>
> 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.
yes, 'hwrng_fillfn' does not consider O_NONBLOCK and can result in mutex wait
for other tasks. What if we pass zero for wait to 'hwrng_fill' to return early
if there is no data?
--- a/drivers/char/hw_random/core.c
+++ b/drivers/char/hw_random/core.c
@@ -403,7 +403,7 @@ static int hwrng_fillfn(void *unused)
break;
mutex_lock(&reading_mutex);
rc = rng_get_data(rng, rng_fillbuf,
- rng_buffer_size(), 1);
+ rng_buffer_size(), 0);
mutex_unlock(&reading_mutex);
put_rng(rng);
if (rc <= 0) {
Thanks,
Pankaj
>
> Thanks.
>
> --
> Dmitry
>
