Date: Wed, 20 Feb 2008 11:39:43 +0100
From: Jens Axboe <jens.axboe@oracle.com>
To: "Johann Felix v. Soden-Fr." <johfel@gmx.de>
Cc: Patrick McManus <mcmanus@ducksong.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] splice: fix problem with sys_tee and SPLICE_F_NONBLOCK
Message-ID: <20080220103941.GW23197@kernel.dk>
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On Wed, Feb 20 2008, Johann Felix v. Soden-Fr. wrote:
> On Wed, Feb 20, 2008 at 10:35:28AM +0100, Jens Axboe wrote:
> > 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.
> > > > 
> > 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;
> > -			}
> >  		}
> >  	}
> > 
> 
> Thanks! This works great.
> Add if you want: Tested-by: Johann Felix Soden <johfel@users.sourceforge.net>

Thanks for testing that it works as expected, I'll commit and add your
tested-by.

-- 
Jens Axboe

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