read() on relayfs channel returns premature 0

kingsley
2005-03-23T09:12:17Z

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Hi

I'm using relayfs to relay data from a kernel module to user space on
a SuSE 2.6.5 kernel. I'm not absolutely sure what version of relayfs
has been back ported to it.

While reading data from the channel I've been seeing read() return 0
prematurely. However, the 0 does not signify that the file is being
closed for there is still data available afterwards.

I've noticed that zeros occur when roughly one page of data has been
read. I suspect that they occur whenever there is a read across the
relayfs sub-buffers.

Now I understand that this is not the latest release of relayfs (there
are the redux patches, which I have yet to try). Nonetheless I'd like
to know whether this behaviour is deliberate. Is it?

Thanks,
--
Kingsley

P.S. I've been able to get around this by deliberately modifying
do_read() with the attached patch. I'm not absolutely sure its
correct but it seems to work.

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Index: fs/relayfs/relay.c
===================================================================
RCS file: /export/cvsroot/SuSE-Kernel-2.4.21/fs/relayfs/Attic/relay.c,v
retrieving revision 1.1.2.1
diff -u -r1.1.2.1 relay.c
--- fs/relayfs/relay.c 18 Feb 2005 00:00:51 -0000 1.1.2.1
+++ fs/relayfs/relay.c 23 Mar 2005 09:00:42 -0000
@@ -1445,26 +1445,22 @@

avail_offset = cur_idx = relay_get_offset(rchan, &max_offset);

+ last_buf_byte_offset = (read_bufno + 1) * buf_size - 1;
if (cur_idx == read_offset) {
if (atomic_read(&rchan->suspended) == 1) {
- read_offset += 1;
+ read_offset = last_buf_byte_offset + 1;
if (read_offset >= max_offset)
read_offset = 0;
*actual_read_offset = read_offset;
} else {
- *new_offset = read_offset;
+ *new_offset = read_offset;
return 0;
}
- } else {
- last_buf_byte_offset = (read_bufno + 1) * buf_size - 1;
- if (read_offset == last_buf_byte_offset) {
- if (unused_bytes != 1) {
- read_offset += 1;
- if (read_offset >= max_offset)
- read_offset = 0;
- *actual_read_offset = read_offset;
- }
- }
+ } else if ((read_offset + unused_bytes) > last_buf_byte_offset) {
+ read_offset = last_buf_byte_offset + 1;
+ if (read_offset >= max_offset)
+ read_offset = 0;
+ *actual_read_offset = read_offset;
}

read_bufno = read_offset / buf_size;