From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757424AbaIRCnW (ORCPT ); Wed, 17 Sep 2014 22:43:22 -0400 Received: from ozlabs.org ([103.22.144.67]:36745 "EHLO ozlabs.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755705AbaIRCnV (ORCPT ); Wed, 17 Sep 2014 22:43:21 -0400 From: Rusty Russell To: Amos Kong , virtualization@lists.linux-foundation.org Cc: kvm@vger.kernel.org, herbert@gondor.apana.org.au, m@bues.ch, mb@bu3sch.de, mpm@selenic.com, amit.shah@redhat.com, linux-kernel@vger.kernel.org Cc: "Linus Torvalds" Subject: Re: [PATCH v2 2/3] hw_random: fix stuck in catting hwrng attributes In-Reply-To: <1410796949-2221-3-git-send-email-akong@redhat.com> References: <1410796949-2221-1-git-send-email-akong@redhat.com> <1410796949-2221-3-git-send-email-akong@redhat.com> User-Agent: Notmuch/0.17 (http://notmuchmail.org) Emacs/24.3.1 (x86_64-pc-linux-gnu) Date: Thu, 18 Sep 2014 12:13:08 +0930 Message-ID: <87wq91odhf.fsf@rustcorp.com.au> MIME-Version: 1.0 Content-Type: text/plain Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Amos Kong writes: > I started a QEMU (non-smp) guest with one virtio-rng device, and read > random data from /dev/hwrng by dd: > > # dd if=/dev/hwrng of=/dev/null & > > In the same time, if I check hwrng attributes from sysfs by cat: > > # cat /sys/class/misc/hw_random/rng_* > > The cat process always gets stuck with slow backend (5 k/s), if we > use a quick backend (1.2 M/s), the cat process will cost 1 to 2 > minutes. The stuck doesn't exist for smp guest. > > Reading syscall enters kernel and call rng_dev_read(), it's user > context. We used need_resched() to check if other tasks need to > be run, but it almost always return false, and re-hold the mutex > lock. The attributes accessing process always fails to hold the > lock, so the cat gets stuck. > > User context doesn't allow other user contexts run on that CPU, > unless the kernel code sleeps for some reason. This is why the > need_reshed() always return false here. > > This patch removed need_resched() and always schedule other tasks > then other tasks can have chance to hold the lock and execute > protected code. OK, this is going to be a rant. Your explanation doesn't make sense at all. Worse, your solution breaks the advice of Kernighan & Plaugher: "Don't patch bad code - rewrite it.". But worst of all, this detailed explanation might have convinced me you understood the problem better than I did, and applied your patch. I did some tests. For me, as expected, the process spends its time inside the virtio rng read function, holding the mutex and thus blocking sysfs access; it's not a failure of this code at all. Your schedule_timeout() "fix" probably just helps by letting the host refresh entropy, so we spend less time waiting in the read fn. I will post a series, which unfortunately is only lightly tested, then I'm going to have some beer to begin my holiday. That may help me forget my disappointment at seeing respected fellow developers monkey-patching random code they don't understand. Grrr.... Rusty. > Signed-off-by: Amos Kong > --- > drivers/char/hw_random/core.c | 3 +-- > 1 file changed, 1 insertion(+), 2 deletions(-) > > diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c > index c591d7e..263a370 100644 > --- a/drivers/char/hw_random/core.c > +++ b/drivers/char/hw_random/core.c > @@ -195,8 +195,7 @@ static ssize_t rng_dev_read(struct file *filp, char __user *buf, > > mutex_unlock(&rng_mutex); > > - if (need_resched()) > - schedule_timeout_interruptible(1); > + schedule_timeout_interruptible(1); > > if (signal_pending(current)) { > err = -ERESTARTSYS; > -- > 1.9.3