From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755587AbaIRMtG (ORCPT ); Thu, 18 Sep 2014 08:49:06 -0400 Received: from mx1.redhat.com ([209.132.183.28]:42515 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751294AbaIRMtE (ORCPT ); Thu, 18 Sep 2014 08:49:04 -0400 Date: Thu, 18 Sep 2014 20:47:45 +0800 From: Amos Kong To: Rusty Russell Cc: virtualization@lists.linux-foundation.org, 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, Linus Torvalds Subject: Re: [PATCH v2 2/3] hw_random: fix stuck in catting hwrng attributes Message-ID: <20140918124745.GB29803@air.redhat.com> References: <1410796949-2221-1-git-send-email-akong@redhat.com> <1410796949-2221-3-git-send-email-akong@redhat.com> <87wq91odhf.fsf@rustcorp.com.au> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87wq91odhf.fsf@rustcorp.com.au> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Sep 18, 2014 at 12:13:08PM +0930, Rusty Russell wrote: > 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. Hi Rusty, > 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'm sorry about the misleading. > 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. Got it now. The catting hang bug was found when I try to fix unhotplug issue, the unhotplug issue can't be reproduced if I try to debug by gdb or printk. So I forgot to debug cat hang ... but spend time to misunderstand schedle code :( > 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. I just posted a V2 with two additional fixes, hotunplugging works well now :) > Grrr.... Enjoy your holiday! Amos > 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 -- Amos.