On Tue, Sep 26, 2017 at 02:36:57AM -0400, Pankaj Gupta wrote: > > > > > A bit late to a party, but: > > > > On Mon, Dec 8, 2014 at 12:50 AM, Amos Kong <akong@xxxxxxxxxx> wrote: > > > From: Rusty Russell <rusty@xxxxxxxxxxxxxxx> > > > > > > There's currently a big lock around everything, and it means that we > > > can't query sysfs (eg /sys/devices/virtual/misc/hw_random/rng_current) > > > while the rng is reading. This is a real problem when the rng is slow, > > > or blocked (eg. virtio_rng with qemu's default /dev/random backend) > > > > > > This doesn't help (it leaves the current lock untouched), just adds a > > > lock to protect the read function and the static buffers, in preparation > > > for transition. > > > > > > Signed-off-by: Rusty Russell <rusty@xxxxxxxxxxxxxxx> > > > --- > > ... > > > > > > @@ -160,13 +166,14 @@ static ssize_t rng_dev_read(struct file *filp, char > > > __user *buf, > > > goto out_unlock; > > > } > > > > > > + mutex_lock(&reading_mutex); > > > > I think this breaks O_NONBLOCK: we have hwrng core thread that is > > constantly pumps underlying rng for data; the thread takes the mutex > > and calls rng_get_data() that blocks until RNG responds. This means > > that even user specified O_NONBLOCK here we'll be waiting until > > [hwrng] thread releases reading_mutex before we can continue. > > I think for 'virtio_rng' for 'O_NON_BLOCK' 'rng_get_data' returns > without waiting for data which can let mutex to be used by other > threads waiting if any? > > rng_dev_read > rng_get_data > virtio_read As I said in the paragraph above the code that potentially holds the mutex for long time is the thread in hwrng core: hwrng_fillfn(). As it calls rng_get_data() with "wait" argument == 1 it may block while holding reading_mutex, which, in turn, will block rng_dev_read(), even if it was called with O_NONBLOCK. Thanks. -- Dmitry
