Re: [Xenomai] Unable to Read from Pipe

[Philippe Gerum <rpm@xenomai.org>]

On 07/20/2015 11:31 PM, C Smith wrote:
> I found a workaround for this, but I believe it illustrates a bug in
> xenomai 3.0rc5, which is also present in in xenomai 2.6.4.
> 
> The problem is that if the userspace writer of a pipe closes the pipe
> immediately after writing, the data does not get into the pipe reliably and
> the real-time reader process usually doesn't get the data. This is true
> even though the write() returns the expected number of bytes indicating
> success.
> 

This is intended. We must allow the regular side to force a shutdown on
the pipe since this is a valid use case, which would not be possible if
waiting for the rt input to drain was the only option before the kernel
actually closes the channel.

O_SYNC can be passed to open() to have the writer wait for the rt side
to start consuming the messages on the other end. In addition, we could
make the close() call wait until the rt input is fully consumed with
O_SYNC too, which is not the case so far (we currently sync on write()
only). This patch has been scrupulously untested.

diff --git a/ksrc/nucleus/pipe.c b/ksrc/nucleus/pipe.c
index df587ab..5367c4c 100644
--- a/ksrc/nucleus/pipe.c
+++ b/ksrc/nucleus/pipe.c
@@ -697,6 +697,14 @@ static int xnpipe_release(struct inode *inode,
struct file *file)

 	xnlock_get_irqsave(&nklock, s);

+	if ((file->f_flags & O_SYNC) != 0 && !emptyq_p(&state->inq)) {
+		if (xnpipe_wait(state, XNPIPE_USER_WSYNC, s,
+				emptyq_p(&state->inq))) {
+			xnlock_put_irqrestore(&nklock, s);
+			return -ERESTARTSYS;
+		}
+	}
+
 	xnpipe_dequeue_all(state, XNPIPE_USER_WREAD);
 	xnpipe_dequeue_all(state, XNPIPE_USER_WSYNC);

-- 
Philippe.