On Thu, Sep 11, 2014 at 09:08:03PM +0930, Rusty Russell wrote:

Amos Kong [off-list ref] writes:

quoted

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.

quoted

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 <redacted>
---
 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.