Linux Virtualization - Sep 2014 - [PATCH 2/2] virtio-rng: fix stuck in catting hwrng attributes

Amos Kong <akong at 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().

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 at 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

On Thu, Sep 11, 2014 at 09:08:03PM +0930, Rusty Russell
wrote:
> Amos Kong <akong at 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 at 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.

On Sun, Sep 14, 2014 at 01:12:58AM +0800, Amos Kong
wrote:
> On Thu, Sep 11, 2014 at 09:08:03PM +0930, Rusty Russell wrote:
> > Amos Kong <akong at 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.
%s/they/why/

Hi Rusty,


Reference:
http://www.linuxgrill.com/anonymous/fire/netfilter/kernel-hacking-HOWTO-2.html

read() syscall of /dev/hwrng will enter into kernel, the read operation is
rng_dev_read(), it's userspace context (not interrupt context).

Userspace context doesn't allow other user contexts run on that CPU,
unless the kernel code sleeps for some reason.


In this case, the need_resched() doesn't work.

My solution is removing need_resched() and use an appropriate delay by 
schedule_timeout_interruptible(10).

Thanks, Amos
  
> > 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 at 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;
> > > --