On Monday 22 May 2006 13:38, you wrote:
> On Mon, 22 May 2006 13:25:10 +0200, Michael Buesch said:
> > On Monday 22 May 2006 13:15, you wrote:
>
> > > It looks to me line the old code stayed in a while() loop in rng_dev_read
> > > until it had fulfilled the read request (including possibly multiple
> > > calls to need_resched() and friends). The new code will bail on an -EAGAIN
> > > as soon as the *first* poll fails, rather than waiting until something
> > > is available - even if it is NOT flagged O_NONBLOCK.
> >
> > Yeah. That is how it works. I am wondering why userspace doesn't
> > simply retry, if it receives an EAGAIN.
> > Should we return ERESTARTSYS or something like that instead?
>
> That's not the way it worked in previous kernels, and it's not the way that
> the current rng-utils RPM in Fedora expects it to work.
>
> Here's a patch that makes it work the way it used to. Adding the test
> for O_NONBLOCK is the biggie - the old code did a resched test at that
> point in the loop, so I added it here too.
>
> --- linux-2.6.17-rc4-mm3/drivers/char/hw_random/core.c.rnd_fix 2006-05-22 07:23:34.000000000 -0400
> +++ linux-2.6.17-rc4-mm3/drivers/char/hw_random/core.c 2006-05-22 07:22:29.000000000 -0400
> @@ -125,7 +125,7 @@ static ssize_t rng_dev_read(struct file
> mutex_unlock(&rng_mutex);
>
> err = -EAGAIN;
> - if (!bytes_read)
> + if (filp->f_flags & O_NONBLOCK && !bytes_read)
> goto out;
>
> err = -EFAULT;
> @@ -138,6 +138,9 @@ static ssize_t rng_dev_read(struct file
> data >>= 8;
> }
>
> + if (need_resched())
> + schedule_timeout_interruptible(1);
> +
Andrew's comment on this:
What's going on with the need_resched() tricks in there? (Unobvious, needs
a comment). From my reading, it'll cause a caller to this function to hang
for arbitrary periods of time if something if causing heavy scheduling
pressure.
So I decided to remove it and return -EAGAIN, so userspace can retry.
But seems like it it does not. I thought glibc would handle that.
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