From mboxrd@z Thu Jan 1 00:00:00 1970 From: Rusty Russell Subject: Re: [PATCH 2/2] virtio-rng: fix stuck in catting hwrng attributes Date: Thu, 11 Sep 2014 21:08:03 +0930 Message-ID: <8738byie04.fsf@rustcorp.com.au> References: <1410340027-15373-1-git-send-email-akong@redhat.com> <1410340027-15373-3-git-send-email-akong@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: amit.shah@redhat.com, kvm@vger.kernel.org To: Amos Kong , virtualization@lists.linux-foundation.org Return-path: In-Reply-To: <1410340027-15373-3-git-send-email-akong@redhat.com> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: virtualization-bounces@lists.linux-foundation.org Errors-To: virtualization-bounces@lists.linux-foundation.org List-Id: kvm.vger.kernel.org Amos Kong writes: > When I check hwrng attributes in sysfs, cat process always gets > stuck if guest has only 1 vcpu and uses a slow rng backend. > > Currently we check if there is any tasks waiting to be run on > current cpu in rng_dev_read() by need_resched(). But need_resched() > doesn't work because rng_dev_read() is executing in user context. I don't understand this explanation? I'd expect the sysfs process to be woken by the mutex_unlock(). If we're really high priority (vs. the sysfs process) then I can see why we'd need schedule_timeout_interruptible() instead of just schedule(), and in that case, need_resched() would be false too. You could argue that's intended behaviour, but I can't see how it happens in the normal case anyway. What am I missing? Thanks, Rusty. > This patch removed need_resched() and increase delay to 10 jiffies, > then other tasks can have chance to execute protected code. > Delaying 1 jiffy also works, but 10 jiffies is safer. > > 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..b5d1b6f 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(10); > > if (signal_pending(current)) { > err = -ERESTARTSYS; > -- > 1.9.3