why does linux 0.11 use read lock when doing block reading? - linux-kernel

Question

I have read linux source code about block device part of version 0.11 and i found if two processes do reading block operation, one will lock the buffer(bh),another one has to wait this lock, my question is why reading will block each other by using such kind of locker? isn't it supposed to be write operation?
make_request (int major, int rw, struct buffer_head *bh)
{
struct request *req;
int rw_ahead;
if (rw_ahead = (rw == READA || rw == WRITEA))
{
if (bh->b_lock)
return;
if (rw == READA)
rw = READ;
else
rw = WRITE;
}
if (rw != READ && rw != WRITE)
panic ("Bad block dev command, must be R/W/RA/WA");
lock_buffer (bh);
if ((rw == WRITE && !bh->b_dirt) || (rw == READ && bh->b_uptodate))
{
unlock_buffer (bh);
return;
}
repeat:
if (rw == READ)
req = request + NR_REQUEST;
else
req = request + ((NR_REQUEST * 2) / 3);
/* find an empty request */
while (--req >= request)
if (req->dev < 0)
break;
if (req < request)
{
if (rw_ahead)
{
unlock_buffer (bh);
return;
}
sleep_on (&wait_for_request);
goto repeat;
}
req->dev = bh->b_dev;
req->cmd = rw;
req->errors = 0;
req->sector = bh->b_blocknr << 1;
req->nr_sectors = 2;
req->buffer = bh->b_data;
req->waiting = NULL;
req->bh = bh;
req->next = NULL;
add_request (major + blk_dev, req);
}