Re: [PATCH 2/2] virtio-rng: fix stuck in catting hwrng attributes
From: Rusty Russell <hidden>
Date: 2014-09-11 11:38:03
Also in:
kvm
Amos Kong [off-list ref] 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.
quoted hunk ↗ jump to hunk
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