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    SubjectRe: [PATCH v5 REPOST 1/6] hw_random: place mutex around read functions and buffers.
    A bit late to a party, but:

    On Mon, Dec 8, 2014 at 12:50 AM, Amos Kong <akong@redhat.com> wrote:
    > From: Rusty Russell <rusty@rustcorp.com.au>
    >
    > 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@rustcorp.com.au>
    > ---
    ...
    >
    > @@ -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.

    > if (!data_avail) {
    > bytes_read = rng_get_data(current_rng, rng_buffer,
    > rng_buffer_size(),
    > !(filp->f_flags & O_NONBLOCK));
    > if (bytes_read < 0) {
    > err = bytes_read;
    > - goto out_unlock;
    > + goto out_unlock_reading;
    > }
    > data_avail = bytes_read;
    > }

    Thanks.

    --
    Dmitry

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