From: Masami Hiramatsu <mhiramat@redhat.com>
Date: Fri, 7 Sep 2007 15:47:44 -0400
Subject: [misc] Fix relay read start position
Message-id: 46E1AAE0.4030907@redhat.com
O-Subject: [RHEL5.2 PATCH 1/2] Fix relay read() start position
Bugzilla: 250706

Hello,

This patch fixes a bug in the relay read interface causing the
number of consumed bytes to be set incorrectly.

When the relay file is closed and opened again, the read position is set to 0.
However, since the relay does not care this situation, it read again
the buffers which are already read.
To solve this problem, this patch re-calculate the read position from the
consumed bytes(which shows how many bytes are already read) if the
read position is 0.

The original patch was merged into upstream kernel.
Commit:     8d62fdebdaf9b866c7e236a8f5cfe90e6dba5773
http://www.mail-archive.com/git-commits-head@vger.kernel.org/msg14969.html

This patch can be applied for 2.6.18-44.el5 and resolves bz #250706

Testing: I built a test kernel, made a test script for this patch and tested:

---(testfill1.sh)
#!/bin/sh

stap - -m fillup -p4 << EOF
global counter=0
probe timer.ms(1) {
counter++;
printf("%08d\n", counter);
}
EOF
mv fillup.ko /tmp/fillup.ko
staprun -L /tmp/fillup.ko -o /dev/null
sleep 1
count1=`cat /sys/kernel/debug/systemtap/fillup/trace0 | tail -n 1`
sleep 1
count2=`cat /sys/kernel/debug/systemtap/fillup/trace0 | head -n 1`
/sbin/rmmod fillup.ko
rm /tmp/fillup.ko

if [ `expr $count1 + 1` -eq $count2 ]; then
        echo "test success"
else
        echo "test fail:" $count1 $count2
fi

Acked-by: Pete Zaitcev <zaitcev@redhat.com>
Acked-by: Josef Bacik <jbacik@redhat.com>

diff --git a/kernel/relay.c b/kernel/relay.c
index 85786ff..026dee7 100644
--- a/kernel/relay.c
+++ b/kernel/relay.c
@@ -829,7 +829,10 @@ static size_t relay_file_read_start_pos(size_t read_pos,
 	size_t read_subbuf, padding, padding_start, padding_end;
 	size_t subbuf_size = buf->chan->subbuf_size;
 	size_t n_subbufs = buf->chan->n_subbufs;
+	size_t consumed = buf->subbufs_consumed % n_subbufs;
 
+	if (!read_pos)
+		read_pos = consumed * subbuf_size + buf->bytes_consumed;
 	read_subbuf = read_pos / subbuf_size;
 	padding = buf->padding[read_subbuf];
 	padding_start = (read_subbuf + 1) * subbuf_size - padding;