From mboxrd@z Thu Jan  1 00:00:00 1970
From: Amos Kong <akong@redhat.com>
Subject: Re: [PATCH 2/2] virtio-rng: fix stuck in catting hwrng attributes
Date: Sun, 14 Sep 2014 01:12:58 +0800
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Cc: amit.shah@redhat.com, kvm@vger.kernel.org,
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To: Rusty Russell <rusty@rustcorp.com.au>
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On Thu, Sep 11, 2014 at 09:08:03PM +0930, Rusty Russell wrote:
> Amos Kong <akong@redhat.com> 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().

But actually sysfs process's not woken always, this is they the
process gets stuck.
 
> 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 <akong@redhat.com>
> > ---
> >  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

-- 
			Amos.