rvEeTQkp/VfgZ6z98JRW7S6l6eophoWs0/ZyRfOm+QVSqRfFZdxdP2PlGeIFM C3fXJgygXJkFPyWkVElr76JTbtSHsGWbt6xUlYHKXWo+xf9WgtLeby3cfSkEchACrxDrQpj+ Jt/JFP+q997dybkyZ5IoHWuPkn7uZGBrKIHmBunTco1+cKSuRiSCYpBIXZMHCzPgVDjk4viP brV9NwRkmaOxVvye0vctJeWvJ6KA7NoAURplIGCqkCRwg0MmLrfoZnK/gRqVJ/f6adhU1oo6 z4p2/z3PemA0C0ANatgHgBb90cd16AUxpdEQmOCmdNnNJF/3Zt3inzF+NFzHoM5Vwq6rc1JP jfC3oqRLJzqAEHBDjQFlqNR3IFCIAo4SYQRBdAHBCzkM4rWyRhuVABEBAAGJAh8EGAECAAkF AlYFJhkCGwwACgkQ8ww4vT8vvjwg9w//VQrcnVg3TsjEybxDEUBm8dBmnKqcnTBFmxN5FFtI WlEuY8+YMiWRykd8Ln9RJ/98/ghABHz9TN8TRo2b6WimV64FmlVn17Ri6FgFU3xNt9TTEChq AcNg88eYryKsYpFwegGpwUlaUaaGh1m9OrTzcQy+klVfZWaVJ9Nw0keoGRGb8j4XjVpL8+2x OhXKrM1fzzb8JtAuSbuzZSQPDwQEI5CKKxp7zf76J21YeRrEW4WDznPyVcDTa+tz++q2S/Bp P4W98bXCBIuQgs2m+OflERv5c3Ojldp04/S4NEjXEYRWdiCxN7ca5iPml5gLtuvhJMSy36gl U6IW9kn30IWuSoBpTkgV7rLUEhh9Ms82VWW/h2TxL8enfx40PrfbDtWwqRID3WY8jLrjKfTd R3LW8BnUDNkG+c4FzvvGUs8AvuqxxyHbXAfDx9o/jXfPHVRmJVhSmd+hC3mcQ+4iX5bBPBPM oDqSoLt5w9GoQQ6gDVP2ZjTWqwSRMLzNr37rJjZ1pt0DCMMTbiYIUcrhX8eveCJtY7NGWNyx FCRkhxRuGcpwPmRVDwOl39MB3iTsRighiMnijkbLXiKoJ5CDVvX5yicNqYJPKh5MFXN1bvsB kmYiStMRbrD0HoY1kx5/VozBtc70OU0EB8Wrv9hZD+Ofp0T3KOr1RUHvCZoLURfFhSQ= Message-ID: <3ecdbf3e-abb1-b74c-7751-9740b5a2e4fc@redhat.com> Date: Thu, 13 Jun 2019 10:53:16 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.0 MIME-Version: 1.0 In-Reply-To: <8c2d26799074a46fb5f2aaae7dc4e951ec8318a2.camel@infradead.org> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.38]); Thu, 13 Jun 2019 08:53:20 +0000 (UTC) X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] X-Received-From: 209.132.183.28 Subject: Re: [Qemu-devel] [RFC] virtio-rng: add a watchdog X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: "Michael S. Tsirkin" , Amit Shah Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 12/06/2019 09:03, Amit Shah wrote: > On Tue, 2019-06-11 at 19:20 +0200, Laurent Vivier wrote: >> The virtio-rng linux driver can be stuck in virtio_read() on a >> wait_for_completion_killable() call if the virtio-rng device in QEMU >> doesn't provide data. >> >> It's a problem, because virtio_read() is called from rng_get_data() >> with >> reading_mutex() held. The same mutex is taken by >> add_early_randomness() >> and hwrng_fillfn() and this brings to a hang during the boot sequence >> if >> the virtio-rng driver is builtin. >> Moreover, another lock is taken (rng_mutex) when the hwrng driver >> wants to switch the RNG device or the user tries to unplug the >> virtio-rng >> PCI card, and this can hang too because the virtio-rng driver is only >> able >> to release the card if the virtio-rng device sends back the virtqueue >> element. >> >> # echo -n virtio_rng.1 > /sys/class/misc/hw_random/rng_current >> [ 240.165234] INFO: task kworker/u2:1:34 blocked for more than 120 >> seconds. >> [ 240.165961] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" >> disables this message. >> [ 240.166708] kworker/u2:1 D >> ffffffffb86b85a8 0 34 2 0x00000000 >> [ 240.166714] Workqueue: kacpi_hotplug acpi_hotplug_work_fn >> [ 240.166716] ffffa0e8f3c0b890 0000000000000046 ffffa0e8f3c00000 >> ffffa0e8f3c0bfd8 >> [ 240.166717] ffffa0e8f3c0bfd8 ffffa0e8f3c0bfd8 ffffa0e8f3c00000 >> ffffffffb86b85a0 >> [ 240.166719] ffffffffb86b85a4 ffffa0e8f3c00000 00000000ffffffff >> ffffffffb86b85a8 >> [ 240.166720] Call Trace: >> [ 240.166725] [] >> schedule_preempt_disabled+0x29/0x70 >> [ 240.166727] [] >> __mutex_lock_slowpath+0xc7/0x1d0 >> [ 240.166728] [] mutex_lock+0x1f/0x2f >> [ 240.166730] [] hwrng_register+0x32/0x1d0 >> [ 240.166733] [] virtrng_scan+0x19/0x30 >> [virtio_rng] >> [ 240.166744] [] virtio_dev_probe+0x1eb/0x290 >> [virtio] >> [ 240.166746] [] >> driver_probe_device+0x145/0x3c0 >> ... >> >> In some case, the QEMU RNG backend is not able to provide data, and >> the virtio-rng device is not aware of that: >> - with rng-random using /dev/random and no entropy is available, >> - with rng-egd started with a socket in "server,nowait" mode and >> no daemon connected, >> - with rng-egd and an egd daemon that is not providing enough data, >> - ... >> >> To release the locks regularly, this patch adds a watchdog in QEMU >> virtio-rng device that sends back to the guest the virtqueue buffer >> with a 0 byte payload. This case is expected and correctly managed by >> the hwrng core. > > I'm wondering if it makes more sense to rework the way the kernel > driver requests for seeding entropy during probe. The kernel side was my first angle of attack. I tried first to not block in add_early_randomness(): "hwrng: core - don't block in add_early_randomness()" https://patchwork.kernel.org/patch/10877571/ But I agree with the maintainer, the problem must be fixed at virtio-rng level. > The virtio_read call is killable, so it can take signals when initiated > by userspace. For the initial probe, specifying a timeout / watchdog > in the driver is better. Yes, I think also it's better, I tried to do something like that: --- a/drivers/char/hw_random/virtio-rng.c +++ b/drivers/char/hw_random/virtio-rng.c @@ -77,10 +77,7 @@ static int virtio_read(struct hwrng *rng, void *buf, size_t size, bool wait) register_buffer(vi, buf, size); } - if (!wait) - return 0; - - ret = wait_for_completion_killable(&vi->have_data); + ret = wait_for_completion_timeout(&vi->have_data, wait ? MAX_SCHEDULE_TIMEOUT : HZ); if (ret < 0) return ret; But I have a problem doing the timeout / watchdog at driver level: once the buffer is submitted to the virtqueue, how to cancel it? Is there a way to ask the QEMU device to not process the element in the virtqueue we have stopped to wait for because of the timeout (or for the signal: I don't understand how it works in this case. How it is canceled?)? Thanks, Laurent