Re: 2.6.17-rc4-mm3

[Valdis . Kletnieks]

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);
+
 		if (signal_pending(current))
 			goto out;
 	}

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