Re: [PATCH] splice: fix problem with sys_tee and SPLICE_F_NONBLOCK

[Jens Axboe <jens.axboe@oracle.com>]

On Tue, Feb 19 2008, Johann Felix Soden wrote:
> 
> Am Dienstag, den 19.02.2008, 22:25 +0100 schrieb Jens Axboe:
> > On Tue, Feb 19 2008, Johann Felix Soden wrote:
> > > > On Tue, Feb 19 2008, Johann Felix Soden wrote:
> > > > > From: Johann Felix Soden <johfel@users.sourceforge.net>
> > > > > 
> > > > > With SPLICE_F_NONBLOCK sys_tee should return number of duplicated bytes,
> > > > > not only -EAGAIN on success.
> > > > 
> > > > ?
> > > > 
> > > > The current behaviour is to return bytes tee'd, or return -EAGAIN for
> > > > zero bytes if SPLICE_F_NONBLOCK is set. It doesn't return "-EAGAIN on
> > > > success", not sure what you mean there.
> > > > 
> > > Sorry, my patch description was not correct. 
> > > 
> > > The new behavior of sys_tee with my patch is: 
> > > 	- return -EAGAIN if there are no data in the pipe, but writer
> > > 	  connected to the pipe, 
> > > 	- return 0 if there are not writers connected
> > > 	- else return number of duplicated byte 
> > > 
> > > The old behavior was: return -EAGAIN or the number (>0) of duplicated
> > > bytes.
> > 
> > Your patch has an odd way of achieving that goal, modify the real
> > location of the assignment instead of overriding something. That has the
> > potential to turn into another confusing bug later on, wondering why the
> > heck your return value isn't being passed back.
> > 
> > Improvement is welcome though, you can't distuingish -EAGAIN on the
> > input side from the output side currently.
> > 
> 
> I thought again about the problem and my patch: you are right, the patch
> is nonsense. I have learnt, that the correctness of a patch is not
> guaranteed by the (bad, but anyhow working) solution of the problem the
> patch was written for.
> Sorry for wasting your time.

Don't worry, it's not a waste of time even though your solution isn't
the correct one.

When non-blocking is set, ideally we want to return 0 if there's no hope
of anymore data and EAGAIN if trying later may yield some data. So how
about this instead?

diff --git a/fs/splice.c b/fs/splice.c
index 9b559ee..0670c91 100644
--- a/fs/splice.c
+++ b/fs/splice.c
@@ -1669,6 +1669,13 @@ static int link_pipe(struct pipe_inode_info *ipipe,
 		i++;
 	} while (len);
 
+	/*
+	 * return EAGAIN if we have the potential of some data in the
+	 * future, otherwise just return 0
+	 */
+	if (!ret && ipipe->waiting_writers && (flags & SPLICE_F_NONBLOCK))
+		ret = -EAGAIN;
+
 	inode_double_unlock(ipipe->inode, opipe->inode);
 
 	/*
@@ -1709,11 +1716,8 @@ static long do_tee(struct file *in, struct file *out, size_t len,
 		ret = link_ipipe_prep(ipipe, flags);
 		if (!ret) {
 			ret = link_opipe_prep(opipe, flags);
-			if (!ret) {
+			if (!ret)
 				ret = link_pipe(ipipe, opipe, len, flags);
-				if (!ret && (flags & SPLICE_F_NONBLOCK))
-					ret = -EAGAIN;
-			}
 		}
 	}
 

-- 
Jens Axboe