On Thu, Sep 11, 2014 at 09:08:03PM +0930, Rusty Russell wrote: > Amos Kong <[email protected]> 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 <[email protected]> > > --- > > 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. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html
