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@xxxxxxxxxx> 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@xxxxxxxxxx> > > > --- > > > 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; > > > -- -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
