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

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@xxxxxxxxxxxxxxxxxxxxx>

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@xxxxxxxxxxxxxxxxxxxxx>

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

--
Jens Axboe

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