In soreceive_generic() remove the optimization for the case when

MSG_WAITALL is set, and it is possible to do the entire receive
operation at once if we block (resid <= hiwat). Actually it might make
the recv(2) with MSG_WAITALL flag get stuck when there is enough space
in the receiver buffer to satisfy the request but not enough to open
the window closed previously due to the buffer being full.

The issue can be reproduced using the following scenario:

On the sender side do 2 send(2) requests:

1) data of size much smaller than SOBUF_SIZE (e.g. SOBUF_SIZE / 10);
2) data of size equal to SOBUF_SIZE.

On the receiver side do 2 recv(2) requests with MSG_WAITALL flag set:

1) recv() data of SOBUF_SIZE / 10 size;
2) recv() data of SOBUF_SIZE size;

We totally fill the receiver buffer with one SOBUF_SIZE/10 size request
and partial SOBUF_SIZE request. When the first request is processed we
get SOBUF_SIZE/10 free space. It is just enough to receive the rest of
bytes for the second request, and soreceive_generic() blocks in the
part that is a subject of this change waiting for the rest. But the
window was closed when the buffer was filled and to avoid silly window
syndrome it opens only when available space is larger than sb_hiwat/4
or maxseg. So it is stuck and pending data is only sent via TCP window
probes.

Discussed with:	kib (long ago)
MFC after:	2 weeks

1 files changed, 2 insertions, 8 deletions

diff --git a/sys/kern/uipc_socket.c b/sys/kern/uipc_socket.c
index afbf41c85a13..c82b0de715e6 100644
--- a/sys/kern/uipc_socket.c
+++ b/sys/kern/uipc_socket.c
@@ -1496,17 +1496,11 @@ restart:
 	 * If we have less data than requested, block awaiting more (subject
 	 * to any timeout) if:
 	 *   1. the current count is less than the low water mark, or
-	 *   2. MSG_WAITALL is set, and it is possible to do the entire
-	 *	receive operation at once if we block (resid <= hiwat).
-	 *   3. MSG_DONTWAIT is not set
-	 * If MSG_WAITALL is set but resid is larger than the receive buffer,
-	 * we have to do the receive in sections, and thus risk returning a
-	 * short count if a timeout or signal occurs after we start.
+	 *   2. MSG_DONTWAIT is not set
 	 */
 	if (m == NULL || (((flags & MSG_DONTWAIT) == 0 &&
 	    so->so_rcv.sb_cc < uio->uio_resid) &&
-	    (so->so_rcv.sb_cc < so->so_rcv.sb_lowat ||
-	    ((flags & MSG_WAITALL) && uio->uio_resid <= so->so_rcv.sb_hiwat)) &&
+	    so->so_rcv.sb_cc < so->so_rcv.sb_lowat &&
 	    m->m_nextpkt == NULL && (pr->pr_flags & PR_ATOMIC) == 0)) {
 		KASSERT(m != NULL || !so->so_rcv.sb_cc,
 		    ("receive: m == %p so->so_rcv.sb_cc == %u",