Re: [PATCH 2/2] virtio-rng: fix stuck in catting hwrng attributes

[Rusty Russell <rusty@rustcorp.com.au>]

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().

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