Amos Kong <akong@xxxxxxxxxx> writes:
> From: Rusty Russell <rusty@xxxxxxxxxxxxxxx>
>
> current_rng holds one reference, and we bump it every time we want
> to do a read from it.
>
> This means we only hold the rng_mutex to grab or drop a reference,
> so accessing /sys/devices/virtual/misc/hw_random/rng_current doesn't
> block on read of /dev/hwrng.
>
> Using a kref is overkill (we're always under the rng_mutex), but
> a standard pattern.
>
> This also solves the problem that the hwrng_fillfn thread was
> accessing current_rng without a lock, which could change (eg. to NULL)
> underneath it.
>
> V2: reduce reference count in signal_pending path
OK, I changed it to do the put_rng before the check, so instead of:
> @@ -208,17 +256,19 @@ static ssize_t rng_dev_read(struct file *filp, char __user *buf,
>
> if (signal_pending(current)) {
> err = -ERESTARTSYS;
> + put_rng(rng);
> goto out;
> }
> +
> + put_rng(rng);
> }
> out:
> return ret ? : err;
> -out_unlock:
> - mutex_unlock(&rng_mutex);
> - goto out;
> +
> out_unlock_reading:
> mutex_unlock(&reading_mutex);
> - goto out_unlock;
> + put_rng(rng);
> + goto out;
> }
We have:
mutex_unlock(&reading_mutex);
put_rng(rng);
if (need_resched())
schedule_timeout_interruptible(1);
if (signal_pending(current)) {
err = -ERESTARTSYS;
goto out;
}
}
out:
return ret ? : err;
out_unlock_reading:
mutex_unlock(&reading_mutex);
put_rng(rng);
goto out;
}
Cheers,
Rusty.
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